Most if not all stars host planetary systems, but the structures of these systems and the properties of their planets are diverse (Chapter 2) and often unlike those of the Solar System. The physical and chemical processes that produce planetary bulk compositions, surfaces, and atmospheres are complex, and the guiding principles that determine the outcome have not yet been determined. One of the major goals of exoplanet science is as follows:
Goal 1: To understand the formation and evolution of planetary systems as products of the process of star formation, and characterize and explain the diversity of planetary system architectures, planetary compositions, and planetary environments produced by these processes.
To achieve this goal, the exoplanet science community needs to do the following:
- Determine the range of planetary system architectures by surveying planets at a variety of orbital separations and searching for patterns in the structures of multiplanet systems;
- Characterize the diversity of bulk compositions and atmospheric compositions;
- Identify the parameters that determine which stars can form certain types of planetary systems; and
- Identify relationships between the planet formation process and the resulting planetary evolution, bulk composition, and atmospheric properties.
A key goal of the theory of planet formation is to produce population synthesis models that tie observationally constrained properties of stars and circumstellar disks to the observed population of planets. Major open questions in planet formation theory include the following: How do planetesimals with sizes of order kilometers to hundreds of kilometers form? Do all gas giants form through the core accretion process or is an additional population produced by gravitational instability? How does recent work demonstrating that gas drag can facilitate efficient accretion of pebble-size planetesimals change classic core accretion scenarios? What determines whether a planet grows to be a terrestrial, super-Earth, or giant planet? How do chemical transitions at the ice lines of abundant volatile species affect the compositions of planets that form at different distances from their stars? What determines whether planets undergo substantial migration? Broadly stated, how do the parameters of disks, including their
masses, sizes, angular momenta, chemical structures, and dispersal time scales, result in the diversity of planetary systems? Pursuing these questions requires an understanding of the distribution of both disk properties and planets.
Characterizing the population and properties of giant planets is central to this endeavor: giant planets dominate the dynamics and planetary mass of their host systems. To the extent that the population of gas giants has been constrained, it has already provided important clues to the physical parameters that control planet formation: For example, high-metallicity stars are known to host more close-in giants (Gonzalez, 1997; Fischer and Valenti, 2005; Santos et al., 2005) and more planets on highly elliptical orbits at moderate separations from their stars (Dawson and Murray-Clay, 2013). These results support the core accretion model of planet formation, in which disks hosting more solid material more easily form solid cores massive enough to accrete giant envelopes. However, scientists do not yet understand the range of environments in which gas giants can form (must it be in a cold, ice-rich location?) or how they form (do all giant planets first begin as solid cores of a specified mass, and then accrete gas?). Importantly, most of the mass of gas giants is contained in their gas envelopes, meaning that their formation time scale is closely tied to that of disk gas dissipation.
The location and time scale of the formation of giant planets, in turn, can have an enormous impact on the prospects for forming small planets in the same system. The relationship between the inner and the outer parts of planetary systems constrains models of formation, with implications for migration histories and compositions. For example, no consensus yet exists about whether the super-Earths with orbital periods less than 1 year, that are known to orbit perhaps a third of stars, are icy bodies formed in outer disks that then migrate to their present locations or if they form in situ from primarily rocky material.
The desire to understand planet formation cannot be satisfied by finding only the small planets close to their stars (as discussed below, neither can the desire to understand habitability). Only by first measuring the full architecture of planetary systems can the correlations between architectures, compositions, and stellar properties be established to ultimately lead to an understanding of planet formation. For example, do stars with high ratios of silicon to iron form more solid material, and are they ultimately more efficient at building the cores of large planets? Do less massive stars form less massive planets, revealing that initial disk mass is an important controlling parameter? Do systems hosting close-in super-Earths typically also contain distant gas giants, suggesting that they are products of disks with large masses in solids? And alternatively, are super-Earths anti-correlated with gas giants, suggesting that they form when most disk solids are able to migrate into the inner system? Are multiple cold planets not close to resonance while multiple warm planets are, suggesting that migration distances in disks are larger in inner than outer disks? Are stellar binaries of certain mass ratios and separations incapable of forming planetary systems? Or is planet formation so inevitable that it is largely insensitive to binarity?
The chemistry of planets is inherited from the disks from which they form. The Solar System bodies provide substantial clues to expectations about disk inheritance, including more volatile-rich material in the outer regions at large distances from the Sun, substantial mixing in the early solar nebula that leads to high-temperature condensates in comets and similar organics in both comets and asteroids (Keller et al., 2008), and evidence for giant impacts late in the formation process. While exoplanetary systems can have vastly different architectures than the Solar System, these chemical segregation and mixing processes depend on physics that should operate in all disks. Merely knowing of the existence of planets (and their bulk properties) is not sufficient: a statistical sample of atmospheric compositions at a range of orbital distances is also required, to test the link between planet composition, location of formation, and migration distance.
Major facilities to study planetary atmospheres should ensure that instrument design and survey design allow for characterization of a diverse planet population. Eventually, the goal is for population synthesis models to incorporate accurate disk physics and predict the correct distribution of planets in a statistical sense. With limited spectral coverage, interpretation of planetary atmospheric compositions will likely be ambiguous. Planet formation studies can help resolve this ambiguity, but only if enough empirical information is provided across a range of systems.
Finding: Current knowledge of the demographics and characteristics of planets and their systems is substantially incomplete. Advancing an understanding of the formation and evolution of planets requires two surveys: First, it requires a survey for planets where the census is most incomplete, which includes the parameter space occupied by most planets of the Solar System. Second, it requires the characterization of the atmospheres and bulk compositions of planets spanning a broad range of masses and orbits.
Determining the demographics of exoplanets is challenging for two reasons: First, knowing how the planet population varies as a function of a number of parameters, such as stellar mass, stellar metallicity, planet mass, and orbital period, is needed; and exploring each new dependency is resource intensive. Second, different detection methods are sensitive to different kinds of stars, planets, and orbital periods. Exacerbating the challenge, the ranges of sensitivities often do not overlap, requiring extrapolation to connect the populations inferred by different methods (Figure 3.1).
