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Question 12: Exoplanets

The past decade has seen extraordinary growth in our knowledge of planetary systems around other stars, as well as in the conditions of planet formation and the remarkable diversity and abundance of exoplanets.¹ These advances have been supported by continuing exoplanet discoveries, ground-breaking observations afforded in part by new facilities, and a progression from planet detection to detailed study and characterization of individual planets. Observations of protoplanetary disks around young stars have now begun to reveal the entire planet formation process, from the earliest accumulation of grains into large agglomerates to accretion onto growing protoplanets.

The observational approaches for exoplanets are quite disparate compared to those for solar system science, as illustrated in Figure 15-1, which highlights some key examples of this remarkable progress. High-resolution images of protoplanetary disks show evidence for thin disks and multiple rings and gaps, some of them owing to the presence of forming planets. Two accreting planets were discovered around young star PDS 70, with evidence for a circumplanetary disk around PDS 70 c. The orbital motions of directly imaged planetary systems can be followed accurately, as in the case of Beta Pictoris and HR 8799. We have discovered that there are more planets than stars in our galaxy, as exemplified by the discovery of Proxima Centauri b, a ~3 Earth mass planet orbiting within the “habitable zone” (i.e., a distance from the star where water at the planetary surface, if present, may be liquid) of the nearest star to the Sun. Detailed exoplanetary spectra have been obtained, both for young and massive directly imaged planets and for close-in planets. The presence of clouds and differential rotation is becoming discernable, as shown for the close brown dwarf Luhman 16B. Similarly, high-spectral resolution observations with the Doppler imaging technique can be used to measure wind speeds in hot Jupiters or the spin of planet Beta Pictoris b.

The planetary systems that have been discovered—as exemplified by the seven-planet system around TRAPPIST-1, only about 12 pc away—are extremely diverse, and study of the demographics of large numbers of exoplanets has led to several advances in understanding, including the recognition that many small-mass planets possess hydrogen atmospheres. Progress in observational technology will enable discovery of an even greater number of systems, as shown in Figure 15-2 for the forthcoming Nancy Grace Roman Space Telescope, and much expanded characterization of individual exoplanets by, for example, the James Webb Space Telescope. Overall, the ability to image exoplanets both

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¹ A glossary of acronyms and technical terms can be found in Appendix F.

when they are forming and in their mature stage, the ability to characterize these exoplanets and their atmospheres, and the immense wealth of data available on more than 4,000 exoplanets (and counting), is providing us with new opportunities to understand planetary systems in the universe and to compare and contrast them with the solar system’s planets.

The Exoplanet Science Strategy (ESS; NASEM 2018) identified two overarching goals:

• 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.
• Learn enough about the properties of exoplanets to identify potentially habitable environments, their frequency, and connect these environments to the planetary systems in which they reside. Furthermore, researchers need to distinguish between the signatures of life and those of nonbiological processes, and search for signatures of life on worlds orbiting other stars.

Questions 1 through 11 in this report have identified priority science questions for exploring the solar system over the next decade. Exoplanetary data, which increasingly reflect a multiplicity of objects, are an invaluable complement to the detailed observations and theories developed for the solar system’s planets. Conversely, understanding derived from solar system studies can be compared to and contrasted with wider-reaching exoplanet observations. The synergy between the ESS goals and the goals for planetary science and astrobiology outlined in this decadal survey provides strong motivation for a new era of collaborative research. This can occur not only through existing cross-disciplinary research and analysis programs, but also through collaborative efforts in mission design and implementation, telescope observations, data analysis, and laboratory and experimental research.

In this chapter, the committee structures its discussion around the 11 priority science questions for (Chapters 4–14) solar system science, providing for each question examples of how it relates to and can benefit from exoplanetary studies, including the identification of strategic research activities.

