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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics (2011)

Chapter: 4 Report of the Panel on Planetary Systems and Star Formation

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Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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4
Report of the Panel on Planetary Systems and Star Formation

Only one generation in the history of the human species is privileged to live during the time those great discoveries are first made; that generation is ours.


—Carl Sagan

The Pale Blue Dot: Earth, as seen in 1990 from a distance of 40.6 AU, by Voyager 1. SOURCE: NASA/JPL.

The Pale Blue Dot: Earth, as seen in 1990 from a distance of 40.6 AU, by Voyager 1. SOURCE: NASA/JPL.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

SUMMARY

There is an opportunity in the coming decade to make fundamental advances in understanding the origins of stars and planets, and to ascertain the frequency of potentially habitable worlds. These compelling scientific opportunities have far-reaching implications in areas ranging from cosmic evolution and galaxy formation to the origins of life. The paths by which star-forming clouds produce stars and planet-forming disks have become much clearer over the past decade, and a startling diversity of planets orbiting nearby stars has been discovered. We now stand on the verge of determining whether habitable worlds are common in the galaxy. Moreover, there exists the immediate possibility of identifying any such worlds circling nearby very cool stars and of characterizing their physical properties and atmospheres as the search for signs of habitation is carried out. Now is the time to take advantage of this progress to answer some of the key questions of our cosmic origins that have inspired scientists and fascinated the public.

The Astro2010 Science Frontiers Panel on Planetary Systems and Star Formation was charged to consider science opportunities in the domain of planetary systems and star formation—including the perspectives of astrochemistry and exobiology—spanning studies of molecular clouds, protoplanetary and debris disks, and extrasolar planets, and the implications for such investigations that can be gained from ground-based studies of solar system bodies other than the Sun.1 The panel identifies four central questions that are ripe for answering and one area of unusual discovery potential, and it offers recommendations for implementing the technological advances that can speed us on our way. The questions and the area of unusual discovery potential are these:

  • How do stars form?

  • How do circumstellar disks evolve and form planetary systems?

  • How diverse are planetary systems?

  • Do habitable worlds exist around other stars, and can we identify the telltale signs of life on an exoplanet?

  • Discovery area: Identification and characterization of nearby habitable exoplanets.

How Do Stars Form?

The process of star formation spans enormous ranges of spatial scales and mass densities. The first stage involves the formation of dense structures that con-

1

The Astronomy and Astrophysics 2010 Survey of which this panel report is a part does not address solar system exploration, which is the subject of a parallel decadal survey.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

stitute only a small fraction of the volume and mass of a typical molecular cloud. Knowing how these dense regions form and evolve is vital to understanding the initiation of star formation, and it has implications for galactic and cosmic evolution. Yet the mechanisms controlling these processes are not well understood. To make further progress in characterizing the internal dynamical states of molecular clouds over a wide range of spatial scales and environments, the panel recommends the following:

  • Extensive dust and molecular-line emission surveys of massive giant molecular clouds spanning spatial scales from 100 to 0.1 parsec (pc) at distances greater than 5 kiloparsec (kpc), and

  • Complementary studies of the young stellar populations spawned in these regions, conducted by means of infrared surveys with spatial resolution at least 0.1 arcsec to reduce source confusion in clusters, with probing sufficiently faint to detect young brown dwarfs.

In the next stage of star formation, the dense structures in molecular clouds fragment into self-gravitating “cores” that are the direct progenitors of stars. There is mounting evidence from nearby star-forming regions that the distribution of core masses may be directly related to the resulting distribution of stellar masses, although some subsequent fragmentation likely produces binaries and very low mass objects. This may occur especially during the final stage of star formation through disk accretion. To explore this evolution and to improve core-mass spectra and characterize the core properties that may lead to subsequent fragmentation into stars, the panel recommends the following:

  • Deep surveys of cores down to sizes of 0.1 pc at millimeter and submillimeter wavelengths in diverse star-forming environments out to distances of several kiloparsecs, using both interferometers and large single-dish telescopes, far-infrared imaging and spectroscopy from spaceborne telescopes, and polarimetry to determine the role of magnetic fields.

An essential test of the understanding of star formation requires a definitive answer to the question of whether the initial mass function (IMF)—that is, the relative frequency with which stars of a given mass form—is independent of environment. This is a topic of great importance to an understanding of the development of galaxies and the production of heavy elements over cosmic time (see the discussion in Chapter 2, “Report of the Panel on Galaxies Across Cosmic Time”) in this volume. Initial investigations of massive young clusters using the Hubble Space Telescope (HST) and other large instruments have suggested that the IMF may be “top-heavy” (with larger fractions of massive stars) in very dense regions,

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

such as might prevail in starburst galaxies. To explore IMFs in more extreme environments, such as dense galactic regions and the nearest low-metallicity systems (the Magellanic Clouds), the panel recommends the following:

  • Near-infrared surveys with less than 0.1-arcsec resolution to limit source confusion in the galaxy and 0.01-arcsec resolution for the Magellanic Clouds.

Major theoretical efforts will be necessary to develop a fundamental understanding of these new observations, including improved treatment of thermal physics for an understanding of fragmentation and the origin of the IMF, along with better models for the chemical evolution of collapsing protostellar cores. More realistic calculations of the effects of massive stars on their environments (most dramatically in supernova explosions) are also needed to contribute to an understanding of how this feedback limits star-formation efficiencies. To facilitate these advances in the theoretical understanding of star formation and to enable the interpretation of complex data sets, the panel recommends the following:

  • The development of improved algorithms, greater computational resources, and investments in laboratory astrophysics for the study of the evolution of dynamics, chemistry, and radiation simultaneously in time-dependent models of star-forming regions.

How Do Circumstellar Disks Evolve and Form Planetary Systems?

Circumstellar disks are the outcome of the collapse of rotating protostellar cores. Both central stars and planets are assembled from disks. Major advances were made over the past decade in characterizing evolutionary timescales of protoplanetary disks, but their masses and structure are much less certain. In the coming decade, improved angular resolution will routinely yield resolved images of disks, providing keys to their mass, physical and chemical structure, and mass and angular momentum transport mechanisms, crucial to the understanding of both star and planet formation.

The superb new high-resolution, high-contrast imaging capabilities of the Atacama Large Millimeter Array (ALMA), the James Webb Space Telescope (JWST), and large optical/infrared ground-based telescopes with adaptive optics (AO) will revolutionize the present understanding of disks. Resolved submillimeter-wavelength measurements of dust emission will help constrain dust opacities and improve the understanding of disk masses and mass distributions. The direct detection of spiral density waves resulting from gravitational instabilities would enable independent estimates of disk masses and establish their role in mass and angular momentum transport. Spiral waves and gaps can also be produced by

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

forming planets; the latter may be directly detected within these gaps owing to their high luminosities during formation. Detections of forming planets would enable monumental advances in the understanding of planet formation. To achieve these goals, the panel recommends the following:

  • Studies of protoplanetary disks in nearby star-forming regions at resolutions below 100 milliarcsec, with every effort to achieve 10-milliarcsec resolution, at millimeter, submillimeter, infrared, and optical wavelengths, in order to map disk structure on spatial scales of approximately 1-10 AU;

  • Searches for infant planets in disk gaps using JWST, extreme-AO near-infrared (near-IR) imaging on 8- to 10-m-class telescopes, and eventually extreme-AO imaging with 30-m-class telescopes.

Improved imaging will also revolutionize the understanding of later-stage debris disks, illuminating planetary system architectures through the detection of structure in the debris formed by the collisions of numerous solid bodies undergoing dynamical evolution. To exploit these possibilities, the panel recommends the following:

  • Imaging debris disks in optical and near-IR scattered light on 8-m-class telescopes and in thermal dust emission at submillimeter wavelengths with ALMA and other interferometric arrays in order to search for resonant structures, gaps, and other features caused by the gravitational perturbations produced by planets, allowing the inference of unseen bodies and constraining their masses.

Similar dynamical instabilities also occurred early in the evolution of our own solar system, as indicated by resonant structures in the Kuiper belt. To improve vastly the understanding of the evolution of our solar system as well as to provide an essential link to the understanding of extrasolar debris disk systems, the panel recommends the following:

  • Systematic, whole-sky, synoptic studies to R magnitude ≥24 of Kuiper belt objects (KBOs).

The physics and chemistry of disks, particularly those in the protoplanetary phase, are extremely complex. To make progress in understanding these topics, the panel recommends the following:

  • Expanded theoretical efforts and simulations, with a detailed treatment of observational tracers to test theories, to develop an understanding of mass transport within disks and of the processes of coagulation and accretion that lead to planet formation; and

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
  • Major new efforts in chemical modeling and laboratory astrophysics to contribute to the understanding of the chemistry underlying molecule formation in the wide-ranging conditions in disks. In particular, laboratory studies of molecular spectra in the poorly studied far-infrared and submillimeter-wavelength regions of the spectrum are urgently needed to allow understanding and interpretation of the vast new array of spectral lines that are being detected by the Herschel mission and will be found by ALMA.

How Diverse Are Planetary Systems?

The past decade has seen a dramatic increase in the knowledge of the population and properties of planets orbiting nearby stars. Many more than 300 such exoplanets are now known, along with direct estimates of the densities and atmospheric temperatures for several dozen of these worlds. What has been learned from these exoplanets—mostly gas and ice giants—makes it clear that planetary systems are far from uniform. Yet these results apply just to the 14 percent of stars with close-in giant planets detectable with current techniques. The actual frequency of planetary systems in the galaxy and the full extent of their diversity, especially for small, rocky worlds similar to Earth, await discovery in the coming decade.

The recently commissioned Kepler mission is expected to yield the first estimate for the population of terrestrial exoplanets. However, the scientific return will be fully realized only if mass estimates can be obtained for a significant number of such planets. Therefore, the panel recommends the following:

  • Both a substantial expansion of the telescope time available to pursue radial-velocity work, and the development of advanced radial-velocity techniques with a target precision sufficient to detect an Earth-mass planet orbiting a Sun-like star at a distance of 1 AU.

This investment in radial-velocity precision will also augment the understanding of more massive worlds located at distances of 1-10 AU from their stars, which is the region of giant planets in our own solar system. Another promising approach is the detection of microlensing, which does not require that data be gathered over a full orbital cycle and can thus relatively rapidly provide detailed statistics on the masses and orbital separations of planets in the outer as well as inner reaches of planetary systems.

Thus the findings from Kepler combined with the results of a space-based microlensing survey will provide the essential statistics to test astronomers’ grand picture of how planetary systems form and whether the solar system is a commonplace occurrence or a cosmic rarity. Although the fundamental basis for understanding exoplanet diversity rests on measuring orbits and masses, and radii when possible, the chemistries, structures, and dynamics of exoplanet

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

atmospheres can be explored with spectra. Therefore, the panel recommends the following:

  • Extension of the eclipsing techniques currently employed with HST and the Spitzer Space Telescope to JWST, and

  • Extreme-contrast-ratio imaging with both the extant ground-based observatories and the next generation of giant segmented-mirror telescopes (GSMTs) in order to image planets with dynamical mass estimates and to calibrate models predicting emission from planets as a function of mass and age.

Do Habitable Worlds Exist Around Other Stars, and Can We Identify the Telltale Signs of Life on an Exoplanet?

One of the deepest and most abiding questions of humanity is whether there exist inhabited worlds other than Earth. Discovering whether or not such a planet exists within the reach of Earth’s astronomical observatories will have ramifications that surpass simple astronomical inquiry to impact the foundations of many scholarly disciplines and irrevocably to alter our essential picture of Earth and humanity’s place in the universe.

The goal of detecting life on other worlds poses daunting technological challenges. A Sun-like star would be 100 times larger, 300,000 times more massive, and 10 million to 10 billion times brighter than a terrestrial planet with an atmosphere worthy of studying. Although several techniques have been proposed to achieve detection of biomarkers, it is currently premature to decide the technique and scope of such a mission. Rather, the panel endorses the finding of the National Science Foundation-National Aeronautics and Space Administration-U.S. Department of Energy (NSF-NASA-DOE) Astronomy and Astrophysics Advisory Committee (AAAC) Exoplanet Task Force2 that two key questions that will ultimately drive the technical design must first be addressed:

  1. What is the rate of occurrence of Earth-like planets in the habitable zones of Sun-like stars, and hence at what distance will the target sample lie?

  2. What is the typical brightness of the analogs of the zodiacal light disks surrounding solar analogs; in particular, do a significant fraction of stars have dust disks that are so bright as to preclude the study of faint Earth-like planets?

Kepler will address the first question, but the means to answer the second, perhaps through ground-based interferometry or space-based coronagraphy, has

2

The full report, ExoPlanet Task Force, Worlds Beyond: A Strategy for the Detection and Characterization of Exoplanets, Washington, D.C., May 22, 2008, is available at http://www.nsf.gov/mps/ast/aaac/exoplanet_task_force/reports/exoptf_final_report.pdf.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

yet to be fully developed. Provided that Earth analogs are sufficiently common, the panel recommends the following as the preferred means to identify targets with appropriate masses:

  • A space-based astrometric survey of the closest 100 Sun-like stars with a precision sufficient to detect terrestrial planets in the habitable zones.

The characterization effort lies beyond the coming decade, but it could be achieved in the decade following, provided that the frequency of Earth analogs is not too low. The panel recommends the following:

  • A strong program to develop the requisite technologies needed for characterization should be maintained over the coming decade.

Discovery Area: Identification and Characterization of Nearby Habitable Exoplanets

An exciting possibility in the coming decade is the detection of possibly habitable, large, rocky planets (super-Earths) orbiting the abundant and nearby stars that are much less massive than the Sun (less than 0.3 solar masses). The panel deems this to be the single greatest area for unusual discovery potential in the coming decade, as it can be carried out with current methods provided that the necessary resources are made available.

The low luminosities of these cool, low-mass stars in the solar neighborhood ensure that the conditions for liquid water to exist on the surface of an orbiting planet occur at a small separation of planet and star. The small stellar size, low stellar mass, and small orbital separation for habitable conditions all conspire to facilitate the discovery of super-Earths by a combination of the two detection methods that have proven the most successful to date: stellar radial velocities and timing of obscuration due to planetary transits of host stars. These techniques, currently refined for the study of Sun-like stars, need to be adapted for cooler, low-mass stars. Therefore, the panel recommends the following:

  • Increasing the amount of observing time available for radial-velocity studies,

  • Investing in precision radial-velocity techniques at longer wavelengths, and

  • Developing novel methods to calibrate the new, longer-wavelength spectrographs.

A far-reaching outcome of this investment is that the atmospheres of transiting super-Earths would be amenable to spectroscopic study with JWST and a future

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

GSMT, permitting a search for biomarkers in the coming decade. Thus, the panel recommends the following:

  • The closest 10,000 M-dwarfs should be surveyed for transiting super-Earths in their stellar habitable zones in time to ensure that the discoveries are in hand for JWST.

The discovery of even a handful of such worlds would present an enormous scientific return, fundamentally alter our perspective on life in the universe, and offer a hint of what might be expected for the properties of terrestrial worlds around Sun-like stars.

Summary of Requirements

The conclusions of this panel report are summarized in Table 4.1.

TABLE 4.1 Summary of Conclusions of the Panel on Planetary Systems and Star Formation

 

Question 1: How Do Stars Form?

Question 2: How Do Disks Evolve and Form Planetary Systems?

Question 3: How Diverse Are Planetary Systems?

Question 4: Can We Identify the Telltale Signs of Life on an Exoplanet?