As described in Chapter 2, the radial velocity and transit methods have provided a statistical census of the planet population of close-in planets with separations of less than roughly 1 AU and masses or radii greater than that of Earth, and imaging has provided a first look at the population of young (and hence luminous) massive worlds far from their stars. However, other than the several dozen precious detections from microlensing, knowledge of the population of mature planets on long orbits is woefully incomplete. This leaves researchers unable to probe planets as a function of where ices such as water condense (snow lines), although these transitions appear important in the Solar System and in protoplanetary disks. Furthermore, the population of known planetary systems in the parameter space of planets’ mass (or radius) and semimajor axis (or period) is nearly completely disjoint from the parameter space covered by the planets in the Solar System. As discussed in Chapter 2, the detection methods that are best positioned to contribute to improving a statistical census of planets, particularly analogues of the planets in the Solar System, are astrometry, microlensing, and direct imaging.
The Gaia astrometric mission (Perryman et al., 2001) is currently operating and, by the end of its mission, it will have an astrometric accuracy sufficient to discover roughly 20,000 planets with masses at least as great as Jupiter, and in orbital periods of less than 6 years (Perryman et al., 2014). The region of parameter space probed by Gaia will overlap significantly with that of radial velocity surveys, but Gaia will inform us of masses and the relative inclinations of the orbits in multiplanet systems. Furthermore, Gaia will be sensitive to planets orbiting stars more massive (and, thus, at different evolutionary states) than those probed by radial velocity surveys. And,
because Gaia’s yield of planets is substantially larger than any other previous exoplanet survey, it will be uniquely sensitive to rare systems, which often provide deep insight into the processes of planet formation and evolution (e.g., HD 80606; Naef et al., 2001).
Nonetheless, Gaia will not, in general, provide access to the population of planets that are either lower mass or more distant than Jupiter, particularly for Sun-like stars (Casertano et al., 2008). Therefore, Gaia will not probe the majority of the parameter space that is spanned by the Solar System planets with the exception of Jupiter analogues, or indeed a region of parameter space that has not already been surveyed by radial velocity surveys. An understanding of planet formation is critically informed by the existence of planets with masses less than, and separations greater than, Jupiter.
A Space-Based Microlensing Survey
As described in Chapter 4, in the section “Expanding the Statistical Census of Exoplanets,” the essential survey that will largely complete the statistical census of exoplanets is the Wide-Field Infrared Survey Telescope (WFIRST) microlensing survey. Combining the findings of WFIRST with previous transit and radial velocity surveys will
- Measure the frequency of planets with masses greater than Earth at all relevant separations;
- Measure the frequency of planets with masses as low as that of Mars out to several AU;
- Be sensitive to bodies with masses as low as several times the lunar mass, including objects orbiting Earth-mass planets;
- Determine if the frequency of planets in the galactic bulge is significantly different from that in the neighborhood of the Sun; and
- Measure the mass function of free-floating planetary mass objects, whose occurrence rate and mass function likely provide a strong indicator of the dominant formation processes of exoplanetary systems.
A limitation of WFIRST microlensing is that it is typically sensitive to only one planet in each system (for some events, it will detect multiple planets, but not all planets). The committee notes the complementarity here with the direct imaging method, which is discussed later. Direct imaging can detect planets over a broad range of separations, and thereby tie the population of outer planets (i.e., analogues of Jupiter and Saturn) with the presence of any worlds in the habitable zone. Given that the existence of a giant planet may or may not affect the habitability of the terrestrial world, this feature of direct imaging surveys is quite important. Direct imaging facilities will deliver detailed characterization of a comparatively smaller number of planetary systems, which will provide important constraints on the physics of planetary atmospheres and interiors, and on the architectures of planetary systems, including the frequency with which terrestrial planets have outer giant planets.
What Gaps Will Remain in the Planetary Census?
Although the execution of Gaia and WFIRST will greatly expand the statistical census of exoplanets, gaps in knowledge of the demographics of exoplanets in certain regions of parameters space will nevertheless remain.
- Protoplanetary and debris disk observations probe dust grains up to a millimeter in size, whereas the exoplanet detection techniques are sensitive to planets with masses greater than that of roughly the moon. This is a 36-order-of-magnitude gap between observations of protoplanetary disks and mature planetary systems; thus, there exists a very large gap in the mass and radius of solid bodies.
- Understanding of the demographics of planets in binary stars is incomplete. Many planets orbiting one member of a wide binary have been discovered, and a handful of circumbinary planets, but the physical processes that govern the formation of planets in these systems remains poorly understood.
- The demographics of planets orbiting young stars would be exceptionally valuable. This could constrain the initial conditions of planet formation after gas dispersal, thereby potentially cleanly separating the effects of
- gas dynamics from N-body dynamics. The activity of young stars makes planet detection difficult, except for the method of direct imaging (Pollack et al., 1996). If calibrated by future observations of disks and planets in the same systems, features in the dust distributions of protoplanetary and young debris disks may provide an additional window into this population.
- The dynamical characteristics of its resident planetesimals and satellites have greatly constrained models of the Solar System’s formation. For example, resonance structures in the asteroid and Kuiper belts reveal that the Solar System’s giant planets migrated from their formation locations. The dust produced by planetesimal belts in extrasolar systems can be probed with direct imaging instruments. However, satellites of exoplanets are difficult to detect, and none has yet been confirmed. In principle, one can detect satellites via timing variations or flux anomalies for transiting planets (Kipping et al., 2009). Microlensing is also sensitive to large-mass moons, but the sensitivity is poor for typical planet-moon separations (Bennett and Rhie, 2002).
The study of exoplanet atmospheres and interiors presents a complementary way to advance knowledge of planet formation and evolution, and provides a unique window into the physics and chemistry of planetary environments. Some of the pressing questions in planet formation and evolution are outlined below, with the associated physics and chemistry of their atmospheres. The diversity of exoplanets presents both a challenge and opportunity for characterizing the atmospheres and interiors of these objects. Diversity presents a challenge, because it means that individual planets are not fully representative of a broad class of objects, and large samples are likely needed to reveal the properties of distinct populations. On the other hand, diversity is an opportunity: by looking at similar objects in different regimes, and in regimes not represented by the Solar System planets, the effects of the different physical processes can, hopefully, be untangled.
Giant exoplanets present the opportunity to understand the growth of planets in gas-rich protoplanetary disks. The core nucleated accretion model for giant planet formation has been largely validated for planets that are found within 5 AU of their stars. Nevertheless, key questions remain, including the role of the core, the location of formation, and the effect of migration.