Q12.1 EVOLUTION OF THE PROTOPLANETARY DISK

The discovery and characterization of disks around other stars have revolutionized our understanding of disk evolution and planet accretion, providing a unique window into the processes that occurred within the Sun’s

protoplanetary nebula. Coordinating research between studies of these disks and models of solar system formation can greatly advance our understanding of planetary system formation.

Q12.1a How Does a Disk’s Bulk Composition Affect the Diversity of Resulting Planetary Materials?

Meteoritic studies tell us that the bulk composition of the solar nebula was similar to that of the Sun’s photosphere, providing direct evidence that stellar composition can be used to constrain overall circumstellar disk composition (e.g., Johnson et al. 2012). However, we know from the diversity of meteorites that the composition of early accreted solids and planetesimals in the solar system also depended on localized disk conditions such as temperature and oxygen fugacity. Observations of protoplanetary disks can reveal disk properties across a wide range of stellar types and ages, including variations in dust and gas compositions at different evolutionary stages, providing a powerful complement to meteoritic studies and solar nebula compositional models. Key open issues include the role of stellar metallicity and disk structure in determining planetesimal composition.

The conditions in the solar nebula can only be inferred from what remains today of the gas and dust. We do not know how well-mixed the other heavy elements were in the disk from which the solar system formed (see Q3.2). Observations of bands in cometary material suggest the existence of crystalline silicate material in the cold outer regions where comets formed. The distinct oxygen isotope signatures of terrestrial, asteroidal, and martian materials indicate that the solar system was not well-mixed. Meteorites represent (at least) two distinct reservoirs of material, and perhaps the early formation of Jupiter may have played a role in isolating these reservoirs. Mapping radial compositional gradients in the dusty component of young disks can potentially constrain the origin and mixing of compositional reservoirs.

The surprising discovery of two interstellar objects (ISOs) passing through the inner solar system, 1I/‘Oumuammua and 2I/Borisov, raises the possibility of direct analysis of materials formed in the disks of other stars. 1I/‘Oumuammua appeared asteroidal, in that it did not show comet-like activity (Meech et al. 2017), while 2I/Borisov exhibited evidence of a cometary color and tail (Guzik et al. 2020). Constraints on the mineralogy and elemental abundances of ISOs could provide insight into the composition of their parent disks. Improvements to Pan-STARRS1 and observations by the Vera C. Rubin Observatory will support detection of additional ISOs in coming years, and development of rapid launch capabilities could enable future spacecraft encounters with one of these extrasolar visitors.

Q12.1b How Do Disks Evolve with Time, and How and When Do Macroscopic Particles and Planetesimals Begin to Form?

Dating of meteoritic materials implies that macroscopic solids and planetesimals formed within the first few million years of solar system formation, with the latter established by the age of the oldest materials, the CAIs (calcium-aluminum-rich inclusions). Multi-wavelength observations of molecular clouds and young stellar objects and their disks allow us to develop a comprehensive picture of the timescales and conditions of disk formation and early evolution. Such observations can help address outstanding issues including the survival of molecular cloud components during disk formation, heating mechanisms responsible for CAIs and chondrules, and planetesimal formation mechanisms. Observations can also track variations in disk structure with time, as well as variations in nebular gas and ice, where the latter are not directly recorded in rocky meteoritic materials but are essential for understanding early planetary accretion mechanisms. Such observations can address fundamental open issues including the range of nebular properties and lifetimes and the mechanisms responsible for gas dissipation.

Meteorites show evidence for irradiation of early dust, but questions remain concerning the source, extent, and influence of irradiation on gas, dust, ice, and organics. Observations of radiation sources (e.g., cosmic and stellar winds) for young stellar objects and tracking the effect of radiation on dust and gas can provide insight into the role irradiation plays in the composition of nebular components. This will require stronger constraints on the ages of young stellar objects, and better constraints on the link between CAIs and the astronomical timescale of stellar evolution.