Discovery Area: Identification and Characterization of Nearby Habitable Exoplanets

Facilities expected

EVLA, ALMA, Herschel, SOFIA, JWST, 8- to 10-m telescope with AO

EVLA, ALMA, Herschel, JWST, 8- to 10-m telescope with AO, UV/visible synoptic surveys

1 m sec–1 RV surveys and transit follow-up; Kepler, JWST, Spitzer transits; Gaia astrometry

1 m sec–1 RV surveys and transit follow-up; Kepler, JWST, Spitzer transits

JWST transiting-exoplanet spectroscopy

New facilities needed

30-m submillimeter telescope; 8- to 10-m telescope with MCAO; GSMT with centimeter-wave interferometry on very long baselines

GSMT with extreme AO; near-IR synoptic surveys

0.2 m sec–1 RV; microlensing surveys; GSMT with extreme AO

Earth-like planet frequency ; 10-zody limits on exozodies; 0.1-μas astrometry

Census and transit survey, 104 nearest M-dwarfs; visible/near-IR RV follow-up

Always needed

Support for theoretical efforts, including high-performance computational resources, and laboratory-molecular astrophysics with an emphasis on far-infrared, submillimeter, and millimeter line identifications, along with chemical studies ranging from surface reactions relevant to cold clouds to processes in planetary atmospheres.

NOTE: Acronyms are defined in Appendix C.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

INTRODUCTION

Human beings exist, in part, because we live on a rocky planet that has an atmosphere and water and is warmed appropriately by a long-lived star. As knowledge of the universe has expanded over the centuries, so has speculation about the possibility of life elsewhere. Remarkably, we now stand on the threshold of developing the technology needed to detect other habitable planets, and to determine how common they are in the galaxy. Primed by the discovery over the past 15 years of significant numbers of other planetary systems, by the realization that planet-forming disks result as a natural and frequent by-product of star formation, and by major advances in characterizing the properties of the gas clouds that form stars, scientists are now ready to make fundamental progress on the central questions related to the birth of stars and planets. Following its charge, the Panel on Planetary Systems and Star Formation identified four questions that it considers as ripe for answering in the coming decade, as well as one discovery area:

  • How do stars form?

  • How do circumstellar disks evolve and form planetary systems?

  • How diverse are planetary systems?

  • Do habitable worlds exist around other stars, and can we identify the telltale signs of life on an exoplanet?

  • Discovery area: Identification and characterization of nearby habitable exoplanets.

In the sections that follow, the panel explores each of these questions and identifies key observational and theoretical advances (set apart with bullets and summarized in tables at the end of each major section) that are necessary to make the associated, fundamental advances.

PSF 1. HOW DO STARS FORM?

Star formation plays a crucial role in many important astrophysical processes, ranging from galactic evolution to planet formation. Major investments over the past decade in both observational and computational facilities have brought astronomers to the verge of developing a quantitative understanding of how, where, and when stars form; why planet-forming disks result from protostellar cloud collapse; the ways in which the formation of massive stars differ from that of solar-type stars; and how the energy input from massive stars drives the evolution and destruction of star-forming clouds. New facilities enabling both highly detailed studies of individual, nearby objects and large-scale surveys of diverse star-forming regions will allow scientists to address three key aspects of the star formation pro-

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

cess: What sets the overall rate and efficiency of star formation? What determines the properties of star-forming cloud cores? And finally, is the initial distribution of stellar masses universal or a function of environment?

What Determines Star-Formation Rates and Efficiencies in Molecular Clouds?

The rates at which stars form over the age of the universe strongly affect galactic structure and cosmic evolution (see also, the second key science question, GCT 2, in Chapter 3, “Report of the Panel on Galaxies Across Cosmic Time”). Increasingly detailed studies of external galaxies have led to improved Kennicutt-Schmidt “laws” relating large-scale gas content and other global galactic properties to star-formation rates. To develop a comprehensive theory of how star formation depends on environment, large-scale extragalactic studies need to be complemented by investigations on the much smaller spatial scales on which clouds actually fragment into clusters and stars (Figure 4.1). Here the panel focuses on the opportunities

FIGURE 4.1 Schematic of the hierarchy of star formation. Left: Hubble Space Telescope image of the spiral galaxy M51, with Hα emission (in red) tracing the massive star-forming regions with a pixel scale equivalent to 5 pc. Center: Nearby Orion A molecular cloud traced by 13CO J = 1 → 0 emission, where the colors represent radial Doppler velocity. Right: Orion Nebula Cluster as seen by the IRAC infrared camera on the Spitzer Space Telescope. SOURCE: Left: N.Z. Scoville, M. Polletta, S. Ewald, S.R. Stolovy, R. Thompson, and M. Rieke, High-mass, OB star formation in M51: Hubble Space Telescope Hα and Paα imaging, Astronomical Journal 122(6):3017-3045, 2001, reproduced by permission of the AAS. Center: J. Bally, Overview of the Orion Complex, in Handbook of Star Forming Regions, Vol. I. (B. Reipurth, ed.), Astronomical Society of the Pacific, San Francisco, Calif., 2008, reproduced by kind permission of the Astronomical Society of the Pacific. Right: NASA/JPL-Caltech/University of Toledo.

FIGURE 4.1 Schematic of the hierarchy of star formation. Left: Hubble Space Telescope image of the spiral galaxy M51, with Hα emission (in red) tracing the massive star-forming regions with a pixel scale equivalent to 5 pc. Center: Nearby Orion A molecular cloud traced by 13CO J = 1 → 0 emission, where the colors represent radial Doppler velocity. Right: Orion Nebula Cluster as seen by the IRAC infrared camera on the Spitzer Space Telescope. SOURCE: Left: N.Z. Scoville, M. Polletta, S. Ewald, S.R. Stolovy, R. Thompson, and M. Rieke, High-mass, OB star formation in M51: Hubble Space Telescope Hα and Paα imaging, Astronomical Journal 122(6):3017-3045, 2001, reproduced by permission of the AAS. Center: J. Bally, Overview of the Orion Complex, in Handbook of Star Forming Regions, Vol. I. (B. Reipurth, ed.), Astronomical Society of the Pacific, San Francisco, Calif., 2008, reproduced by kind permission of the Astronomical Society of the Pacific. Right: NASA/JPL-Caltech/University of Toledo.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

over the next decade to advance the understanding of star-formation efficiencies and rates from the study of local (galactic) star-forming regions.

Local molecular clouds convert only a few percent of their mass into stars before they are dispersed, probably due to stellar energy input; efficiencies may be still lower in the massive giant molecular clouds (GMCs) that contain most of the galaxy’s molecular mass. These low star-formation efficiencies are clearly related to the complex structures of molecular clouds, in which only a small fraction of the mass resides in high-density regions prone to gravitational collapse. Understanding how these dense regions arise within much larger volumes of low-density, magnetized, and supersonically “turbulent” gas is an essential first step in star-formation theory, and poses major challenges. Numerical simulations of molecular clouds over the past decade have made great strides, yielding a fraction of gravitationally collapsing dense gas in rough agreement with observations. However, the nature and origin of the supersonic turbulence that produces dense structures are uncertain, and the final efficiency of star formation depends in part on cloud dispersal, which has only recently begun to be addressed numerically.

A quantitative understanding of how dense star-forming cloud structures are produced will rest on the observational characterization of the physical and dynamical states of molecular clouds, from the typical ~100 pc sizes of GMCs to the ≤0.1 pc scales of the densest structures. The panel recommends the following:

  • Surveys to enable a combination of wide- and narrow-field studies of molecular clouds in millimeter and submillimeter dust continuum emission and a range of molecular tracers from low to high density using current and new telescopes under development (e.g., ALMA, Herschel, EVLA), along with the implementation of large-format heterodyne and bolometer arrays on single-dish telescopes and interferometers.

  • Studies beyond the solar neighborhood out to ~8 kpc, which would allow access to the Milky Way’s “molecular ring” and the galactic center, to measure star-formation efficiencies and rates in massive GMCs, which are the sites of most star formation. A spatial scale of 0.1 pc, which would resolve the densest gas, corresponds to 3 arcsec at the galactic center; surveys need to be complete for cores of mass to probe near the peak of the core and stellar mass functions (see the next two subsections below).

  • Polarimetric imaging from millimeter through infrared wavelengths and measurements of the Zeeman effect using key molecules at radio wavelengths to characterize the dynamical role of magnetic fields and relate these measures of gas properties to star-formation rates and efficiencies.

  • Infrared surveys with high spatial resolution, ~0.1 arcsec to minimize source confusion in clusters (assuming conditions similar to the Trapezium region in the

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

Orion Nebula Cluster), and probing down to K = 19 mag to detect young brown dwarfs to characterize the associated stellar populations.

Deriving stellar masses requires comparison to pre-main sequence models and corrections for differential extinction and binarity. Understanding in this area will be greatly advanced by the European Space Agency’s Gaia mission, which will provide accurate distances to vast numbers of stars. In addition, the proper motion measurements from Gaia will provide new insight into the dynamical states of star-forming regions. The Very Long Baseline Array (VLBA) will also contribute by measuring parallaxes for those young stars having strong nonthermal emission, and will be able to map out the locations of star-forming regions throughout the galaxy using water and methanol masers.

Energy input from young stars—ionizing radiation, winds, radiation pressure, and supernova explosions—generates dynamically important turbulence, and clearly limits star-formation efficiencies and rates by disrupting and dispersing molecular clouds. To understand how stellar energy input or feedback inhibits star formation in some regions and triggers it in others, the panel recommends the following:

  • Detailed investigations of GMC complexes—including low-frequency radio surveys for supernova remnants and high-resolution imaging of embedded protostars to measure their ionized winds, H II regions, infrared luminosities, and molecular outflows—to separate the various processes. Studies characterizing the effects of feedback should be able to resolve ultracompact H II regions (typically 0.1 pc) and CO outflows (down to a few times 0.01 pc) in a variety of different environments including the galactic center (8 kpc away), requiring sub-arcsecond imaging at all wavelengths. These measurements are therefore best achieved using interferometers at radio through to submillimeter wavelengths (e.g., ALMA, CARMA, EVLA, future centimeter-wavelength instruments), and near- and mid-infrared imaging and spectroscopy using ground-based telescopes up to 30-m class and JWST.

Improved theoretical tools will be needed to interpret these observations and to develop quantitative theories of the origin of cloud structure leading to star formation. As enormous dynamic ranges in size and density are involved, the panel recommends the following:

  • Investment in improved algorithms and major computational resources, as well as insight into which subproblems can be tackled separately. Major improve-

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

ments are required in treating radiative transfer and thermal physics in simulations (critical to fragmentation), along with better chemical models to facilitate comparison with observations.

Incorporating more complex physics, including magnetic fields and stellar energy input (ionizing radiation, winds, radiation pressure, supernova explosions) is essential to the development of a deep understanding of how pre-stellar cloud cores are assembled.

What Determines the Properties of Pre-Stellar Cloud Cores?

Stars form from dense cores within molecular clouds. Cores represent the mass reservoirs from which stars form, and their angular momentum content is responsible for the formation of protoplanetary disks and probably fragmentation into multiple star systems. Developing a quantitative theory of star formation requires an understanding of how cores form, which in turn necessitates the characterization of individual core properties and the dependence of these properties on environment.

Over the past decade, surveys with millimeter and submillimeter bolometer-array cameras and infrared extinction mapping have identified large enough samples of cores to begin investigating the distribution of core masses. Encouragingly, several different studies have found core mass functions (CMFs) that appear to be similar to the stellar IMF for stars at moderate masses (; Figure 4.2). These findings suggest that, at least in some regimes, there may be a relatively direct mapping from the CMF to the IMF. However, there are significant systematic uncertainties in determining core masses, not least of which is that most cores do not have well-defined boundaries. Fragmentation into multiple systems during the evolution from cores to stars may be especially important in defining the transition from the CMF to the IMF at low masses, while the accretion of intracluster gas may be particularly important in making the highest-mass stars.

Better characterization of the properties of cores and their immediate environments will require measurements of both dust emission and molecular line tracers to disentangle local chemical and radiative transfer effects. The accuracy of core mass estimates from continuum measurements is dependent on the dust emissivity and temperature; multiwavelength (λ ~ 0.4-1.1 mm) continuum observations are needed to provide better estimates of dust emissivities and temperatures in cores that will yield more accurate mass estimates. Kinematic studies, with molecular line observations, of both cores and their immediate environments are especially important, not only to probe the dynamic states of cores and their angular momentum content, but to eliminate “false” cores resulting from line-of-sight projections, distinguish multiple superposed cores, and measure continued accretion of

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
FIGURE 4.2 The core mass function (CMF) of the Orion B molecular cloud, derived from dust continuum emission. As has been found in other studies, the CMF for (dotted line) is similar in slope to the high-mass slope of the stellar IMF (solid line), but the possible turnover at lower masses is uncertain, as completeness limits become important (vertical dashed line). SOURCE: Reprinted with permission from D. Nutter and D. Ward-Thompson, A SCUBA survey of Orion—The low-mass end of the core mass function, Monthly Notices of the Royal Astronomical Society 374:1413-1420, 2007, copyright 2007 Royal Astronomical Society.

FIGURE 4.2 The core mass function (CMF) of the Orion B molecular cloud, derived from dust continuum emission. As has been found in other studies, the CMF for (dotted line) is similar in slope to the high-mass slope of the stellar IMF (solid line), but the possible turnover at lower masses is uncertain, as completeness limits become important (vertical dashed line). SOURCE: Reprinted with permission from D. Nutter and D. Ward-Thompson, A SCUBA survey of Orion—The low-mass end of the core mass function, Monthly Notices of the Royal Astronomical Society 374:1413-1420, 2007, copyright 2007 Royal Astronomical Society.

matter onto cores from their surroundings. To accomplish these studies the panel recommends the following:

  • Millimeter- and submillimeter-wavelength interferometers such as ALMA, observations with large (>15 m) single-dish telescopes at millimeter and submillimeter wavelengths, far-infrared (far-IR) imaging and spectroscopy from spaceborne telescopes, and polarimetry to determine the role of magnetic fields; and

  • High-resolution imaging of cores in far-IR/submillimeter continuum and in high-density molecular line tracers is needed to explore how nonspherical, asymmetric core geometries may produce further fragmentation into multiple star systems. These studies should be complete for cores of and extend to regions forming the full range of stellar masses, to encompass those cores with the potential for forming massive planets.

As is the case for star-formation efficiencies, it is crucially important to extend the current set of CMF determinations, which are derived mostly from the nearest, low-density star-forming regions, to the more distant sites where massive stars and star clusters are being formed. These regions are responsible for most star formation; their study is needed to develop an understanding of whether the CMF (and ultimately the IMF; see the subsection below titled “What Is the Origin of the Stellar Mass Function?”) varies with environment. Initial steps toward this goal have been taken, based on the detection in mid-infrared absorption of massive filaments in distant (few kiloparsec) clouds—the infrared dark clouds (IRDCs)—using the Spitzer Space Telescope (Figure 4.3). Only a relatively small number of IRDCs have been studied in any detail so far, but further studies may lead to an understanding

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
FIGURE 4.3 Three-color (RGB = 24, 8, 3.6 microns) Spitzer image of an Infrared Dark Cloud (IRDC) with white CARMA N2H+ (J = 1-0) contours. Such IRDCs are likely the sites of future massive star and cluster formation. SOURCE: C. Brogan, T. Hunter, R. Indebetouw, and A. Wootten, in preparation. Courtesy of C. Brogan, personal communication.