The core nucleated accretion model posits that the formation of giant planets is seeded by the growth of a solid core that reaches a critical mass that is sufficient to initiate runaway accretion of nebular gas (Pollack et al., 1996). The classic value for the critical core mass is 10 ME, but can vary substantially depending on the details (Rafikov, 2006). Therefore, it would be extremely valuable to know the core masses of giant planets. However, measurements of the absolute core masses of giant planets is a notoriously difficult problem even for the Solar System giants, due to uncertainties in equations of state and model degeneracies (Fortney and Nettelmann, 2010).
The challenge of determining core masses can be somewhat overcome for giant exoplanets by focusing instead on relative rather than absolute values. Under the same model and equation of state assumptions, the relative amount of heavy elements in the interiors of exoplanets can be inferred from knowledge of masses and radii (Miller and Fortney, 2011). As described in Chapter 2, initial studies indicate that planetary heavy element abundances are correlated with the metallicities of the stellar hosts (Thorngren et al., 2016). These studies should be expanded to explore how the bulk heavy element abundance of giant planets changes with planet mass, orbital distance, and host star mass and composition. This requires precise measurements of the masses and radii of a sample of planets that are less irradiated than the ones studied to date.
As described above, the envelopes of giant planets are thought to form by runaway accretion of nebular gas. Therefore, the atmospheric compositions of giant planets should be broadly similar to those of their host stars. On the other hand, the core accretion model is based on the assumption that giant planets form in a region of the disk that is rich in solid planetesimals. These planetesimals sequester heavy elements out of the gas, with their composition depending on the temperature and dynamics of the disk, and the gas, ion, and surface chemistry. Some of the planetesimals will dissolve in the atmosphere and return heavy elements to the gas. Migration can also lead
to giant planets feeding from different parts of the disk. Thus, the ways in which the atmospheric compositions of giant planets differ from their host stars can be used to constrain the chemistry and surface density of solids, the migration pathway, and the overall formation mechanism (e.g., Oberg et al., 2011; Mordasini et al., 2016).
The atmospheric compositions of the solar-system giant planets have been used for decades as a tracer of their origins. Figure 3.2 summarizes what is known from a combination of Earth-based observations, orbiters and flybys, and in situ measurements (Atreya et al., 2016). Despite the significant investment in studying these planets, it is surprising how little is known about their elemental makeup. There is good reason for this, which is that the solar system giants are relatively cold and thus most key chemical species have condensed out of their observable atmospheres. Jupiter is the exception solely because of the in situ measurements by the Galileo entry probe (Owen et al., 1999).
Exoplanets offer several important advantages in the study of giant planet atmospheric compositions. First, the atmospheres of hot giant exoplanets can be studied, and for these planets many of the sought-after chemical species are in the gas phase and observable. A much wider range of elemental abundances can be measured for exoplanets than for the solar system giants. It should be possible to determine the carbon, nitrogen, and oxygen abundances, which is important because these elements make up the bulk of the solid planetesimals that are crucial for giant planet formation.
Figure 3.3 shows what is known about the atmospheric metallicities of the solar-system giant planets, and exoplanets. In the solar system, the methane abundances increase toward smaller planet masses, consistent with the expectation from the core accretion model. Water abundances for transiting planets are just now being measured and added to this diagram (e.g., Kreidberg et al., 2014). Upcoming work using both transit and direct imaging techniques should expand these studies to achieve a more complete assessment of metallicity (by measuring more chemical species), and to determine the elemental abundance ratios, such as C/O.
Another advantage of exoplanets is the opportunity to study a much larger sample than that afforded by the solar system. This is important because planet formation represents the stochastic outcome of a variety of physical processes (resulting in, for example, a predicted spread in the planetary mass-metallicity relationship; Fortney et al., 2013). Furthermore, the population of exoplanets affords the possibility to learn how atmospheric composition depends upon orbital semimajor axis and host star mass. Characterization of many planets will be needed to reveal even the simplest trends that are expected from models.
Formation by gravitational instability could play a role in the colder, outer regions of planetary systems. Young planets with ages <100 Myr that are still evolving will retain the imprint of their initial entropy in the form of their radii and temperatures; giant planets formed by disk instability should have higher entropy and thus larger radii and temperatures than similar planets formed by core accretion (Spiegel and Burrows, 2012). Determining the temperatures and surface gravities (a proxy for radius) for a large sample of distant, young, self-luminous planets will constrain the roles of the core accretion and disk instability mechanisms. Exploration via direct imaging of the boundary between wide-separation giant planets and brown dwarf binary companions will also inform whether a population of companions formed through gravitational instability exists (Kratter et al., 2010).
Straddling the Gap
The surprising discovery of an abundance of planets with masses and sizes between that of Earth and Neptune allows us to investigate the crossover regime between rocky terrestrial planets and gas-rich giants. It was previously thought that cores of 10 ME were necessary to initiate runaway gas accretion (e.g., Ida and Lin, 2004), but mass-radius measurements indicating low bulk densities and implying hydrogen-rich envelopes for planets below this
mass have cast doubt on this idea (see Chapter 2). A range of theories for forming low-density super-Earths and mini-Neptunes now exist. Yet the relative importance of gas accretion onto smaller cores, degassing of volatiles, and accretion of ice-rich material from large orbital separations in sculpting the population of low-mass low-density exoplanets remains to be determined. Atmospheric observations will play a pivotal role in addressing these ideas.
Few observations of the atmospheres of sub-Neptune-size exoplanets currently exist. Of the ones that do, most are featureless transmission spectra resulting from aerosol-dominated atmospheres (e.g., Kreidberg et al., 2014; Knutson et al., 2014). However, not all planets are dominated by aerosols (e.g., the recent detection of water absorption in GJ3470; Tsiaras et al., 2018); thus, observations of more planets should reveal the desired gas-phase compositions. Furthermore, broader wavelength and higher spectral resolution transmission spectroscopy observations and thermal emission spectroscopy observations have the potential to see through obscuring aerosols. These observations will allow us to directly probe the composition of planets in the transition regime between giants and terrestrials.
