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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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,
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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: 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? 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.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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-
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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 sec−1 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-
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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.
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Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics 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 10−4-10−5 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.