FIGURE 4.3 Three-color (RGB = 24, 8, 3.6 microns) Spitzer image of an Infrared Dark Cloud (IRDC) with white CARMA N2H+ (J = 1-0) contours. Such IRDCs are likely the sites of future massive star and cluster formation. SOURCE: C. Brogan, T. Hunter, R. Indebetouw, and A. Wootten, in preparation. Courtesy of C. Brogan, personal communication.

of how molecular clouds fragment to produce stellar clusters and how the dense gas fraction (see the subsection above titled “What Determines Star Formation Rates and Efficiencies in Molecular Clouds?”) relates to the properties of the surrounding molecular material on scales up to about 100 pc.

For studies of IRDCs and similar regions the panel recommends the following:

  • A combination of interferometers and array cameras on single-dish millimeter and submillimeter telescopes in order to provide the full dynamic range of spatial scales needed to minimize biases introduced by observing technique.

  • Extending core detections to sufficiently low masses that the turnover in the CMF—and not just a high-mass power-law “tail”—can be detected definitively. Surveys will need to be complete to below for the most distant regions and to in nearby regions to achieve this goal.

In the nearest IRDCs at distances of 2 kpc, a core of spatial scale of 0.01 pc corresponds to a resolution of 1 arcsec. For galactic-center star-forming regions at distance of 8 kpc, the corresponding resolution is 0.3 arcsec.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

Numerical studies over the past decade have demonstrated that turbulence within molecular clouds is likely to be responsible for creating many of the observed (transient) structures at moderate to high densities, with gravity further compressing these structures into protostellar condensations. However, realistic multi-physics models of cloud formation and internal evolution (including feedback) are only now becoming possible. CMFs seen in numerical simulations—including turbulence, self-gravity, and diffusion of partially ionized gas through magnetic fields—increasingly resemble observations, but much work is still needed in order to develop a predictive theory connecting large-scale cloud properties (which vary widely) and the CMF. As discussed in the subsection above on starformation rates, the panel recommends the following:

  • Improvements in the radiation transfer and thermal physics of simulations to contribute to the understanding of gravitational fragmentation and consequent core formation; and

  • Better models of chemical evolution to enable detailed tests of theories by comparison with observations.

Almost all far-infrared, submillimeter, and radio observations rely on the use of molecular lines as probes of molecular gas evolution. The interpretation of these data therefore requires knowledge of the relevant molecular spectra in the laboratory and is particularly important for understanding warm regions of the interstellar medium where the spectral density in the millimeter-wave and submillimeter-wave regions is high, and precise wavelengths are required. Comparisons of line observations and chemical simulations containing both gas-phase and grain-surface chemistry have been used to constrain estimates of the lifetimes and physical conditions of evolutionary stages leading to the formation of low-mass stars (e.g., pre-stellar cores) and, to a lesser extent, high-mass stars (e.g., hot cores). Therefore the panel recommends expanded laboratory work in two areas:

  • Spectroscopy in the far-infrared, a poorly studied region, which will be opened up by Herschel and SOFIA; and

  • Studies to improve knowledge of the relevant chemical processes on small grains, especially the surface chemistry, which also requires a detailed understanding of stochastic effects. Chemical simulations will have to be coupled more strongly to heterogeneous and dynamically evolving cloud models.

What Is the Origin of the Stellar Mass Function?

The distribution of masses with which stars form—the IMF—is an essential ingredient in studies of galaxy formation and the evolution of heavy-element abun-

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

dances; the proportion of high-mass stars to the total production of stellar mass controls the cycles of mass and energy, as well as chemical cycles within galaxies. In the preceding subsection the panel discussed the starting point for understanding the origin of the IMF: specifically, the properties of star-forming cores and how they are produced. Here the transformation from CMF to IMF and the dependence of the IMF on the star-forming environment are addressed.

Although the similarity in shape of the CMF to the stellar IMF hints at a direct mapping between the two, dispersal and subsequent core fragmentation into binary and multiple stars must occur; in particular, formation of the lowest-mass objects may require fragmentation in disks. To probe the transformation from the CMF to the IMF will require the following:

  • High-angular-resolution studies of molecular line emission in collapsing cores with ALMA to resolve the transition zones between the infalling envelope of cores and their protostellar, possibly circumbinary, disks on scales of ~100 AU, where further gravitational fragmentation may occur;

  • Further laboratory studies of molecular spectra in the far-infrared and submillimeter regions to contribute to the understanding and interpretation of the vast new array of spectral line observations that will form the basis for sophisticated physical and chemical models; and

  • Improved numerical simulations, including magnetic fields and feedback.

In the nearest star-forming clouds, near-IR observations have reached sufficient angular resolution to distinguish individual stars in crowded regions and to start to resolve wide binaries. As new facilities such as JWST and adaptive optics systems for ground-based 8- to 10-m telescopes become available, such studies can be extended to more distant regions, thereby probing a wider variety of environments. Typical stellar separations in a region with crowding similar to the Trapezium region in the Orion Nebula Cluster are ~0.1 arcsec if viewed at a distance of 8 kpc; a wide binary with separation of 100 AU subtends ~10 mas at this distance. The greater resolving power of GSMT plus AO would make significant improvements in the knowledge of the embedded stellar content of cores out to the distances of massive galactic star-forming regions, although the resolution of protostellar multiplicity will likely remain confined to relatively nearby clouds. Gaia will complement this effort by providing accurate astrometry for the detection of binary companions.

Estimates of star-formation rates in high- and intermediate-redshift galaxies typically assume a specified, usually invariant, form of the IMF. It is crucial to determine whether the IMF is in fact universal or whether it is top-heavy in the densest regions, as suggested by studies of the galactic center and some galactic massive clusters (Figure 4.4). To eliminate major uncertainties introduced by this

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
FIGURE 4.4 Left: Hubble Space Telescope image of the dense star cluster NGC 3603, at a distance of ~6 kpc. Right: the mass function derived for NGC 3603 is slightly flatter than that derived for the local stellar population. SOURCE: Left: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration. Acknowledgment: Jesús Maíz Apellániz (Instituto de Astrofísica de Andalucía, Spain) and Davide de Martin (skyfactory.org). Right: A. Stolte, W. Brandner, B. Brandl, and H. Zinnecker, The secrets of the nearest starburst cluster. II. The present-day mass function in NGC 3603, Astronomical Journal 132:253-270, 2006, reproduced by permission of the AAS.

FIGURE 4.4 Left: Hubble Space Telescope image of the dense star cluster NGC 3603, at a distance of ~6 kpc. Right: the mass function derived for NGC 3603 is slightly flatter than that derived for the local stellar population. SOURCE: Left: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration. Acknowledgment: Jesús Maíz Apellániz (Instituto de Astrofísica de Andalucía, Spain) and Davide de Martin (skyfactory.org). Right: A. Stolte, W. Brandner, B. Brandl, and H. Zinnecker, The secrets of the nearest starburst cluster. II. The present-day mass function in NGC 3603, Astronomical Journal 132:253-270, 2006, reproduced by permission of the AAS.

assumption, much larger and more sensitive surveys of the IMF in a wider variety of environments are needed, probing down to substellar masses. In particular, the panel recommends the following:

  • Measurements of the IMF in the lower-metallicity environment of the Large Magellanic Cloud. At ~50 kpc, approximately 10 times better angular resolution is needed compared with what is required for the galactic center, and sensitivities down to K ~ 23 mag are needed to probe a similar range of the IMF.

It is challenging to obtain an accurate local IMF across the full mass range. High-mass stars are rare except in massive star-forming regions, but at the distance of most such regions, lower-mass stars are difficult to resolve. In older clusters, dynamical effects on the population, such as mass segregation and ejection, are important. Thus investigations must be done in young—often, heavily extinguished—regions. Furthermore, while the IMF of massive stars on radiative tracks may be

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

determined adequately from photometric infrared luminosity functions, accurate mass distributions for the low-mass stars and brown dwarfs require theoretical and observational calibrations of mass-luminosity relations as a function of age, which also requires good distance determinations using trigonometric parallaxes and spectroscopic identification.

To obtain improved determination of IMFs down to substellar masses over a range of environments, the panel recommends the following:

  • High-resolution infrared imaging in the galaxy on 8- to 10-m-class telescopes with wide-field AO (Figure 4.5) and near-IR integral-field spectrographs (IFUs);

FIGURE 4.5 Wide fields, high spatial resolution, and high image contrast are needed to cover large areas while separating clusters into individual stars. Multiconjugate adaptive optics can potentially achieve these goals. Left: Simulated observation of an open stellar cluster at H (1.65 μm) with a Gemini 8-m telescope, using classical (single-phase-reference) adaptive optics and the proposed Gemini MCAO instrument. The image is well corrected over the central 30 arcsec of the classical AO image, and over the entire 2.75-arcmin field in the MCAO image. Right: Slice through the center of a typical point-spread function (PSF) at H, simulated for Gemini MCAO. The PSF diameter is 0.05 arcsec (FWHM). Also plotted are the relative brightnesses of stars with that of an O5 star scaled to the peak of the PSF, indicating the proximity to O stars at which fainter stars can be detected. SOURCE: Courtesy of Gemini Observatory/AURA.

FIGURE 4.5 Wide fields, high spatial resolution, and high image contrast are needed to cover large areas while separating clusters into individual stars. Multiconjugate adaptive optics can potentially achieve these goals. Left: Simulated observation of an open stellar cluster at H (1.65 μm) with a Gemini 8-m telescope, using classical (single-phase-reference) adaptive optics and the proposed Gemini MCAO instrument. The image is well corrected over the central 30 arcsec of the classical AO image, and over the entire 2.75-arcmin field in the MCAO image. Right: Slice through the center of a typical point-spread function (PSF) at H, simulated for Gemini MCAO. The PSF diameter is 0.05 arcsec (FWHM). Also plotted are the relative brightnesses of stars with that of an O5 star scaled to the peak of the PSF, indicating the proximity to O stars at which fainter stars can be detected. SOURCE: Courtesy of Gemini Observatory/AURA.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
  • Similar instrumentation on a 30-m-class telescope to extend observations to the Magellanic Clouds;

  • More investment in theoretical pre-main sequence evolutionary tracks and their calibration;

  • Deep X-ray surveys to identify low-mass populations, particularly for members without infrared excesses, with sensitivities to X-ray luminosities down to a few times 1028 erg s–1 for emission at T > 107 K at the galactic center to detect low-mass members without disks and thus establish the full range of the IMF; and

  • Wide-field, high-precision relative astrometry of the cluster with respect to the background field for establishing cluster membership.

Conclusions: Star Formation

Table 4.2 summarizes the panel’s conclusions on activities to address its first science question.

PSF 2. HOW DO CIRCUMSTELLAR DISKS EVOLVE AND FORM PLANETARY SYSTEMS?

Disks are integral both to the formation of stars over a wide range of masses and to their planetary systems. Formed as a consequence of the angular momentum in a molecular cloud core undergoing gravitational collapse, disks initially build stars during a phase of active accretion. Later, as the mass reservoir from the collapsing core is dissipated, the thinning disk provides a fertile environment for the growth of planets. Images of disks around young stars, both from optical/near-infrared light scattered by small grains along the surface and from submillimeter-and millimeter-wavelength emission from large grains in the midplane, show that many disks have sizes at least as large as that of our own solar system if not much larger. The ubiquity of potentially planet-forming disks around the youngest stars, demonstrated most recently by a complete photometric census of young clusters closer than a kiloparsec with Spitzer, surpasses the current exoplanet detection frequency, suggesting that the majority of nearby planetary systems await discovery.

Astronomers’ understanding of the evolution of protoplanetary disks advanced dramatically in the past decade. It is now known that by 3 Myr, dust in the inner few tens of astronomical units is strongly depleted in about half the stars in a cluster (Figure 4.6), in pace with the depletion of disk gas and the diminution of stellar accretion. By 6 Myr, nearly all inner disks have disappeared; this limits the timescale for planet formation. About 10 percent of disks around stars younger than 3 Myr have breaks in their spectral energy distributions indicative of dust-free inner holes or gaps 1 to 40 AU across, with a few of the largest confirmed by submillimeter and

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

TABLE 4.2 Panel’s Conclusions Related to the Study of Star Formation

Observation

Type Area

Angular Resolution

Sensitivity

Instrumentation

Science Area

Dust continuum emission survey

Galactic plane (103 deg2); detailed studies of selected star formation (SF) regions (102 deg2)

1-20″

Galactic-plane surveys: (≤5 mJy/beam at λ = 0.4-1.1 mm for dust at 2 kpc); detailed studies:

ALMA; Herschel; large-format (103-104 pixels) bolometer array cameras on large, single-dish millimeter/submillimeter/far-IR telescopes

SF rates/SF efficiencies; core properties, CMF; physics of CMF/IMF connection

Molecular and atomic line emission surveys

Galactic plane (103 deg2); detailed studies of selected SF regions (102 deg2)

1-20″

0.05 K at ∆v = 0.1 km/s

ALMA; Herschel; SOFIA; EVLA: heterodyne focal plane arrays on millimeter interferometers and large single-dish telescopes; wide-field imaging capabilities and large collecting areas for centimeter-wave interferometers

SFRs/SFEs, feedback; core properties, CMF; physics of CMF/IMF connection

Radio continuum survey

Selected SF regions (102 deg2)

≤1″

1-100 μJy/beam at 1-50 GHz

EVLA; large collecting areas for other centimeter-wave interferometers

Role of ionized gas in feedback; physics of CMF/IMF connection

Infrared, submillimeter, and millimeter polarization imaging

Selected SF regions (102 deg2)

0.5″-1″

1-σ IR polarization fraction ~0.1% to infer magnetic field direction from measured position angle; 1-σ millimeter/submillimeter polarization fraction <1%

O/IR polarimeter on 8-m-class telescope; MIR polarimeter on SOFIA; polarimeters on submillimeter/millimeter telescopes

Role of B-fields in SFR/SFE; core properties; physics of CMF/IMF connection

Zeeman measurements

Selected SF regions (102 deg2)

~10 mas for masers; 0.5″-30″ for disks to cores

1 mK rms with ∆v = 0.1 km/s to measure small Zeeman shifts for typical magnetic fields in star-forming clouds

VLBI, ALMA, EVLA, and larger-area radio interferometers, heterodyne focal plane arrays on millimeter interferometers and large single-dish telescopes

Role of B-fields in SFR/SFE; core properties; physics of CMF/IMF connection

Near-IR stellar census imaging and spectroscopic survey

Galactic plane (103 deg2), Magellanic Clouds

≤20 mas

K ~ 19, 23 mag for hydrogen burning limit in the galactic center, LMC, respectively

30-m-class telescope plus AO with multiple object IFU; JWST

SFR/SFEs; IMF studies

X-ray imaging of clusters

All clusters within Local Group

0.1″

1028 erg s−1 for T > 107 K at galactic center

New X-ray satellite

Stellar content of clusters for SFR/SFEs; IMF studies

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

Observation Type

Area

Angular Resolution

Sensitivity

Instrumentation

Science Area

Trigonometric parallaxes

Selected SF regions

few mas

≤2-μas positional accuracy

Increased sensitivity on very long centimeter-wave interferometer baselines; optical interferometer in space (Gaia)

Stellar populations, distance measurements for IMF calibration

Theory support

Development of new efficient algorithms for following molecular cloud evolution, core formation, and collapse to protostars spanning many orders of magnitude in size and density with improvements to radiation transfer and thermal physics, and inclusion of magnetic fields; support for improvements to stellar evolutionary tracks

Laboratory astrophysics

Studies of far-infrared and submillimeter molecular spectra, ranging from diatomics to prebiotic molecules, especially to aid in-line identification; determination of chemical rates involving granular surface processes and unusual gas-phase reactions such as radiative association for star-forming regions

NOTE: Acronyms are defined in Appendix C.