The shift from gas-rich to rocky planets is apparent in the bimodal radius distribution of exoplanets discovered by Kepler (Chapter 2). While this transition is now well defined in radius, it will be illuminating to study it in terms of density and atmospheric composition. By exploring the mass-radius relation for planets smaller than Neptune and their atmospheric compositions, the true range of outcomes for planet formation and evolution can be studied in an intermediate size range that is not represented in the Solar System. Outstanding questions that researchers hope to answer for these planets include the following:
- Over what range of masses, insolation, and formation time scale are planets able to retain hydrogen-rich atmospheres?
- What are the relative roles of outgassing, accretion, and atmospheric escape in forming the atmospheres of sub-Neptune-size exoplanets?
- Do intermediate-size exoplanets come only in the form of mini-Neptunes and super-Earths, or do water-dominated hydrogen-poor worlds also exist?
The secondary outgassed atmospheres of terrestrial exoplanets are expected to be far more compositionally diverse than those of the larger gas giants. The atmospheric composition of a terrestrial exoplanet results from complex processes related to the dynamical history of the planet, its accretion history (both of solids and gas), its thermal history, and its interactions with its host star via high-energy photons and stellar winds.
The required observations of terrestrial exoplanet atmospheres do not yet exist due to the tiny size of their atmospheric signatures against the background of their host stars. Even the James Webb Space Telescope (JWST) will be hard pressed to make meaningful observations of temperate terrestrial planetary atmospheres, except for in the few most optimal cases of planets orbiting mid-to-late M dwarfs. As discussed in Chapter 4, in the section “The Case for Imaging,” flying a large space-based direct imaging mission will be the most productive strategy for performing atmospheric characterization of a large number of terrestrial planets over a range of orbital separations.
Precise measurements of the masses and radii of terrestrial planets will play an essential role in exploring their interior structures. The ratio of the iron core to the rocky mantle can be constrained for terrestrial planets (e.g., Zeng and Sasselov, 2013). The near-term study of the diversity of terrestrial exoplanets will provide the vital context for future studies of habitable exoplanets and the search for life beyond the Solar System.
Physics and Chemistry of Planetary Environments
Atmospheres are the principal window into the physical processes that shape planetary environments. Atmospheres govern planetary climate by mediating the balance between stellar irradiation, reradiated flux, and a planet’s intrinsic luminosity. Heat transport moves energy vertically and horizontally, but strong temperature gradients can persist in altitude, longitude, and latitude. Testing theories of heat transport requires determining how the albedos, bolometric luminosities, thermal structures, and atmospheric circulation vary with planetary irradiation, surface
gravity, and rotation rate. Current observations focused mostly on transiting hot Jupiters have raised the question of how temperature inversions arise on highly irradiated planets and how such planets transport energy from their permanent daysides to their permanent nightsides (e.g., Parmentier et al., 2018; Parmentier and Crossfield, 2017). These preliminary results demonstrate the need for broad wavelength coverage to accurately determine bolometric luminosities and albedos, spectroscopic mapping to reveal the full planetary context, and more advanced models that incorporate a wider range of physics and chemistry.
Chemical gradients often follow temperature gradients, with condensation/evaporation, dissociation/recombination, and oxidation/reduction states varying. Furthermore, nonequilibrium effects like photochemistry and quenching can alter the composition as well. For example, carbon chemistry is a sensitive probe of atmospheric conditions. For hydrogen-rich atmospheres CH4 is expected to be the dominant carbon-bearing molecule at low temperatures and CO at high temperatures. However, CH4 is also readily photolyzed, so it may be absent from atmospheres in which it would otherwise be expected from equilibrium chemistry considerations. For the oxidizing atmospheres of terrestrial exoplanets, CO2 is expected to become the primary carbon-bearing species. For nitrogen, a similar balance occurs between N2 and NH3, the latter of which is also photochemically sensitive and may be converted to either N2 or HCN. Ultimately, measuring the abundances of a wide range of chemical species for a wide range of planets will be needed to fully explore the various chemical pathways in exoplanet atmospheres.
The formation of aerosols both by equilibrium and nonequilibrium processes is an especially pernicious problem in planetary atmosphere chemistry. Through their opacities, aerosols impact the observability of gas-phase chemical species and influence planetary energy budgets. Condensation more generally can also sequester key elements out of view as described above for the giant planets of the Solar System.
While problematic from the standpoint of spectroscopic observability, the presence of aerosols are also signposts of the physical and chemical processes occurring in planetary atmospheres. For example, photochemical hazes indicate disequilibrium chemistry and photolysis processes occurring in the upper atmospheres. For condensate clouds, diagnosing their composition can be used to probe both the chemical makeup and the thermal structure of an atmosphere. Aerosols also serve as tracer particles to determine planetary rotation periods and dynamics (e.g., Apai et al., 2017).
At the lower boundary, atmospheres are coupled to the planetary interior, including through outgassing and drawdown processes at the surface interface for terrestrial planets. Terrestrial planets can also be impacted by biological activity. Therefore, atmospheric compositions are an important diagnostic of planetary geophysics, habitability, and life (see the following section).
Finding: Characterizing the masses, radii, and atmospheres of a large number of exoplanets with a range of physical and orbital parameters for a diverse set of parent stars will yield fundamentally new insights into the formation and evolution of planets and the physics and chemistry of planetary environments.
Recent advances toward understanding the context for biosignatures, as well as the factors that make a planet hospitable to life, underpin the second major goal of exoplanet science:
Goal 2: To learn enough about the properties of exoplanets to identify potentially habitable environments and their frequency, and connect these environments to the planetary systems in which they reside. Furthermore, scientists need to distinguish between the signatures of life and those of nonbiological processes, and search for signatures of life on worlds orbiting other stars.
To achieve this goal and support the search for life in the galaxy, the following two areas of interdisciplinary research need to be developed:
- A multiparameter habitability assessment for target selection and
- A comprehensive framework for biosignature assessment.
In parallel with these theoretical underpinnings, a sequence of observational milestones needs to be achieved to
- Identify and rank exoplanet targets;
- Characterize the environment, including the atmosphere, surface, and interior of the planet, and the spectrum and variability of the star; and
- Search for life in the context of the planetary and stellar environment.
The habitable zone (Kasting et al., 1993; Kopparapu et al., 2013) has been a practical but simplistic way to assess exoplanet habitability, depending upon only two readily observable characteristics: the planet-star distance, and the type of star. Going forward, a more comprehensive multiparameter habitability assessment, combining laboratory and theoretical computer modeling with constraints from observations, will be needed to rank exoplanets for observational searches for biosignatures. Similarly, a framework for biosignature assessment based on laboratory and field measurements, as well as theoretical modeling to predict key observables needed to discriminate between abiotic and biological sources, will allow researchers to interpret putative biosignatures in the context of their environment, and thereby increase the credibility of their interpretation. As part of this framework, novel biosignatures need to be considered and abiotic processes that could mimic the signs of life need to be identified.
These precursor studies will both inform the measurement requirements for future instruments and will be necessary to interpret the resulting data. In the near term, the search for habitable environments and signs of life will focus on transiting M dwarf planets (see below and the section “Opportunities to Characterize Planets Through Transits,” in Chapter 4). In the longer term, direct imaging facilities will search for indicators of habitability and life on planets orbiting stars closer in mass to the Sun (see below and the section “The Case for Imaging,” in Chapter 4). These studies will be complemented by statistical surveys on larger samples of terrestrial planets that provide information on planet demographics and bulk properties for terrestrial planets both interior to and outside the habitable zone. Additionally, the ability to confidently identify habitable planets and life will rely on the expertise of several disciplines outside of traditional astronomy (see below and the sections “Astrobiology” and “Mechanisms to Achieve Interdisciplinarity,” in Chapter 4).
An improved understanding of the planetary-system-wide impacts on habitability requires research that explores exoplanets as systems, including the exchange between the interior and atmosphere, the interaction between the planet and its parent star, and the planetary system architecture (see Figure 3.4). The following processes, including those occurring on solar system planets, should be studied to better understand how a planet can acquire and maintain habitability.
The exoplanet science community needs to expand its knowledge of intrinsic planetary properties that make habitability more likely and identify observational discriminants for those properties (Figure 3.4). For example, as outlined in Chapter 2, in the section “The Search for Life on Exoplanets,” a habitable planet is most likely terrestrial with a solid surface that supports a liquid water ocean formed from initial volatile content, and with orbital parameters that allow a habitable surface temperature. It must also have lost most or all of its primary H2-dominated atmosphere (Owen and Mohanty, 2016; Pierrehumbert and Gaidos, 2011) and resisted the loss of its ocean and secondary atmosphere (Meadows and Barnes, 2018).
The presence of a planetary atmosphere strongly impacts habitability by providing the surface pressure required to maintain a surface liquid ocean against escape processes (Hunten, 1973; Wordsworth and Pierrehumbert, 2014; Garcia-Sage et al., 2017). The composition of that atmosphere—including the noncondensable bulk composition, greenhouse gases, clouds, and hazes—modifies the atmospheric thermal structure and could provide a habitable surface temperature (Rauer et al., 2011). Atmospheric gases and aerosols help shield the planetary surface from UV flux (Segura et al., 2003, 2005; Rugheimer et al., 2015; Rugheimer and Kaltenegger, 2018; Arney et al., 2016).
Both atmospheric pressure and composition play a role in buffering day-night and seasonal temperature differences, including reducing the probability of atmospheric collapse via freeze-out on the nightside of synchronously rotating planets (Joshi and Haberle, 2012; Turbet et al., 2016, 2017). The presence of an ocean (Hu and Yang, 2014) can also buffer day-night temperature differences and help a synchronously rotating planet avoid atmospheric collapse.
Since terrestrial planets are more likely to support a surface ocean, a critical area of future research in planetary habitability will be the loss, maintenance, and replenishment of secondary terrestrial atmospheres. Moreover, the current understanding of the evolution of terrestrial exoplanets, with a diversity of compositions and host star environments, will need to improve. A key component will be understanding degassing for terrestrial planets of different compositions, including potentially more volatile-rich migrated planets that may be found orbiting M dwarfs, both inside and far from the nominal habitable zone (Gillon et al., 2017; Luger et al., 2017; Grimm et al., 2018; Berta-Thompson et al., 2015; Dittmann et al., 2017; Meadows and Barnes, 2018).
The host star’s composition, gravity, and irradiation can strongly influence the formation and orbital, interior, and atmospheric evolution of all classes of planets. For habitable-zone terrestrials, however, the host star’s characteristics can also strongly impact whether or not the planet is able to acquire and maintain a strong impact on the planet’s environment and habitability. In particular, the star’s evolution in luminosity (Baraffe et al., 1998) drives strong climate change and may result in atmospheric or ocean loss (Ribas et al., 2016; Barnes et al., 2018; Dong et al., 2017). The stellar spectrum and activity also influence atmospheric escape and climate, and photochemically modify the atmospheric composition (Meadows and Barnes, 2018). Indeed, the need to understand the illuminating star is paramount for understanding all irradiated planets, whether or not they might be habitable.
For photochemical models to predict and interpret atmospheric conditions, including planetary surface UV flux, knowledge of the high-energy stellar fluxes, spectral slopes, and variability are needed. For habitable-zone terrestrial planets, especially those in close-in orbits around M dwarfs, the high-energy photon and proton flux is even more critical to understand (Segura et al., 2005, 2010; Rugheimer et al., 2015; Tilley et al., 2017). It is therefore essential to obtain stellar UV spectroscopy on different time scales for stars of different stellar type and ages, to characterize stellar UV fluxes and variability. This is especially needed in the near term to prepare for and interpret atmospheric observations of M dwarf exoplanets with JWST (e.g., for the TRAPPIST-1 system and GJ1132b; Lincowski et al., 2018) and large ground-based telescopes (e.g., Proxima Centauri b; Lovis et al., 2017; Snellen et al., 2015).
Stellar activity levels and UV slopes can be quantified with broadband photometry obtained from space-based observatories such as the Galaxy Evolution Explorer or the Hubble Space Telescope (HST), or via CubeSat missions such as the Star-Planet Activity Research CubeSat (Shkolnik et al., 2018) or the Colorado Ultraviolet Transit Experiment (France et al., 2018). Additionally, visible light proxies for UV emission can be obtained from ground-based telescopes. However, wavelength-dependent spectral information on flares and the stellar extreme-ultraviolet (XUV) to near-ultraviolet region can be done currently only with UV spectroscopy from HST, which is nearing the end of its mission lifetime. It is critically important to gather UV information on main sequence stars that may host exoplanets, before this capability is lost. Additional ground-based work characterizing stellar variability and activity (especially for M stars), and enhanced communication with stellar astrophysicists, would advance the field.