FIGURE 4.6 Declining frequency of disks in clusters of ages 0.5 to 11 Myr. SOURCE: J. Hernandez, L. Hartmann, N. Calvet, R.D. Jeffries, R. Gutermuth, J. Muzerolle, and J. Stauffer, A Spitzer view of protoplanetary disks in the γ Velorum Cluster, Astrophysical Journal 686:1195-1208, 2008, reproduced by permission of the AAS.

FIGURE 4.6 Declining frequency of disks in clusters of ages 0.5 to 11 Myr. SOURCE: J. Hernandez, L. Hartmann, N. Calvet, R.D. Jeffries, R. Gutermuth, J. Muzerolle, and J. Stauffer, A Spitzer view of protoplanetary disks in the γ Velorum Cluster, Astrophysical Journal 686:1195-1208, 2008, reproduced by permission of the AAS.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

millimeter imaging at approximately 50 AU resolution (Figure 4.7). The gaps in at least some of these transitional disks may be carved by newborn, giant planets.

The past decade also witnessed major advances in the understanding of later-stage debris disks, composed of optically thin dust generated from collisions between macroscopic bodies. Among solar-type stars of ages 10 to 100 Myr, about 15 percent possess debris disks. Such disks—youthful analogues of the solar system’s Kuiper belt—also hold keys to the process of planet formation. A dozen debris systems are close enough to have been imaged at a resolution of several astronomical units in submillimeter emission and visible/near-IR scattered light, revealing intricate structures: gaps, clumps, and eccentric rings. These features are probably sculpted by giant planets, which await direct detection in the coming decade; some may be imaged already.

Another revolution in the understanding of protoplanetary and debris disks impends, as significant increases in angular resolution are anticipated from near-IR to centimeter wavelengths initially with ALMA, the EVLA, and JWST, and later with a GSMT, along with orders-of-magnitude improvement in the dynamic range, or contrast, of images; the detection and mapping of complex molecules in disks;

FIGURE 4.7 Gap in a transitional disk, probably caused by the gravitation of one or more newborn giant planets, around GM Aurigae, a solar-mass star 140 pc away. Left: Spitzer-IRS spectrum reveals a flux deficit shortward of 20 μm relative to the median in its cluster, for which models indicate an absence of small dust grains in the 5- to 24-AU radial range, with dusty material at smaller and larger radii. Center: Truncation of the disk at 24 AU is verified by central brightness minimum in this SMA image of dust emission at 860 μm. Right: Artist’s conception of the disk, and the gas-giant protoplanets hypothesized to have created the gap. SOURCE: Left: Data from N. Calvet, P. D’Alessio, D.M. Watson, R. Franco-Hernández, E. Furlan, J. Green, P.M. Sutter, W.J. Forrest, L. Hartmann, K.I. Uchida, L.D. Keller, et al., Disks in transition in the Taurus population: Spitzer IRS spectra of GM Aurigae and DM Tauri, Astrophysical Journal Letters 630:L185-L188, 2005. Center: A.M. Hughes, S.M. Andrews, C. Espaillat, D.J. Wilner, N. Calvet, P. D’Alessio, C. Qi, J.P. Williams, and M.R. Hogerheijde, A spatially resolved inner hole in the disk around GM Aurigae, Astrophysical Journal 698:131-142, 2009, reproduced by permission of the AAS. Right: NASA/JPL-Caltech/T. Pyle (SSC).

FIGURE 4.7 Gap in a transitional disk, probably caused by the gravitation of one or more newborn giant planets, around GM Aurigae, a solar-mass star 140 pc away. Left: Spitzer-IRS spectrum reveals a flux deficit shortward of 20 μm relative to the median in its cluster, for which models indicate an absence of small dust grains in the 5- to 24-AU radial range, with dusty material at smaller and larger radii. Center: Truncation of the disk at 24 AU is verified by central brightness minimum in this SMA image of dust emission at 860 μm. Right: Artist’s conception of the disk, and the gas-giant protoplanets hypothesized to have created the gap. SOURCE: Left: Data from N. Calvet, P. D’Alessio, D.M. Watson, R. Franco-Hernández, E. Furlan, J. Green, P.M. Sutter, W.J. Forrest, L. Hartmann, K.I. Uchida, L.D. Keller, et al., Disks in transition in the Taurus population: Spitzer IRS spectra of GM Aurigae and DM Tauri, Astrophysical Journal Letters 630:L185-L188, 2005. Center: A.M. Hughes, S.M. Andrews, C. Espaillat, D.J. Wilner, N. Calvet, P. D’Alessio, C. Qi, J.P. Williams, and M.R. Hogerheijde, A spatially resolved inner hole in the disk around GM Aurigae, Astrophysical Journal 698:131-142, 2009, reproduced by permission of the AAS. Right: NASA/JPL-Caltech/T. Pyle (SSC).

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

and synoptic surveys in both the optical and the IR. Here the panel poses some key questions about disks that can be answered definitively in the coming decade, provided the requisite observational capabilities and theoretical developments are mustered.

What Is the Nature of the Planet-Forming Environment?

There are still many unknowns regarding the mass, structure, and evolution of disks in the pre-planetary state. How does disk mass evolve with time? How are mass and angular momentum transported to form stars and cause planets to migrate? To what extent is steady accretion punctuated by violent episodes? What are the thermal and chemical structures of disks? Where are the molecules regarded as prebiotic—that is, where are the molecules formed of the simple sugars and amino acids from which living organisms derive? Are the most massive stars also assembled from disks and thus likely to harbor planets?

Essential to answering these questions are improvements in angular and spectral resolution and imaging contrast, especially at wavelengths from the near-IR through the millimeter. To date some 60 protoplanetary disks have been imaged at >20-AU resolution by means of emission from gas and dust with the current generation of millimeter/submillimeter aperture-synthesis arrays (SMA, CARMA, PdBI) and by means of scattered light with HST and AO on ground-based telescopes. A major goal for the next decade is to improve significantly the spatial resolution in imaging such disks. Initially ALMA and JWST will achieve resolutions around 10 AU for the thousand nearest disks, but the panel recommends the following:

  • Every effort should be made to push to 1 AU (10 mas) resolution, with upgrades for ALMA and with a GSMT, in order to achieve the biggest scientific return.

One outcome of AU-resolution imaging of protoplanetary disks would be a clear definition of gaps and small-scale structures that will illuminate the dynamics of planet-disk interaction and early planetary migration. Of special significance would be the detection in disks of gravitational instabilities, such as spiral density waves, that would indicate self-gravitating disks. Compression amplitudes in gravitational instabilities should exceed a factor of 10, making their contrast with respect to the disk very large—brighter than the unperturbed disk—in molecular lines that trace high-density gas (Figure 4.8). Currently the role of self-gravity in disk evolution is a controversial topic, but if density waves are discovered, disk masses could be derived independently of uncertain gas-to-dust ratios and grain-size distributions, and the role of such instabilities in giant-planet formation could be assessed. Detecting waves in disks with the normal range of accretion rates will

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
FIGURE 4.8 A gravitationally unstable disk imaged in line emission from very dense gas in HCO+ J = 7-6 (λ = 480 μm) for a 0.09 M disk around a 1 M star at a distance of 140 pc, viewed face-on. Left: Three-dimensional model at full resolution shows a Jovian-mass collapsing fragment in the outer ring. Right: The same scene convolved to the resolution of ALMA in its longest-baseline configuration, with 0.007-arcsec resolution (1 AU at 140 pc). At ALMA’s projected sensitivity, the noise level in this image would be 100 K km/sec in 16 hours of observation; the protoplanetary fragment would be marginally detected (3 sigma) in about 1 hour. SOURCE: D. Narayanan, C.A. Kulesa, A. Boss, and C.K. Walker, Molecular line emission from gravitationally unstable protoplanetary disks, Astrophysical Journal 647:1426-1436, 2006, reproduced by permission of the AAS.

FIGURE 4.8 A gravitationally unstable disk imaged in line emission from very dense gas in HCO+J = 7-6 (λ = 480 μm) for a 0.09 M disk around a 1 M star at a distance of 140 pc, viewed face-on. Left: Three-dimensional model at full resolution shows a Jovian-mass collapsing fragment in the outer ring. Right: The same scene convolved to the resolution of ALMA in its longest-baseline configuration, with 0.007-arcsec resolution (1 AU at 140 pc). At ALMA’s projected sensitivity, the noise level in this image would be 100 K km/sec in 16 hours of observation; the protoplanetary fragment would be marginally detected (3 sigma) in about 1 hour. SOURCE: D. Narayanan, C.A. Kulesa, A. Boss, and C.K. Walker, Molecular line emission from gravitationally unstable protoplanetary disks, Astrophysical Journal 647:1426-1436, 2006, reproduced by permission of the AAS.

also enable an evaluation of the role of self-gravity in the disk’s accretion flow. Breakthroughs in the theory of accretion disks will require disk thermodynamics to be treated more realistically. For example, gravito-turbulent accretion is driven by net cooling, so the next generation of disk models should include in their energy budgets the feedback from stellar irradiation.

The panel recommends several key measurements and measurement capabilities that will advance the understanding of the structure and evolution of pre-planetary disks, as follows:

  • Sensitive millimeter-wave imaging with ALMA at few-AU resolution plus high-spectral-resolution observations by ALMA and Herschel in column-density tracers and density-sensitive molecular lines to provide the first maps of the opacity, density, thermal, and chemical structure of disks. In turn this will improve estimates of disk masses, enable measurements of the abundances of molecular

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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ions and complex prebiotic molecules that have so far proved impossible, and test chemical models.

  • Longer-wavelength imaging with the EVLA, and high-resolution, molecular-vibration spectroscopy with SOFIA, to provide important constraints on the chemical and radiation environments. These observations should also yield measurements of the disk ionization fraction, which is crucial for assessing the role of magnetic fields in disk transport and distinguishing actively accreting gas from dead zones. The latter may promote the survival of giant planets by acting as barriers to planet migration.

  • High-resolution imaging at near-IR wavelengths, initially with JSWT and later with a GSMT to trace disk structure by means of scattering by small grains residing on the disk surface. Gap contrasts can be several times higher than for thermal emission from the midplane (Figure 4.9). The near-IR will also be a fertile ground for identifying gravitationally induced density waves, which can appear at a contrast of about 0.1 relative to the rest of the disk.

  • High-contrast imaging with extreme adaptive-optical (ExAO) correction to see the disk against the glare of the star in the near-IR. Combining extreme AO with the sensitivity and resolution of a GSMT will be required to probe the innermost regions, where disk/star contrast factors at 10-AU range from about 10−3 at 1 μm (see Figure 4.9) to about 10−1 at 10 μm.

  • Combining high spatial and spectral resolution (λ/∆λ > 50,000) in the near- and mid-IR, coupled with chemical models to characterize the chemistry and ionization in the warm inner disk, which is where our own terrestrial planets reside. An integral field spectrometer with λ/∆λ ≥ 5,000 working at 10-mas resolution in the optical and near-IR would be able to profile the mineralogy and location of small solids in partially depleted disks, enabling an assessment of dust grain growth and crystallization, and testing models of coagulation by grain-grain collisions. This capability would also be invaluable in mapping the structure and ionization of accretion-powered jets on astronomical unit scales, elucidating the onset of collimation and possibly also the origin of the accretion-outflow connection that has so far eluded scientists’ grasp.

Achieving 10-mas resolution at submillimeter and near-IR wavelengths would also illuminate the role of disks in the formation of high-mass stars, by resolving distant, crowded young clusters (100 AU at 10 kpc). Disks around young, high-mass stars could be identified by means of direct imaging, and the properties of these necessarily massive, short-lived, and externally ionized disks could be studied for the first time and could be used to discriminate among models for massive star formation.

A further goal in the study of protoplanetary disks is to characterize their accretion history by means of wide-field long-term synoptic surveys in both the

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 4.9 Surface brightness distributions and gap contrasts in both scattered light and thermal emission of the inner 20 AU of a protoplanetary disk around a 1-Myr-old solar mass star, tipped by 45° to the line of sight. The upper panels show the unperturbed disk, at 1 μm (left) and at 1 mm (right). In the lower panels the disk has a 4-AU-wide gap at 10 AU, created by a 100-Earth-mass planet, showing a contrast ratio of about 0.1 in scattered light and 0.5 in thermal emission with respect to the rest of the disk. At the distance of the Taurus star-formation region each image is 0.28 arcsec wide. Assuming a GSMT with diameter 30 m operating at 1 μm, the instrumental point-spread function would be 1.1 AU in diameter (FWHM), and the surface brightness of 1 Jy arcsec−2 corresponds to a contrast, with respect to the stellar image, of about 3 × 10−3. Expected noise levels at 1 Jy arcsec−2 are on the order of 1 hour with ALMA; GSMT times are much shorter, depending on backgrounds. SOURCE: Courtesy of H. Jang-Condell, personal communication.

FIGURE 4.9 Surface brightness distributions and gap contrasts in both scattered light and thermal emission of the inner 20 AU of a protoplanetary disk around a 1-Myr-old solar mass star, tipped by 45° to the line of sight. The upper panels show the unperturbed disk, at 1 μm (left) and at 1 mm (right). In the lower panels the disk has a 4-AU-wide gap at 10 AU, created by a 100-Earth-mass planet, showing a contrast ratio of about 0.1 in scattered light and 0.5 in thermal emission with respect to the rest of the disk. At the distance of the Taurus star-formation region each image is 0.28 arcsec wide. Assuming a GSMT with diameter 30 m operating at 1 μm, the instrumental point-spread function would be 1.1 AU in diameter (FWHM), and the surface brightness of 1 Jy arcsec−2 corresponds to a contrast, with respect to the stellar image, of about 3 × 10−3. Expected noise levels at 1 Jy arcsec−2 are on the order of 1 hour with ALMA; GSMT times are much shorter, depending on backgrounds. SOURCE: Courtesy of H. Jang-Condell, personal communication.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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optical and near-IR, probing both optically visible and heavily extincted systems. This monitoring can be carried out with conventional resolution and sensitivity (limiting K magnitude of 18 per epoch), but it would ideally include measures of stellar accretion rate, such as near-IR hydrogen recombination lines. Such studies would clarify how important eruptive FU Ori-like events are in the buildup of stellar mass, and whether planet formation may be periodically disrupted.

High-resolution spectroscopy in the ultraviolet (UV) and X-ray regimes would significantly sharpen the understanding of how accretion flows arrive on the surface of a star, with implications for the star’s angular momentum, and for its accretion luminosity, which controls the ionization of the planet-forming regime in the disk. Ground-based near-IR interferometry also has a vital role in the study of protoplanetary disks, probing the inner 0.1 AU, where the stellar magnetosphere and radiation from stellar accretion spots can greatly affect the disk structure. In the coming decade it will be important to extend the sensitivity of this technique so that more than just the few brightest systems can be observed.

How Do Giant Planets Accrete from and Interact with Disks, and What Are These Young Planets Like?

An especially exciting discovery potential that is presented with high-angular-resolution imaging in the near-IR is the possibility of directly detecting infant giant planets (<5 Myr) within disk gaps that they have excavated. In nearby young clusters (<500 pc), there are more than 200 disks whose spectra suggest gaps or inner holes, for which near-IR direct detection and characterization of companions are realizable goals. This is illustrated in Figure 4.10 for the case of hypothetical 1-MJ protoplanets residing in known disk gaps. At this young age, Jovian planets are relatively luminous .