Planetary System Architecture and Evolution
Other components of a planetary system, such as Jovian planets, asteroid and Kuiper belts, and nearby sibling planets, can also affect the potential habitability of a habitable-zone planet and provide clues to its formation and evolution history (Raymond et al., 2008; O’Brien et al., 2014; Meadows and Barnes, 2018). In particular, the masses, orbits, and migration history of Jovian planets should be characterized to the extent possible, as they can affect volatile delivery to forming terrestrial planets, and eccentric Jovians could result in the formation of water-poor terrestrials (Raymond et al., 2004, 2007; Lissauer, 2007; Meadows and Barnes, 2018). Nearby sibling planets can also modify orbital parameters, including eccentricity and obliquity. Belts of minor planets analogous
to the asteroid and Kuiper belts (and detectable as infrared excesses) serve as a reservoir for water-rich bodies and impactors, and the disk’s dust distribution can reveal the gravitational signature of unseen planets (Chambers, 2001; Raymond et al., 2011). Exomoons also influence habitability by damping large obliquity oscillations, but remain challenging to detect (Meadows and Barnes, 2018).
Star-Planet-Planetary System Interactions and Habitability
While the characteristics of planet, star, and planetary system described above can increase or decrease the probability that a planet is habitable, they do so via interactions between the planet, its host star, and its planetary system (Meadows and Barnes, 2018). An improved understanding of these interactions is needed to fully understand the impact on a planet’s environment and predict observable consequences. Some of the key interactions affecting planetary habitability are described below.
Stellar Composition, Planet Formation, and the Delivery of Volatiles
Planet formation, migration, and the delivery of volatiles are key processes that determine the composition and structure of a planet, and whether or not it can be habitable. Importantly, reliable planet formation models can provide essential constraints on planetary properties that are difficult or impossible to observe directly. If models or observations suggest that a planet is more likely to have formed with a very low volatile abundance, then it has less chance of being habitable. For example, while initial modeling of the formation of M-dwarf terrestrial planets suggested that they might form with little water (Raymond et al., 2007; Lissauer, 2007), recent measurements of the density of Earth-size planets orbiting the M8V TRAPPIST-1 star suggest that they are instead volatile rich (Grimm et al., 2018). This may be due to a history of forming in a more volatile-rich birth orbit and then migrating inward, which is suggested by their resonant chain of orbits (Luger et al., 2017). An improved knowledge of stellar compositions is also required to constrain planetary evolution models and for habitability assessment (Young et al., 2018). Measurements of stellar composition can provide insight into the nature of the planets themselves, and can be combined with the planet’s mass and radius, and planet formation and differentiation models to provide clues to the planet’s composition and interior structure (Dorn et al., 2015; Unterborn et al., 2015; Meadows and Barnes, 2018).
Star-Planet and Planet-Planet Orbital and Tidal Interactions
Planets that orbit close to their parent stars (a < 0.1 AU) are affected by tides, and this may include planets in the habitable zones of smaller stars. In addition to tidal locking (Dole, 1964; Barnes, 2017), which can result in synchronous rotation, tides may impact habitability via orbital circularization and migration (Rasio et al., 1996; Jackson et al., 2008), obliquity erosion (Goldenreich, 1966; Heller et al., 2011), and tidal heating (Jackson et al., 2008; Barnes et al., 2013). Synchronous rotation may, in turn, increase the chance of atmospheric collapse (Joshi, 2003; Turbet et al., 2016), but it also can be avoided if a nonzero eccentricity or obliquity can be maintained via perturbation by another planet (Barnes et al., 2010). Planets in noncircular orbits can be tidally heated, changing their internal properties and outgassing rates. Consequently, any constraints on a planet’s orbital and rotation state, and its age, will provide key clues to processes that will impact habitability. These constraints may eventually be obtained via time-dependent multiwavelength mapping using direct imagers to constrain planetary rotation, and from improved stellar ages.
Star-Planet Radiative Interactions and Evolution
Venus, Earth, and Mars exhibit secondary atmospheres that are likely composed of fractionated remnants of the primordial atmospheres, augmented by outgassed volatiles from the planetary interior and volatile delivery from other bodies in the system (Pepin, 2006; Meadows and Barnes, 2018). If the secondary atmosphere is lost at a rate that outstrips replenishment, then the atmosphere would be lost, and surface habitability would be precluded. To assess the potential for exoplanet habitability, especially for planets orbiting M dwarfs, it is critical
that interdisciplinary studies are undertaken that explore the interactions of stellar luminosity, activity evolution and the stellar wind, with planetary atmospheres, interior/atmosphere exchange, and magnetic fields.
As described by Meadows and Barnes (2018), atmospheric escape can be driven by radiation from the star, solar wind interactions, and impact erosion (Ahren, 1993; Quintana et al., 2016). Comparison of Solar System bodies with and without atmospheres, along with exoplanets, suggests that there is an empirically derived “cosmic shoreline” as a function of planetary insolation and escape velocity that appears to be governed by thermal escape processes, and which divides planets with and without atmospheres (see Figure 3.5; Zahnle and Catling, 2017). Larger-mass planets will better resist atmospheric loss due to XUV and impacts, whereas planets with magnetic fields can deflect losses due to the solar wind (Meadows and Barnes, 2018). Because of their extended superluminous pre-main-sequence phase and high activity levels, M-dwarf planets may be particularly vulnerable to atmospheric and ocean loss via hydrodynamic escape processes (Lammer et al., 2008; Luger and Barnes, 2015; Meadows et al., 2018; Barnes et al., 2018; Meadows and Barnes, 2018). Observations and models that can confirm the presence of an atmosphere, as may be possible with transmission spectroscopy or direct imaging, or identify the factors most likely to contribute to atmospheric loss (such as stellar XUV flux and evolution) will help to rank exoplanet targets for study and biosignature searches.
The interaction between planet and star can also fundamentally modify the composition of a planetary atmo-
sphere through photochemistry, which in turn can impact climate by destroying or creating greenhouse gases. Photochemistry also contributes to atmospheric loss by removing H from high-altitude water vapor, allowing the H to escape to space. The stellar UV spectrum drives planetary photochemistry, and the resulting mix of gases depends on the initial atmospheric composition, the total amount of UV emitted by the star, and the wavelength dependence of the star’s UV output (Meadows and Barnes, 2018). Consequently, to be able to interpret the impact of the star’s radiation on planetary atmospheric composition, it is important to develop generalized photochemical/climate models to understand the diversity of terrestrial exoplanet environments. As input to these models, it is critical to characterize planetary host stars sufficiently to provide realistic high-energy stellar fluxes, both at the present time and over their history.
Life as a Planetary Process
Finally, it should be noted that life itself is a planetary process that can strongly impact the global environment; indeed, such changes lie at the heart of remotely detectable biosignatures. There are several examples in Earth’s history where life has significantly modified the environment, with perhaps the most commonly known being the rise of atmospheric oxygen in the Great Oxidation Event at 2.3 Gya, due in part to the production of abundant O2 by cyanobacteria (see review by Lyons et al., 2014). Consequently, it may be difficult to understand the evolution of a habitable terrestrial planet without understanding life’s impact on planetary processes, and more research is needed in this area. More generally, the early Earth provides information on alien habitable environments with different atmospheric compositions and climates (Lyons et al., 2014), populated by metabolisms with potential biosignatures (Pilcher, 2003; Arney et al., 2018) that are very different from those produced by oxygenic photosynthesis on Earth today). These alternative Earth environments also inform the search for habitable exoplanets and biosignatures, as they may be more indicative of conditions found on other, especially younger, habitable exoplanets (Arney et al., 2017).
Finding: The concept of the habitable zone has provided a first-order technique for identifying exoplanets that may be able to harbor life. A multiparameter holistic approach to studying exoplanet habitability, using both theory and observation, is ultimately required for target selection for biosignature searches.
Understanding of biosignatures has advanced significantly (Kaltenegger, 2017; Schwieterman et al., 2018), but it still requires a significant investment to enable the search for life on exoplanets. While O2 is considered the most well-studied biosignature to search for on terrestrial exoplanets (Meadows, 2017), and will likely be the first biosignature searched for, the field has expanded to encompass other biosignatures and supporting observations. There has been a growing understanding from studies of the early Earth (Reinhard et al., 2017), and exoplanet modeling efforts (e.g., Wordsworth and Pierrehumbert, 2014; Luger and Barnes, 2015; Domagal-Goldman et al., 2014; Gao et al., 2015; Harman et al., 2015), that biosignatures need to be interpreted in the context of their environment, as planetary processes may suppress, or even mimic, potential biosignatures (Meadows et al., 2018; Catling et al., 2018). In particular, there are two main questions that guide the future of exoplanet biosignature research:
- How are new potential biosignatures discovered?
- How is confidence in the interpretation of biosignature candidates increased?
Recent research, especially for O2, has illuminated the nature of false negatives—namely, the destruction, removal, or sequestration of biogenic gases by the planetary environment—and false positives—namely, nonbiotic processes (primarily photochemistry of CO2 or H2O) that can generate O2. These discoveries have driven the need to develop a comprehensive framework for biosignature identification and interpretation that takes into account confounding planetary processes (Meadows et al., 2018; Catling et al., 2018).
New potential biosignatures also need to be identified, and treated with the same rigor as O2 as far as iden-
tification of false positives and negatives. The observational discriminants (including stellar characteristics) for processes that produce biosignature gases abiotically need to be further identified (e.g., Schwieterman et al., 2016). This research will inform measurement requirements and guide the development of tiered observing strategies to identify potential biosignatures, and systematically rule out abiotic processes that could have generated them.
Identifying New Biosignatures
There are multiple paths forward in identifying new biosignatures, and all involve interdisciplinary laboratory, fieldwork, and modeling. Research on existing Earth metabolisms could identify new biogenic gases or reflectivity signals that may dominate in environments different from modern Earth’s (Schwieterman et al., 2015, 2018), and observations of modern Earth as an exoplanet can be used to search for global impacts due to life. Similarly, Earth’s past can be explored using biological and geological constraints to understand the likely environments of the ancient Earth, and when particular metabolisms may have become dominant (Stüeken et al., 2017), to understand their potentially observable impact on the ancient environment that hosted them. Biological modeling of photosystems could also be used to improve predictions of how photosynthesis would be expressed for different planetary and stellar environments. Models that couple interactions between the planetary interior, surface, ocean, atmosphere, and biosphere can be used to look at the survivability of potential biosignatures in different contexts.
Agnostic biosignatures look for patterns of complexity or aspects of the environment that cannot be explained by normal planetary processes such as volcanism or photochemistry. The advantage of agnostic biosignatures is that they do not presuppose a known Earth metabolism, but their disadvantage is that the environment also needs to be characterized to be able to identify them. For exoplanets, possible agnostic biosignatures may include atmospheric and surface disequilibria, and the latter requires detecting the presence of an ocean, which is best done in direct imaging (Krissansen-Totton et al., 2016; Cowan et al., 2009; Robinson et al., 2010), or the complexity of atmospheric chemical networks (Walker et al., 2018).
Developing a Comprehensive Framework for Biosignature Interpretation
Increased confidence in the interpretation of biosignatures that may be observed is needed. In part, this confidence will stem from an ability to identify and rule out false positives. Additionally, understanding the variety of environmental contexts that can either strengthen or weaken the interpretation of planetary phenomena as biosignatures will be crucial. The following are key areas for future research to help develop this framework:
- Observing hot Earths (e.g., GJ 1132b, TRAPPIST-1 b, c, and d) to identify bulk atmospheric composition and photochemistry that may produce false positives for O2.
- Determining likely false negative and false positive processes for biosignatures, and identifying their observational discriminants.
A comprehensive observational framework would use signs in the planetary environment of the possibility of false negatives to inform target selection for biosignature searches. The subsequent search for life would then include not only the observation of the putative biosignature, but a search for false positive discriminants, as well as signs of secondary confirmation of the likely metabolism. For example, in the case of O2, false positive discriminants include O2-O2 collisionally induced absorption (O4; Hermans et al., 1999) and absorption from CO2, CH4, and CO, as well as a thorough characterization of the host star’s UV spectrum. For a transiting exoplanet, time-resolved observation of refraction at transit ingress and egress with extremely large telescopes could also help identify vertical distribution of gases, to isolate photochemically generated O2 in a planet’s stratosphere (Misra et al., 2017). If the search for false positive indicators rules out several methods of abiotic production, then the interpretation of the potential biosignature detection becomes more credible. To make the detection even more convincing, a search could be made for secondary confirmation in the planetary environment of the likely metabolism—for example, in the case of oxygenic photosynthesis suspected due to atmospheric O2, absorption from photosynthetic surface pigments, or seasonal variability in CO2 or O2 (or O3; Olson et al., 2018), could
provide corroborating information. These frameworks need to be developed for other biosignatures, and work to determine their observational feasibility for proposed telescopes needs to be undertaken. Specific investigations would include identifying and quantifying the abiological production processes for potential biosignatures such as methane, nitrous oxide, methyl chloride, and others.