  • Thus the biggest detection challenge for directly detecting infant giant planets is imaging contrast, which needs to be 105 or better. A handful of targets may be detectable with JWST and extreme-AO near-IR images on 8- to 10-m telescopes, but most of the known candidates will require the higher resolution of 30-m telescopes, fitted with similar high-contrast instrumentation.

Finding newborn giant planets will enable astronomers to test, for the first time, theories of how these planets nucleate from the disk and feed from it. The infrared luminosity from a young giant planet traces the planet’s age and primordial heat of formation, and as such can be used to decide how these planets formed: whether in a “cold-start” nucleated instability around a rock-ice core (known as “core-accretion” formation), or in a “hot-start,” high-entropy, gravitationally unstable clump of disk gas. Characterization of the atmospheres and neighbor-

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 4.10 Photometric contrast with respect to parent star required for detection in the astronomical H band of hypothetical, ringless, Jovian-mass protoplanets in the gaps of known transitional disks (blue diamonds), compared to the 5σ contrast sensitivity predicted for two proposed extreme-adaptive-optical instruments (solid curves); detection space lies above these curves. The protoplanets are assumed to be the same age as the host star and to lie just inside the outer edges of the inferred gaps. Also shown are the domains in which one could observe older Jovian planets and smaller terrestrial planets. SOURCE: Sensitivities for the Gemini Planet Imager (GPI) and the proposed Thirty Meter Telescope’s Planet Formation Instrument (PFI) were adapted from B. Macintosh, M. Troy, R. Doyon, J. Graham, K. Baker, B. Bauman, C. Marois, D. Palmer, D. Phillion, L. Poyneer, I. Crossfield, et al., Extreme adaptive optics for the Thirty Meter Telescope, Proceedings of SPIE 6272:62720N, 2006, reproduced by permission of SPIE. Additional data from J.M. Brown, G.A. Blake, C.P. Dullemond, B. Merín, J.C. Augereau, A.C.A. Boogert, N.J. Evans II, V.C. Geers, F. Lahuis, J.E. Kessler-Silacci, K.M. Pontoppidan, and E.F. van Dishoeck, Cold disks: Spitzer spectroscopy of disks around young stars with large gaps, Astrophysical Journal Letters 664:L107-L110, 2007; K.H. Kim, D.M. Watson, P. Manoj, E. Furlan, J. Najita, W.J. Forrest, B. Sargent, C. Espaillat, N. Calvet, K.L. Luhman, M.K. McClure, J.D. Green, and S.T. Harrold, Mid-infrared spectra of transitional disks in the Chamaeleon I Cloud, Astrophysical Journal 700:1017-1025, 2009; K.H. Kim, personal communication; B.A. Macintosh, J.R. Graham, D.W. Palmer, R. Doyon, J. Dunn, D.T. Gavel, J. Larkin, B. Oppenheimer, L. Saddlemyer, A. Sivaramakrishnan, J.K. Wallace, B. Bauman, D.A. Erickson, C. Marois, L.A. Poyneer, and R. Soummer, The Gemini Planet Imager: From science to design to construction, Proceedings of. SPIE 7015:701518-701518-13, 2008.

FIGURE 4.10 Photometric contrast with respect to parent star required for detection in the astronomical H band of hypothetical, ringless, Jovian-mass protoplanets in the gaps of known transitional disks (blue diamonds), compared to the 5σ contrast sensitivity predicted for two proposed extreme-adaptive-optical instruments (solid curves); detection space lies above these curves. The protoplanets are assumed to be the same age as the host star and to lie just inside the outer edges of the inferred gaps. Also shown are the domains in which one could observe older Jovian planets and smaller terrestrial planets. SOURCE: Sensitivities for the Gemini Planet Imager (GPI) and the proposed Thirty Meter Telescope’s Planet Formation Instrument (PFI) were adapted from B. Macintosh, M. Troy, R. Doyon, J. Graham, K. Baker, B. Bauman, C. Marois, D. Palmer, D. Phillion, L. Poyneer, I. Crossfield, et al., Extreme adaptive optics for the Thirty Meter Telescope, Proceedings of SPIE 6272:62720N, 2006, reproduced by permission of SPIE. Additional data from J.M. Brown, G.A. Blake, C.P. Dullemond, B. Merín, J.C. Augereau, A.C.A. Boogert, N.J. Evans II, V.C. Geers, F. Lahuis, J.E. Kessler-Silacci, K.M. Pontoppidan, and E.F. van Dishoeck, Cold disks: Spitzer spectroscopy of disks around young stars with large gaps, Astrophysical Journal Letters 664:L107-L110, 2007; K.H. Kim, D.M. Watson, P. Manoj, E. Furlan, J. Najita, W.J. Forrest, B. Sargent, C. Espaillat, N. Calvet, K.L. Luhman, M.K. McClure, J.D. Green, and S.T. Harrold, Mid-infrared spectra of transitional disks in the Chamaeleon I Cloud, Astrophysical Journal 700:1017-1025, 2009; K.H. Kim, personal communication; B.A. Macintosh, J.R. Graham, D.W. Palmer, R. Doyon, J. Dunn, D.T. Gavel, J. Larkin, B. Oppenheimer, L. Saddlemyer, A. Sivaramakrishnan, J.K. Wallace, B. Bauman, D.A. Erickson, C. Marois, L.A. Poyneer, and R. Soummer, The Gemini Planet Imager: From science to design to construction, Proceedings of. SPIE 7015:701518-701518-13, 2008.

hoods—protosatellite disks or rings—of infant giant planets could be carried out with low-resolution spectrometers and polarimeters on the same facilities that imaged them.

Some infant giant planets may be detectable along with their cooler accretion streams, by ALMA, as illustrated in Figure 4.11. Imaging of the planet’s immediate environment, together with high-resolution theoretical studies of planet-disk

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 4.11 Simulation of an 8-hour, 345 GHz ALMA observation of a circumstellar disk with a recently formed planet of 1 MJ, at the 12 o’clock position, orbiting 5 AU away from a star (not shown). The distance to this hypothetical system is 100 pc. Most of the flux detected at the planet’s position is from material about to be accreted by the planet. SOURCE: S. Wolf and G. D’Angelo, On the observability of giant protoplanets in circumstellar disks, Astrophysical Journal 619: 1114-1122, 2005, reproduced by permission of the AAS.

FIGURE 4.11 Simulation of an 8-hour, 345 GHz ALMA observation of a circumstellar disk with a recently formed planet of 1 MJ, at the 12 o’clock position, orbiting 5 AU away from a star (not shown). The distance to this hypothetical system is 100 pc. Most of the flux detected at the planet’s position is from material about to be accreted by the planet. SOURCE: S. Wolf and G. D’Angelo, On the observability of giant protoplanets in circumstellar disks, Astrophysical Journal 619: 1114-1122, 2005, reproduced by permission of the AAS.

interaction, should help to decide how disk non-axisymmetries torque planetary orbits, and should help to determine the sign and speed of orbital migration and eccentricity evolution.

An important milestone will be the measurement of the masses of infant planets through stellar reflex motion. This will be extremely challenging even in the nearest young clusters, either through radial-velocity (∆v ≈ 10 m sec−1) or astrometric (∆r ≈ 0.001 AU) means. Magnetic activity and high rotation rates impose fundamental limits to radial-velocity precision on very young stars, which would only be sensitive to close-in or very massive planets. Astrometry faces less severe limitations, but the small reflex motions involved (≤7 μas) will require space-based facilities. Future space-based astrometric surveys could also further extend the census of infant planets in the 1-5 AU region, where direct imaging is most challenging.

Somewhat older giant planets (10-100 Myr), including those producing gaps in nearby debris disks, also await direct detection by high-contrast imagers. Non-axisymmetric structures seen in these disks (d < 50 pc)—clumps, warps, and eccentricities—are attributed to sculpting by planets. Planets at stellocentric distances of 20-200 AU, where a handful of debris disks have been resolved and where high-contrast imagers operate best, could be the result of long-range migration, but also might provide long-sought examples of planet formation by gravitational instability. It is here, in the outermost reaches of disks, that fragmentation by gravitational instability during the protoplanetary phase is most viable, since cooling times are

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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shortest and orbital times longest. Indeed a system of such planets has recently been reported, orbiting an A4V star with a debris disk, HR 8799. In the future, as higher angular resolution and high contrast imaging become available, the inner regions of these systems can also be searched for young planets. In this age range, a Jovian planet luminosity is predicted to be . Planets in some 200 nearby debris-disk systems are thus within the grasp of the next generation of high-contrast instruments on 8- to 10-m telescopes (see Figure 4.10).

Major advances in the theoretical understanding of planet formation, migration, and interactions with their disks—both gaseous and debris—will be required to take advantage of the wealth of observational data that will be obtained in the coming decade. The required efforts span a wide range of problems, including investigations of turbulence driven by magnetic fields in partially ionized gas; coagulation of solids with settling; disk-driven migration; and fragmentation in irradiated, self-gravitating disks. Many if not all of these problems will require major computational resources.

What Can Debris Disks and the Kuiper Belt Demonstrate About the Dynamical Evolution of Planetary Systems?

The orbits of known extrasolar planets reflect a host of dynamical processes—some violent—including migration, planet-planet scattering, and forcing by stars. Our own solar system did not emerge unscathed, as is evident in the hot thick disk of the Kuiper belt, filled with resonant orbits and large-perihelion bodies that suggest a dynamically violent epoch when 99 percent of the solar nebula’s solids were ejected. Similar dynamical evolution is currently underway in extrasolar debris disks, and the variety of structures observed to date in a handful of systems already hints at the diversity of their planetary systems. With increased angular resolution and sensitivity in the O/near-IR and in the submillimeter-wavelength regions, unprecedentedly fine structures in the dust will be revealed (Figure 4.12), and for systems with asymmetric structures, disk pattern speeds will be measured. Careful modeling of the observed disk morphology will enable the masses and orbits of sculpting bodies to be inferred. New theoretical approaches need development, since extrasolar debris disk densities are so large that interparticle collisions outweigh radiation drag, with consequences for disk structure that are only beginning to be explored. The panel recommends the following:

  • Future models should track not only forcing by planets, but also particle fragmentation in collisional cascades and momentum exchanges between colliding particles.

Multi-epoch observations will break modeling degeneracies by measuring proper motions of debris disk clumps—that is, speeds of wave patterns driven by plan-

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
FIGURE 4.12 Planet-sculpted features in Vega’s debris disk. Left: IRAM image at 1.3 mm with 2-deg resolution. Center: Model based on resonant trapping of planetesimals by migration of Neptune-mass planet. Right: Simulated ALMA observation at 350 GHz with a 1.5-arcsec beam from the 12-element ACA neutron star configuration, plus three additional compact configurations, for 11 hours. Total flux density in the simulated image is about 42 mJy. SOURCE: Left: D.J. Wilner, M.J. Holman, M.J. Kuchner, and P.T.P. Ho, Structure in the dusty debris around Vega, Astrophysical Journal Letters 569:L115-L119, 2002, reproduced by permission of the AAS. Center: M.C. Wyatt, Resonant trapping of planetesimals by planet migration: Debris disk clumps and Vega’s similarity to the solar system, Astrophysical Journal 598:1321-1340, 2003, reproduced by permission of the AAS. Right: Courtesy of R. Reid, L. Mundy, and A. Wooten, personal communication.

FIGURE 4.12 Planet-sculpted features in Vega’s debris disk. Left: IRAM image at 1.3 mm with 2-deg resolution. Center: Model based on resonant trapping of planetesimals by migration of Neptune-mass planet. Right: Simulated ALMA observation at 350 GHz with a 1.5-arcsec beam from the 12-element ACA neutron star configuration, plus three additional compact configurations, for 11 hours. Total flux density in the simulated image is about 42 mJy. SOURCE: Left: D.J. Wilner, M.J. Holman, M.J. Kuchner, and P.T.P. Ho, Structure in the dusty debris around Vega, Astrophysical Journal Letters 569:L115-L119, 2002, reproduced by permission of the AAS. Center: M.C. Wyatt, Resonant trapping of planetesimals by planet migration: Debris disk clumps and Vega’s similarity to the solar system, Astrophysical Journal 598:1321-1340, 2003, reproduced by permission of the AAS. Right: Courtesy of R. Reid, L. Mundy, and A. Wooten, personal communication.

ets (for planets orbiting at 100 AU, the pattern speed is approximately 1 AU/yr). Characterizing debris disks of different ages will enable the understanding of how planets and destructive collisions scour disks of their smallest planetesimals, leaving only large bodies.

A more secure understanding of the dynamical history of our own solar system will come from an unbiased characterization, both kinematic and physical, of the Kuiper belt. Evidence that the orbits of the ice giants Uranus and Neptune have undergone dramatic changes, involving migration and/or violent planet-planet scatterings, is imprinted in the distribution of orbits of known Kuiper belt objects (KBOs), especially in the well-populated mean-motion resonances. The current census of KBOs is pieced together from a patchwork of surveys, each with different areal coverage and limiting magnitude, giving an incomplete picture at best.

  • The panel recommends a repeated systematic whole-sky synoptic survey to a limiting red magnitude of R = 24, sensitive to 30- to 50-km bodies at 40 AU that will provide a large enough unbiased sample (>20,000 KBOs) to test dynamical evolution models, yielding the relative populations of the resonances, the abundance of scattered and detached populations, and the incidence of retrograde objects and binaries.

Surface compositions will also provide important clues to the chemistry of planetesimal formation. KBOs show a broad range of colors, suggesting a wide

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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variety of compositional types from ice-rich to organic-rich. About half possess ultra-red matter that may be extremely pristine, preserved only in the deep freeze of the Kuiper belt. Key questions include whether there is a radial composition gradient across the belt and whether resonantly trapped KBOs are of similar composition or have migrated from different source regions. To answer these questions requires the following:

  • Spectra at resolving power λ/∆λ ≈ 500 from 0.3 to 2.5 μm to measure the depth and width of bands such as methane for the brightest objects, and multi-wavelength precision photometry for the faintest ones. Optical colors need to be accurate to ±0.03 magnitudes, and O/near-IR color indices to ±0.06 magnitudes, for objects as faint as R = 24 in order to discriminate among surface properties and examine correlations between colors and dynamical properties. Time-resolved photometry with similar precision can reveal the shapes and rotation states of the KBOs, two further constraints on their origins and collisional histories.

Conclusions: Disk Evolution and Planet Formation

Table 4.3 summarizes the panel’s conclusions about activities to address its second science question.

PSF 3. HOW DIVERSE ARE PLANETARY SYSTEMS?

The great potential of the coming decade is to achieve a census of the planetary bodies orbiting the Sun’s neighboring stars. With a commitment of sufficient resources, scientists’ efforts will be rewarded with no less a prize than the knowledge of the population of small, rocky, Earth-like planets. The detection and characterization of these worlds will invigorate the efforts both to understand their origin in the grand context of star formation (science question PSF 1) and protoplanetary disk evolution (science question PSF 2), and to compare their compositions, structures, and atmospheres with those of the planets of the solar system. It is this great quest for detection and characterization that concerns the panel in the present section. In the subsequent section on science question PSF 4, the panel focuses on the particular question of life on these worlds, arguably one of the greatest intellectual endeavors ever undertaken by humanity. The panel then identifies a fast-track opportunity that may permit the study of habitable worlds within 5 years (the PSF discovery area), albeit for stars very different from the Sun.

The dominant methods of discovery of planets orbiting other stars (known as exoplanets) are measurements of radial velocities (wherein one infers the existence of a planet through its acceleration of the central star) and photometric transits (in which one measures the periodic dimming of the central star due to the intervening

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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TABLE 4.3 Panel’s Conclusions Regarding the Study of Disk Evolution and Planet Formation

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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passage of an orbiting planet). Together these methods account for nearly all of the more than 400 known planetary systems, in which more than 500 exoplanets have been detected. The nascent methods of microlensing (in which the gravitational field of a planetary system causes the magnification of an unassociated background star) and direct imaging (wherein one spatially separates the light from the planet from the glare of the central star) have each yielded a handful of detections.

Toward an Understanding of Planet Formation

A mere 15 years ago, the knowledge of “normal” planetary systems was restricted to the eight planets orbiting the Sun. The prevailing model for the formation of gas giant planets was that of core accretion (see above the subsection “How Do Giant Planets Accrete”), predicting that such planets would take 10-100 Myr to form and would be found on circular orbits at distances of 5-10 AU from their central star.

In contrast, the past decade of discovery has revealed much shorter timescales for giant-planet formation (see the subsection on giant-planet accretion) and startling diversity in the architectures of mature planetary systems (Figure 4.13). Astronomers have learned of the existence of “hot Jupiters,” planets with masses similar to that of Jupiter, yet orbiting a mere 0.05 AU from their central stars. Although many stars indeed host Jupiter-mass planets at greater separations, the median eccentricity of the population is roughly 0.2, exceeding that of all the solar system planets. Furthermore, many of these eccentric planets are members of multiplanet systems, and in some cases the mutual dynamical interactions of the member planets can be directly observed. There are several strong pieces of

FIGURE 4.13 Orbital eccentricity as a function of orbital semimajor axis, for single exoplanets (diamonds), exoplanets in multiplanet systems (squares), and the solar system (triangles). Data from the Extrasolar Planets Encyclopaedia, hosted by Paris Observatory.

FIGURE 4.13 Orbital eccentricity as a function of orbital semimajor axis, for single exoplanets (diamonds), exoplanets in multiplanet systems (squares), and the solar system (triangles). Data from the Extrasolar Planets Encyclopaedia, hosted by Paris Observatory.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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evidence that the fundamental tenets of the core-accretion model are correct, such as the metal enrichment of exoplanet host stars (Figure 4.14) and the rather high densities of some of the giant exoplanets (Figure 4.15), which demand heavy-element enrichment. Yet it is clear that the hot Jupiters could not have formed in place by core accretion. It also appears that gas and ice giants of our own solar system have moved significantly from their birth place. Such results have underlined the importance of studies of planetary migration and re-opened questions of the basic mechanisms of the formation of giant planets (see the subsection on giant-planet accretion).

Nature has proved far more inventive in forming planets than scientific theories have in predicting their properties. Exoplanetary studies have progressed admirably during the past decade, but it still is not known whether solar systems like ours are commonplace or are cosmically rare. For a true understanding of the process of planet formation, the end states need to be understood: the planetary census must be completed. The fundamental starting point for characterizing planetary systems is the determination of orbital properties and masses; this is vital not only for understanding the dynamics and composition of these systems but also for understanding how planetary systems originate. In addition, a further step should be taken to develop the sensitivity to detect and characterize the bulk properties of rocky planets akin to the solar system terrestrial planets. Novel methods need to be encouraged for the detection of planets far from their stars, at distances where the radial velocity and transit methods are precluded due to the very long orbital periods and low transit probabilities. Astrometry with Gaia will also contribute to the census of massive exoplanets in the coming decade.

FIGURE 4.14 Correlation of stellar Fe/H relative abundance with possession of exoplanets. SOURCE: D.A. Fischer and J. Valenti, The planet-metallicity correlation, Astrophysical Journal 622:1102-1117, 2005, reproduced by permission of the AAS.

FIGURE 4.14 Correlation of stellar Fe/H relative abundance with possession of exoplanets. SOURCE: D.A. Fischer and J. Valenti, The planet-metallicity correlation, Astrophysical Journal 622:1102-1117, 2005, reproduced by permission of the AAS.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 4.15 Observed masses and radii of planets both inside and outside the solar system, compared to models based on differing bulk composition. Left: The original demonstration by Zapolsky and Salpeter that Jupiter and Saturn are predominantly hydrogen and helium (i.e., a near-solar composition), whereas Uranus and Neptune are dominated by heavier elements. Right: A recent rendition in which the solar system’s planets (blue points) are joined by a large number of exoplanets (orange points). The predicted radii of pure H/He bodies are shown for two representative orbital separations to show the expected effect of strong irradiation, but the observed variation exceeds the span of these two models. The two smallest exoplanets shown lie near Neptune and similarly have densities that require a bulk composition dominated by ice and rock. SOURCE: Left: H.S. Zapolsky and E.E. Salpeter, The mass-radius relation for cold spheres of low mass, Astrophysical Journal 158:809-813, 1969, reproduced by permission of the AAS. Right: J. Fortney, University of California, Santa Cruz.

FIGURE 4.15 Observed masses and radii of planets both inside and outside the solar system, compared to models based on differing bulk composition. Left: The original demonstration by Zapolsky and Salpeter that Jupiter and Saturn are predominantly hydrogen and helium (i.e., a near-solar composition), whereas Uranus and Neptune are dominated by heavier elements. Right: A recent rendition in which the solar system’s planets (blue points) are joined by a large number of exoplanets (orange points). The predicted radii of pure H/He bodies are shown for two representative orbital separations to show the expected effect of strong irradiation, but the observed variation exceeds the span of these two models. The two smallest exoplanets shown lie near Neptune and similarly have densities that require a bulk composition dominated by ice and rock. SOURCE: Left: H.S. Zapolsky and E.E. Salpeter, The mass-radius relation for cold spheres of low mass, Astrophysical Journal 158:809-813, 1969, reproduced by permission of the AAS. Right: J. Fortney, University of California, Santa Cruz.

Completing the Planetary Census, 2010-2020

The recently commissioned NASA Kepler mission, with a modest contribution from the CoRoT spacecraft, will offer humanity its first opportunity to study the population of terrestrial exoplanets within 1.5 AU of their stars. This mission employs the transit method and hence will deliver estimates of the planetary radii and orbital periods. However, the scientific return will be fully realized only if mass estimates can be obtained for a significant number of these planets. The de-

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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termination of the radial-velocity orbit serves both as a robust confirmation that a Kepler signal is indeed planetary in origin and also yields the planetary density and thus constrains the composition, a topic of significant importance (see below). The challenges for radial velocities are that the targets are faint and the signal is small: An Earth-mass planet 1 AU from a Sun-like star produces a peak-to-peak radial-velocity difference of only 0.18 m sec−1, five times smaller than the current state-of-the-art precision. Even if the limiting precision were to remain fixed, the panel recommends the following:

  • A substantial expansion of the telescope time available to pursue radial-velocity work, since the mass determination for rocky bodies several times the mass of Earth or at smaller orbital separations is already within reach.

However, given the 10-fold improvement in radial-velocity precision of the past 15 years, the panel finds that it is not unreasonable to aim to accomplish the following:

  • Develop advanced radial-velocity techniques capable of achieving 0.2 m/s precision as a basic requirement (which will only be possible for the most stable stars), with a long-term goal to do better. This will require the development of dedicated and highly specialized spectrographs and novel means for wavelength calibration.

Beyond the vital follow-up of Kepler-detected worlds, this investment in radial-velocity observations will also drive the understanding of more massive worlds located at distances of 1.5-10 AU from their stars, straddling the ice line that is thought to have played so crucial a role in their formation. There is not space here to mention the large number of ongoing radial-velocity projects that will continue to make important contributions to the planetary census; the panel necessarily groups all these efforts together in its endorsement of the expansion of ground-based radial-velocity measurements. If implemented on next-generation large-aperture telescopes (needed to gather a sufficient number of photons), this could permit a survey of the closest 100 Sun-like stars to find the nearest Earth-like planets orbiting within their stellar habitable zones. These systems would be invaluable for subsequent efforts to search for atmospheric biomarkers (science question PSF 4).

The method of microlensing complements that of radial velocities and transits. It does not require that data be gathered over a full orbital cycle and thus can provide in relatively short order the detailed statistics on the masses and orbital separations of planets in the outer reaches of planetary systems. Microlensing is demanding of instrumental stability: a satellite borne survey instrument, for which the point spread function profile would be stable over times very long compared to the duration of a planetary microlensing event, would—compared with a ground-

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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based survey covering the same field of view—detect about 100 times as many planets of given mass and reach planetary orbits about 10 times larger. The results of a space-based microlensing survey would be particularly dramatic for several reasons. First, the sensitivity of Kepler to exoplanets declines sharply beyond 1.5 AU, whereas a microlensing survey in space could reasonably detect a statistically significant population of exoplanets from 0.5 to 15 AU. Second, microlensing will naturally survey a wider range of host stars and will include a large number of low-mass stars, as they are the most numerous type of star in the galaxy. Kepler will survey few low-mass stars owing to their intrinsic dimness. Importantly, a space-based microlensing survey can determine the planetary masses and projected separations in physical units and can frequently detect the light emitted by the host star. This is in contrast to ground-based microlensing surveys, which must generally rely on HST imaging to accomplish this, and there is insufficient time available to follow up a significant number of detections.

To obtain the fundamental data set to test the current picture of how planetary systems form, how their properties depend on the properties of the central star and, by inference, the conditions of the circumstellar disk, the panel recommends a combination of the following:

  • A space-based microlensing survey, which will provide a statistical picture of planetary architectures, and

  • The Kepler findings on terrestrial planet frequency within 1.5 AU of their central stars, augmented by

  • Expanded radial-velocity measurements on 4- to 10-m-class telescopes to provide masses.

More generally, these efforts will demonstrate how typical our own solar system is.

Inferring the Bulk Compositions of Exoplanets

When both photometric and Doppler signals are measured for the same planet-hosting star, the planetary mass and radius are determined directly. These permit, in turn, an estimate of the density and, by inference, the bulk composition and the formation history. Fifty such transiting worlds are now known, and these objects have proven immensely valuable for constraining models of the physical structures of planets as a function of mass, composition, and external environment. Indeed, many show significant discrepancies from the theoretical prediction for a degenerate body with solar abundances (see Figure 4.15). In many cases the planets are larger than expected for their age, indicating that a mechanism to prevent cooling and contraction is in operation. A handful of systems are smaller than expected, indicating the presence of a large fraction of heavy elements, usu-

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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ally interpreted as a core. For nontransiting planets, the radial-velocity method provides only a lower bound on the mass, but

  • The true masses could be determined with space-based astrometry. Since these would be the systems nearest to Earth, knowledge of the true masses for each of the planets in these systems would be extremely valuable.

Progressing downward through the mass range corresponding to Neptune-like bodies, two transiting examples of which are known at the time of writing, the interpretation of radii becomes increasingly ambiguous, because there are three ingredients—gas, ice, and rock—and thus multiple ways to obtain the observed radius and mass. Even lacking unique solutions, the observed diversity yields important constraints on the range of planetary-formation conditions. As one approaches masses that are only modestly larger than Earth’s, one could distinguish between a body that is half ice–half rock (for example) and a body that is almost entirely rock, assuming that the atmosphere contributes only a small fraction of the radius. Deciding whether the atmosphere is small in this sense may prove to be very challenging, but for sufficiently low-mass bodies at modest orbital radii (e.g., 1 AU), strong theoretical arguments can severely limit the presence of a thick hydrogen-rich atmosphere. At the rather low level of accuracy attainable for determining bulk compositions, the present understanding of thermodynamics and pressure-density relationships for candidate materials is adequate for the task. The expected diversity of observations will thus be traceable to the diversity of conditions and environments of planet formation.

  • The tremendously exciting opportunity to make informed estimates of the compositions of perhaps hundreds of Earth-like planets detected by Kepler serves as a compelling motivation for increased radial-velocity precision and the expansion of available observatory time to undertake these measurements.

Characterizing the Atmospheres of Exoplanets

It is essential that spectra be gathered to determine the chemistries, structures, and dynamics of exoplanetary atmospheres. Nearly all of the available data on the atmospheres of exoplanets comes from one of two techniques that are possible only for transiting systems: (1) In transmission spectroscopy, one takes the ratio of a spectrum gathered when the planet is in front of the star with a spectrum of the unocculted star, interpreting any residual absorption features as arising in the atmosphere of the planet. (2) In occultation spectroscopy, one observes the difference between a spectrum gathered when both the planet and star are in view and a spectrum gathered when the star occults the planet. The residual emission is that

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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from the planet. Together these studies have proven immensely valuable, permitting the detection of numerous atoms and molecules including Na, H, H2O, and CH4, as well as clouds, and the direct determination of atmospheric temperatures and studies of temperature inversions. These studies are rendered all the more penetrating by the fact that the masses and radii of the planets are accurately known. More recently, planetary emission has been studied as a function of planetary longitude (Figure 4.16), permitting a direct constraint on the atmospheric dynamics, namely, the degree to which winds circulate energy from the irradiated dayside to the night side.

FIGURE 4.16 A map of the temperature over the surface of the exoplanet HD 189733b. Upper: The light curve at mid-infrared wavelengths, showing the passage of the planet in front of (phase 0) and then behind (phase 0.5) the star. Since the planetary emission is not observed at phase 0.5, the dashed line indicates the stellar flux, and any flux in excess of this value at other phases originates from the planet. Lower: The temperature map of HD 189733b derived from the light curve. The substellar point (corresponding to high noon on the planet) is in the center of the diagram. The hottest point on the planet (indicated with the red arrow) is eastward of the substellar point, indicating the action of strong super-rotating winds on the planet. SOURCE: Reprinted by permission from Macmillan Publishers Ltd.: Nature, H.A. Knutson, D. Charbonneau, L.E. Allen, J.J. Fortney, E. Agol, N.B. Cowan, A.P. Showman, C.S. Cooper, and S.T. Megeath, A map of the day-night contrast of the extrasolar planet HD 189733b, Nature 447:183-186, 2007.

FIGURE 4.16 A map of the temperature over the surface of the exoplanet HD 189733b. Upper: The light curve at mid-infrared wavelengths, showing the passage of the planet in front of (phase 0) and then behind (phase 0.5) the star. Since the planetary emission is not observed at phase 0.5, the dashed line indicates the stellar flux, and any flux in excess of this value at other phases originates from the planet. Lower: The temperature map of HD 189733b derived from the light curve. The substellar point (corresponding to high noon on the planet) is in the center of the diagram. The hottest point on the planet (indicated with the red arrow) is eastward of the substellar point, indicating the action of strong super-rotating winds on the planet. SOURCE: Reprinted by permission from Macmillan Publishers Ltd.: Nature, H.A. Knutson, D. Charbonneau, L.E. Allen, J.J. Fortney, E. Agol, N.B. Cowan, A.P. Showman, C.S. Cooper, and S.T. Megeath, A map of the day-night contrast of the extrasolar planet HD 189733b, Nature 447:183-186, 2007.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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These techniques hold great promise for the future, and indeed the panel foresees a fast-track opportunity to use these methods to search for atmospheric biomarkers such as molecular oxygen. These methods have been developed for HST and Spitzer, and surely will prove even more powerful with JWST. The panel encourages the following:

  • A detailed study of the optimal observing methods with JWST, and if these studies confirm their feasibility, the acquisition of very high signal-to-noise-ratio spectra of hot Jupiters and even-lower-mass planets that are predominantly ice and rock in composition. An extension of the warm phase of Spitzer would bridge the gap, permitting the continued study of exoplanet atmospheres until the launch of JWST.