Bayesian methodology provides a language to define quantitatively the conditional probabilities and confidence levels of future life detection and may help constrain the prior probability of life with or without positive detection (Walker et al., 2018). The empirical and theoretical work described above will help place constraints on the relevant likelihoods, including those emerging from stellar and planetary context, the contingencies of evolutionary history, and the universalities of physics and chemistry (Walker et al., 2018).
More generally, as in all truly new discovery space, humility is prudent when evaluating theoretical expectations about terrestrial planet atmospheres. An empirical census of atmospheres on terrestrial worlds under a wide range of conditions, both in and outside habitable zones, will be needed to validate or adjust current ideas about atmospheric signatures produced through abiotic and biotic processes.
Finding: Inferring the presence of life on an exoplanet from remote sensing of a biosignature will require a comprehensive framework for assessing biosignatures. Such a framework would need to consider the context of the stellar and planetary environment, and include an understanding of false negatives, false positives, and their observational discriminants.
The methods and time scale for identifying potential habitable exoplanets and searching for atmospheric biosignatures are different depending upon the mass and size of the host star.
The M-Dwarf Opportunity
For M dwarfs, the low luminosities of the central stars mean that habitable zones correspond to short orbital periods, and the small sizes mean that the planets and their atmospheres can be detected by the transit method. Indeed, two nearby small stars are already known to host transiting planets that appear terrestrial (TRAPPIST-1, Gillon et al., 2017; LHS1140, Dittmann et al., 2017). Based on the high abundance of such stars and their proclivity to host rocky worlds (Dressing and Charbonneau, 2015), there should be several more systems nearby that will be discovered by Transiting Exoplanet Survey Satellite and ground-based surveys. As described in Chapter 4, in the section “Opportunities to Characterize Planets Through Transits,” the atmospheres of such worlds could be observationally accessible with JWST and the giant segmented mirror telescopes (GSMTs), including the Extremely Large Telescope, the Giant Magellan Telescope and the Thirty Meter Telescope. Transmission spectroscopy of these targets will not be able to probe the near-surface atmosphere and planetary surface, but could reveal atmospheric constituents in the upper troposphere and stratosphere. As described in Chapter 4, in the section “The Case for Imaging,” there are also plans to study the atmospheres of the closer, nontransiting examples (such as Proxima Centauri b; Anglada-Escude et al., 2016) by imaging with the GSMTs. As discussed, M-dwarf terrestrial planets likely undergo very different evolutionary processes than terrestrial planets like Earth orbiting larger stars like the Sun. Yet, the majority of habitable-zone terrestrial planets in the galaxy orbit M dwarfs. M-dwarf exoplanets therefore hold the key to understanding prevalence of life beyond the Solar System. If planets orbiting M dwarfs are indeed able to harbor life, then life may be very common in the galaxy. The fact that targets are already known and that more will be found shortly, and the lack of a need to develop a large, high-contrast imaging space mission to pursue atmospheric studies, means that observational studies of potentially habitable worlds, including potential biosignature gases, could be under way by the mid-2020s.
The G-Dwarf Case
As described earlier, there are numerous reasons to think that the terrestrial worlds orbiting M dwarfs are not habitable; more generally, the need to extrapolate an understanding of habitability grows as one moves away from Sun-like stars. To complement the M-dwarf science case, the study of planets orbiting Sun-like stars will allow the study of planets that may have undergone evolutionary processes more similar to Earth, where it is known that life was able to arise. There is a compelling need to develop the means to detect and spectroscopically characterize temperate terrestrial planets orbiting Sun-like stars; however, the big sizes and large luminosities of such stars and the long orbital periods at the habitable zone mean that such work cannot be done with transit methods. Instead, as described in Chapter 4, in the section “The Case for Imaging,” they require novel imaging space missions far more powerful than any in existence; if research begins now, this investment may bear fruit by the mid-2030s, roughly a decade after the M-dwarf opportunity. Importantly, the direct imaging missions will need to self-discover their own targets, unless these can be discovered in advance by precise radial velocity measurements. Direct imaging observations will potentially allow us to probe the entire atmospheric column and image the planetary surface to search for direct signs of habitability, such as the presence of an ocean.
Both transmission spectroscopy and direct imaging are sensitive to clouds and aerosols, although to differing degrees. High aerosols may stymie transmission observations that are taken through long slant paths in the upper atmosphere, but would provide less opacity to direct imaging observations. If the planet exhibits the partial cloud cover characteristic of convective condensate clouds on Earth, then direct imaging observations may also probe to the surface, even in the presence of clouds.
Requirements for Credible Interpretation of Biosignatures
In summary, the interpretation of biosignatures will likely need the following:
- An expanded interdisciplinary modeling, laboratory, and field effort to understand multifactorial habitability assessment and biosignatures assessment frameworks;
- Studies of a wide range of planetary atmospheres, from gas giants to uninhabitable terrestrials, to improve understanding of the physical and chemical processes that modify planetary environments and provide the context for biosignature interpretation;
- An improved understanding of planet formation deep enough to allow use of a planet’s system architecture, and the compositions of its sibling planets, as discriminants for biosignature assessment.
- Studies of terrestrial worlds that can proceed in the very near future for M dwarfs. For Sun-like stars, substantial investment will be required before observational studies can begin in earnest;
- Knowledge of the planet mass (yet researchers do not currently possess the ability to measure the masses of Earth-like planets orbiting Sun-like stars; see the section “Exoplanet Masses,” in Chapter 4);
- Observational studies of the environments of planets being searched for biosignatures, including characterization of the parent star (see the section “The Need for Detailed Stellar Characterization,” in Chapter 4), and other planets in the system; and
- A multilevel approach to biosignature observations that uses environmental context to rule out false positives, and a statistical approach to assessment that quantifies the likelihood that a given phenomenon is due to life.
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