The methods described above will be limited to short period planets for which transits are likely and frequent. The natural complement is direct imaging, in which the faint light from the orbiting planet is spatially separated from that of the star. These efforts are buoyed by the recent images of two nearby, potentially, planetary systems. Moreover, both systems have debris disks, permitting an unprecedented glimpse at the early phases of planetary systems orbiting stars more massive than the Sun. The panel encourages both of the following:

  • The development of extreme-contrast-imaging capabilities on the currently available 8- to 10-m-class telescopes, as well as their counterparts on the next generation of extremely large observatories. Space-based coronographic spectroscopy may also play an important role in this effort.

A particularly compelling avenue would be to image planets for which dynamical mass estimates can be obtained through radial velocities or astrometry, providing an essential calibration of models that predict the emission from planets as a function of mass and age. The current range of these theoretical tracks varies greatly, depending upon differing assumptions of the starting conditions and the rate of cooling, making it currently unreliable to estimate masses when only brightness measurements are available. The direct spectroscopic study of radial-velocity-detected exoplanets that orbit at distances of several astronomical units from nearby stars will require contrast ratios of 10−9 in visible light and 106 at infrared wavelengths.

Investments in Understanding Planetary Structures and Atmospheres

The anticipated rich yield of exoplanet discoveries and characterization efforts call out for the support of a vibrant research program in theory to interpret the data

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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with respect to models of planet formation, evolution, dynamics, and planetary structures and atmospheres. This includes the following:

  • The development of models to predict the outcome of theories of planet formation in terms of orbital elements, rates of occurrence, multiplicity, and bulk composition;

  • The development of models of exoplanet atmospheres of varying composition and under differing levels of irradiation, including the study of clouds and photochemistry; and

  • The tabulation of molecular-line data supported by laboratory astrophysics as needed, in support of these modeling efforts.

Theoretical Investments in Planetary Dynamics

The abundant harvest of exoplanet discoveries has made it clear that planetary systems are diverse. Commensurate with this observed richness is an increasingly complex array of planet-planet and disk-planet interactions that must be disentangled in attempts to infer the origins of these systems. Algorithmic advances and investments in parallel computing resources are needed to resolve delicate resonant interactions between planets and disks. The most lasting theoretical contributions will be those that can be validated by observations, many of which can be made most readily in the solar system—for example, in planetary rings, which afford miniature, accessible versions of protoplanetary disks, or in the Kuiper belt, which furnishes our own local debris disk.

Conclusions: Planetary-System Diversity

Table 4.4 summarizes the panel’s conclusions on activities to address its third science question.

PSF 4. DO HABITABLE WORLDS EXIST AROUND OTHER STARS, AND CAN WE IDENTIFY THE TELLTALE SIGNS OF LIFE ON AN EXOPLANET?

The gleaming jewel in the crown of exoplanet discoveries would be a world that could sustain life, which would initiate the study of astrobiology on habitable exoplanets as an experimental science. Detection of life elsewhere in the universe remains a compelling goal supported by widespread fascination. However, this specific goal cannot be achieved with only incremental steps in the current areas of progress. The Kepler mission promises the sensitivity to detect Earth-size planets orbiting Sun-like stars at 1 AU. With concerted follow-up observations, the mass, radius, and density of these planets will be known, but precious little, if any, knowl-

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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TABLE 4.4 Panel’s Conclusions Regarding the Study of Planetary-System Diversity

Technique

Requirements

Radial velocity (RV)

A target of 0.2 m sec−1, requiring novel wavelength calibration methods (gas cells, laser combs) Substantial expansion of currently available observing time on 4- to 10-m-class telescopes for Kepler follow-up and other RV survey

Precision photometry/spectroscopy

Microlensing surveys: long-duration near-IR coverage of at least 108 stars, with resolution <0.3 arcsec, and PSF stability <10% of FWHM over >1 month; JWST primary/secondary transit spectroscopy, 10−4-10−5 of host-stellar signal

Direct imaging astrometry

Image contrast ~10–9 in optical, 30 mas—1 arcsec from host star using Gaia; other space mission with 0.1 mas—mission relative positional accuracy

Theory, numerics

Advances in planet/brown dwarf atmospheric models permitting inclusion of dynamics (clouds, zonal flows, etc.) and chemistry

Studies of planetary dynamics, resonances; major computational resources required

NOTE: Acronyms are defined in Appendix C.

edge, of their detailed composition, surface, or atmosphere will exist. One can easily imagine these planets to be just like Earth, or completely unlike Earth: one need not look beyond Earth and Venus to know that similar worlds can differ markedly in their habitability. The faintness of the Kepler sample will prohibit determination beyond such imaginings. However, Kepler will observe a sufficiently large sample to measure accurately the fraction of stars that host Earth-like planets. The section below titled “PSF Discovery Area—Identification and Characterization of a Nearby Habitable Exoplanet” discusses a fast-track approach to enabling some studies of habitable worlds within the coming decade, but this approach is limited to stars that are very different from the Sun. To truly understand life’s origins and to place the life on Earth in context, a new capability is required to search for biomarkers on a twin of Earth orbiting a nearby Sun-like star. This challenge will require sustained investment in technology development and supporting scientific areas.

How Could We Discover the Planets and Characterize Their Atmospheres?

In Astronomy and Astrophysics in the New Millennium3 (AANM), the previous decadal survey committee identified three important precursors to enable a successful Terrestrial Planet Finder (TPF) mission: measurement of the abundance of Earth-like planets, ; measurement of the level of exozodiacal light in the range 10-100 times the solar system’s zodiacal light (“zody”); and a space interferometry mission to discover Earth-mass planets.

The Kepler mission is meeting its required photometric accuracy and is well

3

National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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on the way to delivering the first of these pivotal results. The panel concludes the following:

  • It is essential that sufficient resources be devoted to the analysis and followup of Kepler photometry through ground-based observations, to provide an accurate and definitive estimate of .

Figure 4.17 compares the expected strength of the exozodiacal light to the sensitivity of various observatories. AANM envisioned that the exozodiacal measurements would be accomplished by SIRTF (i.e., Spitzer), the Keck Interferometer (KI), and the Large Binocular Telescope Interferometer (LBTI). Spitzer has achieved its sensitivity goals but did not reach the requisite level for the exozody TPF precursor requirement owing to the extreme accuracy with which the background must be removed and stellar photospheres must be modeled. Measurements with unresolved IR/submillimeter excess from Herschel will provide addi-

FIGURE 4.17 The sensitivity limits to dust around other stars achieved with current state-of-the-art techniques. MIPS and IRS are instruments on Spitzer, and PACS and SPIRE are instruments on Herschel; SCUBA is a ground-based submillimeter camera on JCMT. These are all sensitive to dust levels only several hundred to many thousand times that of the cool Kuiper belt. LBTI and KI are ground-based interferometers and are sensitive to only tens to hundreds of times the zodiacal-dust equivalent. The labels for Kuiper and asteroid belts indicate the approximate level of the zodiacal light. SOURCE: Exoplanet Task Force, Worlds Beyond: A Strategy for the Detection and Characterization of Exoplanets, Washington, D.C., 2008.

FIGURE 4.17 The sensitivity limits to dust around other stars achieved with current state-of-the-art techniques. MIPS and IRS are instruments on Spitzer, and PACS and SPIRE are instruments on Herschel; SCUBA is a ground-based submillimeter camera on JCMT. These are all sensitive to dust levels only several hundred to many thousand times that of the cool Kuiper belt. LBTI and KI are ground-based interferometers and are sensitive to only tens to hundreds of times the zodiacal-dust equivalent. The labels for Kuiper and asteroid belts indicate the approximate level of the zodiacal light. SOURCE: Exoplanet Task Force, Worlds Beyond: A Strategy for the Detection and Characterization of Exoplanets, Washington, D.C., 2008.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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tional constraints, but, ultimately, resolved measurements are required to eliminate the stellar modeling uncertainty. The KI has achieved the nulling of light from the central star but is unlikely to deliver results at the requisite <10-zody level (i.e., 10 times the solar system zodiacal light). The LBTI has not yet been commissioned, but with lower background and higher throughput it will deliver better sensitivity than KI; a low-background GSMT with shearing/nulling interferometry would perform much like LBTI. The panel concludes as follows:

  • Substantial improvement with ground-based instrumentation might require exploiting the cold, dry, and stable atmosphere of the high Antarctic plateau with a nulling interferometer. Owing to the dramatic benefits in IR background and improved seeing, 10-zody sensitivity could be achieved with a relatively modest instrument.

  • Space-based coronagraphy might be the only means to obtain meaningful constraints on the exozodiacal light, but even here relatively large apertures are required to constrain emission at radii of 1 AU for any but the very nearest stars. The problem is that dust is likely sculpted into belts, as in the solar system, so extrapolation inward from large radii is unjustified.

  • The observation of exozodiacal light in candidate targets will eliminate a risk to the scientific viability of direct-detection missions.

The exploration of additional approaches to this problem is warranted to retire the risk of a single-point failure on this scientific path.

The Astronomy and Astrophysics Advisory Committee’s Exoplanet Task Force (ExoPTF) identified contingent strategies to achieve characterization of Earth analogs based on results of and exozody measurements.

  • Central to this strategy was the launch of a space-based astrometric mission to identify specific targets for a planet characterization mission. A planet-finding astrometric mission will provide specific stars with known planets of measured mass.

The knowledge obtained from such a mission would almost certainly have a major impact on the design of a direct-detection mission and would provide certainty that targets exist within the survey sample, rather than simply that it is statistically likely. Moreover, studies of the spectra of these worlds will be far more penetrating if the masses are known.

The recent proliferation of candidate mission concepts offers multiple windows to change fundamentally the present vision of planetary systems. Technology development through the next decade is needed to bring the varied mission concepts to a level at which the scientific priority of competing approaches can be assessed.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Which Measurable Characteristics Define Habitability?

The ubiquity of the organized chemistry that is called life remains a subject of intense interest and speculation. The current working definition of habitability is that liquid water be stable, energy be available, and certain chemical species have non-equilibrium concentrations. Of these factors, the stability of liquid water is the most important diagnostic for planetary habitability. The complexities of this subject are beyond the scope of this report but are explored in great detail in the ExoPTF report4 and the NRC report The Limits of Organic Life in Planetary Systems.5 Ultimately we won’t know how parochial we are until we find the answer.

Given the ultimate focus on finding planets as sites of biology, careful development and understanding of the limitations of biosignatures are required. This will be an activity that requires interdisciplinary collaboration with theory, astrochemistry, biology, and planetary science.6 The interpretation of molecular oxygen or ozone (Figure 4.18) as biological in origin requires the presence of another signature that rules out an abiotic origin. Detection of the red edge of spectra of vegetation would be a particularly remarkable discovery.

During the next decade, observations of exoplanets should be used to direct concepts for future missions. A mission to study an Earth analog will rely on the comprehensive and concerted synergy of the understanding of planetary systems in the universe. To launch a mission that minimizes scientific risk, the efforts of many astronomers will need to be synthesized on questions of , exozodiacal emission, and global understanding of planetary formation and evolution.

Kepler will yield quantitative results on , which are essential for estimating what number of stars will need to be surveyed. High-precision (0.1-μas) astrometry can identify specific host stars, decoupling the task of detection from that of characterization and reducing the risk of scientific failure for a mission that may be correctly scoped based on measurements of but unlucky in the distribution of planets around the target stars. Moreover, astrometry will provide dynamical estimates of planetary masses that will be essential to interpreting the spectra of these distant worlds.

4

Exoplanet Task Force, Worlds Beyond: A Strategy for the Detection and Characterization of Exoplanets, Washington, D.C., 2008.

5

National Research Council, The Limits of Organic Life in Planetary Systems, The National Academies Press, Washington, D.C., 2007.

6

For a more extensive discussion of such cross-disciplinary opportunities, see National Research Council, The Astrophysical Context of Life, The National Academies Press, Washington, D.C., 2005.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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FIGURE 4.18 Spectral signatures of habitability? Left: Full-resolution synthetic disk-averaged albedo spectra of Venus, Earth, and Mars. Synthetic Earth spectra are shown for both uniform high cirrus cloud cover, and as a fit to Earthshine observations of the gibbous Earth. The Venus spectrum was approximated to a disk average and has been multiplied by 0.6 to fit the plot. The Mars and Earth spectra are disk averages of three-dimensional spatially and spectrally resolved Virtual Planetary Laboratory models of Earth and Mars. For the observed Earth, which is ocean dominated with relatively little cloud cover, the Rayleigh scattering (0.45-0.6 μm) is pronounced, but the ozone is less apparent. The ozone absorption is much more pronounced for Earth with cloud cover, increasing the difficulty of identifying the Rayleigh scattering component. Right: Thermal-infrared spectra of Venus, Earth, and Mars. SOURCE: Exoplanet Task Force, Worlds Beyond: A Strategy for the Detection and Characterization of Exoplanets, Washington, D.C., 2008.

FIGURE 4.18 Spectral signatures of habitability? Left: Full-resolution synthetic disk-averaged albedo spectra of Venus, Earth, and Mars. Synthetic Earth spectra are shown for both uniform high cirrus cloud cover, and as a fit to Earthshine observations of the gibbous Earth. The Venus spectrum was approximated to a disk average and has been multiplied by 0.6 to fit the plot. The Mars and Earth spectra are disk averages of three-dimensional spatially and spectrally resolved Virtual Planetary Laboratory models of Earth and Mars. For the observed Earth, which is ocean dominated with relatively little cloud cover, the Rayleigh scattering (0.45-0.6 μm) is pronounced, but the ozone is less apparent. The ozone absorption is much more pronounced for Earth with cloud cover, increasing the difficulty of identifying the Rayleigh scattering component. Right: Thermal-infrared spectra of Venus, Earth, and Mars. SOURCE: Exoplanet Task Force, Worlds Beyond: A Strategy for the Detection and Characterization of Exoplanets, Washington, D.C., 2008.

Conclusions: Discovering Habitable Exoplanets

Table 4.5 summarizes the panel’s conclusions about activities to address its fourth science question.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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TABLE 4.5 Panel’s Conclusions Regarding Discovery of Habitable Planets

Technique

Requirements

Radial velocity (RV)

0.2 m sec−1, or better, requiring novel wavelength calibration methods (gas cells, laser combs)

Substantial expansion of currently available observing time on 4- to 10-m-class telescopes for Kepler follow-up and other RV survey, consistent with ExoPTF reporta

Precision photometry/spectroscopy

(Kepler)

10-zody sensitivity to exozodiacal emission, on as small a scale as possible

JWST primary/secondary transit spectroscopy, 10−4-10−5 of host-stellar signal

Astrometry

0.1-μas relative positional accuracy

Theory, numerics

Advances in planet/brown dwarf atmospheric models permitting positive identification of biomarkers

NOTE: Acronyms are defined in Appendix C.

aExoplanet Task Force, Worlds Beyond: A Strategy for the Detection and Characterization of Exoplanets, Washington, D.C., 2008.

PSF DISCOVERY AREA—IDENTIFICATION AND CHARACTERIZATION OF A NEARBY HABITABLE EXOPLANET

The question of whether or not there exist habitable worlds besides Earth is one of the deepest and most abiding ever posed by humanity. The answer would have ramifications that extend far beyond astronomy. One possibility is that no other inhabited world exists within the reach of modern astronomical observatories and thus that humanity is effectively alone in the cosmos. Another is that ours is but one of many planets where life has flourished. The knowledge of either reality would profoundly affect the scientific fields of astronomy, planetary science, prebiotic chemistry, and evolutionary biology, but it would also inform each individual’s sense of place in the universe. In its discussion above of science question PSF 4, the panel outlines a strategy to search for life on Earth-like planets orbiting in the habitable zones of Sun-like stars. That plan will require a dedicated effort spanning at least two decades. The panel believes that the effort is justified by the transformative power of the question that it seeks to answer. It is natural to wonder what shorter-term opportunities exist that could sustain our efforts in the years ahead.

There is a remarkable, although speculative, fast-track opportunity to finding and characterizing habitable exoplanets: namely, the search for large Earth-analogs (super-Earths) orbiting nearby low-mass, M-dwarf stars, the most common kind of star in the galaxy. This opportunity capitalizes on the most successful techniques for detecting and characterizing exoplanets from the past decade; as such it was unforeseen at the time of the previous astronomy and astrophysics decadal survey. Changes can now be detected in stellar radial velocities as small as 1 m sec1 and decreases in starlight as small as 0.3 percent due to planetary transits. Applied to Sun-like stars, these sensitivities cannot detect solid planets orbiting in their stel-

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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lar habitable zones. However, applied to low-mass stars, they are sufficient for the detection of habitable super-Earths. Moreover, transiting-based follow-up work has yielded a rich set of studies of the structures, compositions, and atmospheres of exoplanets and has fueled the nascent field of comparative exoplanetology. Here again, these techniques are not feasible for the study of habitable, Earth-like planets orbiting Sun-like stars, as the signal of such small planets would be overwhelmed by the photon flux from the star. They would be feasible if applied to nearby M-dwarf stars. It is noted that this opportunity was highlighted in the recent report of the ExoPlanet Task Force to the Astronomy and Astrophysics Advisory Committee.7

This discovery opportunity differs from that related to science question PSF 4 above in several respects. The primary distinction is the scientific question of whether M-dwarfs can indeed form habitable-zone super-Earths and whether or not the properties of M-dwarfs would preclude life on those worlds. On this last point, there are many ways in which M-dwarfs appear unwelcoming to life as we know it. These include their large UV and X-ray fluxes at young ages and the expectation that such planets would be tidally locked; these questions are discussed in more detail later. If indeed it is learned that M-dwarf planets sustain life despite these very different conditions, a fundamental piece of information will have been learned about the robustness of life in the cosmos. If, however, it is found that M-dwarf planets are lifeless, an important bound will have been placed on the conditions in which life can flourish. Clearly the present theoretical understanding of the origin of life is not remotely sufficient to predict whether M-dwarf planets will be inhabited, but the coming 5 years afford the opportunity to move this question into the realm of observational study. The panel also notes two practical ways in which this path differs from that related to PSF 4: First, the methods are based on separating the light of the planet from that of the star in time (through transit and occultation spectroscopy), as opposed to detecting spatial separation of planetary and stellar light (through imaging and interferometry). Second, the timescale for discovery is 5 years (as opposed to 20 years). It is this last point—the idea that the study of life outside the solar system could begin by 2014—that perhaps provides the greatest motivation for this avenue as a separate path from that discussed above in relation to science question PSF 4.

The Small Star Opportunity

Roughly 70 percent of stars in the immediate solar neighborhood are M-dwarfs. For a typical M-dwarf—spectral type M4V, corresponding to a radius and mass only 25 percent of the solar values—the habitable zone is located only 0.07 AU

7

ExoPlanet Task Force, Worlds Beyond: A Strategy for the Detection and Characterization of Exoplanets, Washington, D.C., 2008.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

from the star (Figure 4.19). As a result of the small planet-star separation, small stellar radius, and low stellar mass, the discovery of a large terrestrial planet orbiting within the stellar habitable zone could be achieved using only current radial-velocity and photometric precision, provided that this precision, demonstrated for Sun-like stars, can be obtained on these M-dwarfs. In particular, the transits of an Earth-size body would be 0.13 percent deep, and the radial-velocity signal would be 1.4 m sec–1. Moreover, the transits would occur more frequently—the orbital period would be a mere 15 days—and would be thrice as likely as a habitable-zone planet orbiting a Sun-like star. Perhaps most intriguingly, the planet-to-star contrast ratio in the Rayleigh-Jeans limit would be 0.012 percent, facilitating the study of the infrared spectrum of the planet by occultation spectroscopy. For an M8V primary (with a mass and radius only 10 percent of the solar values), the situation is even more favorable: the habitable zone lies at 0.017 AU, the transits would be 0.84 percent deep and would recur every 2.5 days, and the reflex radial-velocity amplitude would be 4.4 m sec–1. Moreover, the planet-to-star contrast ratio would be 0.11 percent, a helpful increase over the contrast ratio for a super-Earth of ~0.001 percent.

Several authors have considered the signal-to-noise ratio for a mock observing campaign of such a system with various instruments on JWST. These authors conclude that it would require a major investment of JWST time to achieve the requisite signal-to-noise ratio to be able to infer the presence of biomarkers but that it is possible for a super-Earth: an example is shown in Figure 4.20. The prospect that an observatory currently under construction would have the sensitivity to

FIGURE 4.19 Habitable zones around solar-type and M-dwarf stars. The shaded regions denote the range of distances from a G2V star (left) and an M5V star (right) for which the equilibrium temperature of the planet is greater than 0°C and less than 100°C, and hence for which water might be liquid at the surface. This naïve definition ignores the greenhouse effect, which maintains the surface temperature of Earth at roughly +30°C above the equilibrium temperature. Earth’s orbit is indicated by the dashed circle, and the orbit at which a planet would receive the same amount of energy per unit area and unit time is shown as a dashed circle in the right plot. SOURCE: J. Irwin, Harvard-Smithsonian Center for Astrophysics.

FIGURE 4.19 Habitable zones around solar-type and M-dwarf stars. The shaded regions denote the range of distances from a G2V star (left) and an M5V star (right) for which the equilibrium temperature of the planet is greater than 0°C and less than 100°C, and hence for which water might be liquid at the surface. This naïve definition ignores the greenhouse effect, which maintains the surface temperature of Earth at roughly +30°C above the equilibrium temperature. Earth’s orbit is indicated by the dashed circle, and the orbit at which a planet would receive the same amount of energy per unit area and unit time is shown as a dashed circle in the right plot. SOURCE: J. Irwin, Harvard-Smithsonian Center for Astrophysics.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
FIGURE 4.20 A simulated JWST NIRSpec observation (shown as black points) of a habitable-zone super-Earth with a radius of 2.3 and a temperature of 308 K located at 22 pc. The absorption feature due to carbon dioxide is detected with a signal-to-noise ratio of 28 for 85 hours of data in transit and an equal number outside of transit. Both the data and the model (blue curve) are shown at a sampling of λ/300, which would support a spectral resolution of 100 assuming three samples per optical resolution element. SOURCE: Courtesy of D. Demming, personal communication. Adapted from D. Deming, S. Seager, J. Winn, E. Miller-Ricci, M. Clampin, D. Lindler, T. Greene, D. Charbonneau, G. Laughlin, G. Ricker, D. Latham, and K. Ennico, Discovery and characterization of transiting super Earths using an all-sky transit survey and follow-up by the James Webb Space Telescope, Publications of the Astronomical Society of the Pacific 121:952-967, 2009.

FIGURE 4.20 A simulated JWST NIRSpec observation (shown as black points) of a habitable-zone super-Earth with a radius of 2.3 and a temperature of 308 K located at 22 pc. The absorption feature due to carbon dioxide is detected with a signal-to-noise ratio of 28 for 85 hours of data in transit and an equal number outside of transit. Both the data and the model (blue curve) are shown at a sampling of λ/300, which would support a spectral resolution of 100 assuming three samples per optical resolution element. SOURCE: Courtesy of D. Demming, personal communication. Adapted from D. Deming, S. Seager, J. Winn, E. Miller-Ricci, M. Clampin, D. Lindler, T. Greene, D. Charbonneau, G. Laughlin, G. Ricker, D. Latham, and K. Ennico, Discovery and characterization of transiting super Earths using an all-sky transit survey and follow-up by the James Webb Space Telescope, Publications of the Astronomical Society of the Pacific 121:952-967, 2009.

search for atmospheric biomarkers in a terrestrial planet is tremendously exciting, and a spectrum with the fidelity to identify atmospheric composition is clearly at the extremes of the reach of JWST. The more modest goal of simply obtaining a brightness temperature measurement would already be exceptionally interesting. An extension to this would be the determination of temperature as a function of planetary longitude, since a small day-night contrast would be strong evidence for the existence of an atmosphere enshrouding this distant rocky world.

There are two primary reasons why such M-dwarf planets, if they exist, might not be habitable. First, the proximity of the habitable zone places it within the tidal-locking radius, implying that the planet could have fixed day and night sides. This may in turn lead to a freeze-out of volatiles on the night side, at least for thin atmospheres. However, some planets, such as Mercury, may avoid this fate by becoming trapped in spin-orbit resonances, and in any event a thick atmosphere

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

or deep oceans could serve to ensure a moderate day-night temperature contrast. Second, all M-dwarfs are chromospherically active, and their intense stellar winds, combined with a weak or absent magnetic field on the planet owing to its slow rotation, may cause the atmosphere to be stripped away.

The panel takes these points as caveats; the only sure means for progress is measurement. The task of predicting habitability is surely no less complicated than that of modeling planet formation, and thus it bears noting that on the latter question, imagination—prior to the first exoplanet discoveries in 1995—proved wholly inadequate. Even a null result would be quite important: if numerous M-dwarf terrestrial planets are discovered that are similar to Earth in bulk composition, age, and insolation, yet are uninhabited, a fundamental fact about the requirements for life will have been learned.

The Hunt for Inhabited Worlds, 2010-2020

In order to pursue the exciting opportunity described above, the panel recommends the following:

  • Resources to export the radial-velocity precision of 1 m sec−1, currently achieved for Sun-like stars, to large numbers of M-dwarfs;

  • The development of near-IR spectrographs or red optical (700 nm < λ < 1,000 nm) charge-coupled-device-based spectrographs, both suited to capitalize on the spectral regions where M-dwarfs are brightest;

  • The development of new calibration techniques, such as novel gas absorption cells or laser frequency combs, and possibly very high resolution infrared spectrographs on large telescopes;8 and, as noted earlier,

  • A substantial increase in the amount of observatory time to undertake radial-velocity surveys, which was also a key finding for the study of Sun-like stars (science question PSF 3).

The community should pursue a transit survey of the closest 10,000 M-dwarfs for Earth-size planets (or, if conditions dictate, planets with a radius twice Earth’s value) orbiting within the stellar habitable zone. This may require the following:

  • Large-scale ground-based synoptic surveys, or a space-based survey, but the survey by necessity will need to cover a large fraction of the sky;

8

Internal precision of 10 m/sec has recently been reported for near-infrared radial velocity measurements of a very low mass star. See J.L. Bean, A. Seifahrt, H. Hartman, H. Nilsson, A. Reiners, S. Dreizler, T.J. Henry, and G. Weidemann, The proposed giant planet orbiting VB 10 does not exist, Astrophysical Journal Letters 711:L19, 2010.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×
  • A census of M-dwarfs out to 50 pc, and stellar astrophysical studies of these stars to understand their mass, radius, metallicity, distance, and if possible age; and

  • Detailed study of the atmospheric activity of M-dwarfs, to understand the limitations that such variability will impose on searches for planetary companions.

The community should also undertake intensive studies to determine the best observing practices for the study of such objects with JWST, and to determine if similar studies can be pursued with large ground-based telescopes. Accurate estimates of the likely signal-to-noise ratios from JWST (e.g., see Figure 4.20) will be crucial as preparations for the interpretation of such data are made. In conjunction with this work, theoretical expertise in the following areas needs to be fostered:

  • Modeling, in collaboration with the geophysics community, of the physical structures of Earth-size and super-Earth exoplanets with a range of composition, and an emphasis on the interpretation of anticipated data. Such studies may require laboratory investigations into the equations of state of materials that could be significant components in super-Earths.

  • Modeling the atmospheres of habitable Earths and super-Earths. Such studies will include photochemistry and cloud modeling and will require the development of detailed molecular databases.

These studies will likely be interdisciplinary, requiring expertise at the interface of astronomy, planetary science, and biology.

JWST is currently planned for launch in 2014, and it will have a finite lifetime. Because the spectroscopic studies envisioned here will likely extend over several years, it is crucial that the target planets be identified prior to or soon after the launch of the observatory. There is considerable expertise in the exoplanet community with respect to what needs be accomplished to extend the current radial-velocity and transit-survey precision to a sufficient number of M-dwarfs stars to enable this very exciting path toward detecting the first habitable worlds and undertaking a spectroscopic study of its atmosphere to search for biomarkers.

Conclusions: Habitable Planets Around M-Dwarfs

Table 4.6 summarizes the panel’s conclusions on activities to address its selected general area with unusual discovery potential.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
×

TABLE 4.6 Panel’s Conclusions Regarding Detection of Habitable Planets Around M Dwarfs

Technique

Requirements

Radial velocity (RV)

1 m sec−1, on M dwarfs, requiring red-near-IR spectroscopy and new wavelength calibration methods (gas cells, laser combs)

Substantial expansion of currently available observing time on 4- to 10-m-class telescopes for M dwarf survey

Precision photometry/spectroscopy

Transit survey (precision 10−4-105 of host-stellar signal) for 104 closest M dwarfs

Characterization of activity of M dwarfs within 50 pc

JWST primary/secondary transit spectroscopy, 10−4-10−5 of host-stellar signal

NOTE: Acronyms are defined in Appendix C.

Suggested Citation:"4 Report of the Panel on Planetary Systems and Star Formation." National Research Council. 2011. Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics. Washington, DC: The National Academies Press. doi: 10.17226/12982.
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Every 10 years the National Research Council releases a survey of astronomy and astrophysics outlining priorities for the coming decade. The most recent survey, titled New Worlds, New Horizons in Astronomy and Astrophysics, provides overall priorities and recommendations for the field as a whole based on a broad and comprehensive examination of scientific opportunities, infrastructure, and organization in a national and international context.

Panel Reports--New Worlds, New Horizons in Astronomy and Astrophysics is a collection of reports, each of which addresses a key sub-area of the field, prepared by specialists in that subarea, and each of which played an important role in setting overall priorities for the field. The collection, published in a single volume, includes the reports of the following panels:

  • Cosmology and Fundamental Physics
  • Galaxies Across Cosmic Time
  • The Galactic Neighborhood
  • Stars and Stellar Evolution
  • Planetary Systems and Star Formation
  • Electromagnetic Observations from Space
  • Optical and Infrared Astronomy from the Ground
  • Particle Astrophysics and Gravitation
  • Radio, Millimeter, and Submillimeter Astronomy from the Ground

The Committee for a Decadal Survey of Astronomy and Astrophysics synthesized these reports in the preparation of its prioritized recommendations for the field as a whole. These reports provide additional depth and detail in each of their respective areas. Taken together, they form an essential companion volume to New Worlds, New Horizons: A Decadal Survey of Astronomy and Astrophysics. The book of panel reports will be useful to managers of programs of research in the field of astronomy and astrophysics, the Congressional committees with jurisdiction over the agencies supporting this research, the scientific community, and the public.

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