Disk observations have begun to probe the earliest stages of planet formation. Estimates of the total mass in sub-cm-size particles in 1- to 3-million-year-old disks (Ansdell et al. 2016) appear too low to explain observed

exoplanet system masses, suggesting that solids may have already accreted into larger (≥ cm) sizes by this time (Manara et al. 2018). Some observations point to earlier growth of mm-size pebbles in the envelopes and disks of ~10⁵-year-old protostars (Miotello et al. 2014), while isotopically derived ages of the oldest chondrules now suggest that accretion of mm-sized bodies in the solar system began within ~10⁵ years of CAI formation (Bollard et al. 2017). The interplay of meteoritic data with observations of young disks will provide crucial constraints on the initial and perhaps least understood stages of planet assembly.

Strategic Research for Q12.1

• Determine how the nascent planets acquired material from the protosolar disk by measuring abundances and isotopic compositions of noble gases and other key elements (e.g., H, C, O, N, and S) from major planets (e.g., Venus, Mars, and the giant planets), satellites (e.g., Titan), and small bodies via spacecraft data, in situ probes, sample return, and telescopic observations.
• Determine the composition of primordial material that preserves chemical signatures of the protoplanetary disk via sample return, spacecraft data, and telescopic observations.
• Characterize protoplanetary disks around young stars—including their lifetimes, structures, gas versus solid compositions, and properties of any accreted components—with observations and theoretical and modeling studies of disk processes, to include the survival of components of molecular clouds, irradiation and delivery of disk material, formation and evolution of disk components (e.g., dust, ice, and gas), early stages of planetary accretion, and gas disk dissipation.
• Assess the range of conditions and mechanisms that may lead to the commencement of accretion in disks around young stars by model development constrained by diverse disk observations.
• Identify and characterize interstellar objects (e.g., size, dynamical origin, and composition) with spacecraft data, telescopic observations, theoretical and modeling studies of their formation and evolution, and laboratory studies of analogue materials.

Q12.2 ACCRETION IN THE OUTER SOLAR SYSTEM

Outer solar system temperatures allowed for the formation of ice-rock planetesimals, the feedstock for cometary bodies and ice-rich dwarf planets, the solid components of the giant planets, and the outer satellite and ring systems. Delivery of this material to the inner solar system was also a source of terrestrial planet volatiles. Gas accretion led to gas-dominated Jupiter and Saturn, but only modest gas components at Uranus and Neptune. Giant planet gravitational interactions with the background gaseous disk and/or other solid bodies led to dynamical effects felt across the whole solar system, including large-scale giant planet orbital migration. These processes are relevant to exoplanet systems, where diverse system properties demonstrate a complexity of possible outcomes that remain incompletely understood.

Q12.2a How Do Giant Planets Form, and How Does Their Origin Compare to the Formation of Super-Earths and Sub-Neptunes?

Traditional concepts for giant planet growth invoke collisional accretion of a solid core beyond the water ice-line, followed by gas accretion beyond a critical core mass. Meteoritic evidence suggests that Jupiter began forming (up to ~20 Earth-masses) within ~1 million years after CAIs (Kruijer et al. 2017). That Uranus and Neptune accreted only a minority of their mass in gas is attributed to their later formation as the solar nebula was dissipating, and/or to depletion of the local supply of gas owing to its accretion by Jupiter and Saturn. Diverse exoplanet systems display evidence of more complex planetary outcomes. For example, a recent discovery is a relative dearth of planets with radii near 1.8 Earth radii (R_⊕) (see Figure 15-1k), suggesting a difference in either formation or evolution of so-called super-Earths (<1.8 R_⊕) compared to sub-Neptunes (~1.8 to 3.5 R_⊕). Neither class of exoplanet has a clear solar system analog. This distribution of exoplanet radii may reflect a critical core mass needed for gas accretion, and/or the effects of escape of primary atmospheres (see also Q12.6a).

Q12.2b Is the Solar System Architecture, with Multiple Outer Giant Planets That Formed Beyond the Water Ice Line, a Common or Uncommon Outcome of Planet Formation in the Galaxy? How Common Was Giant Planet Migration?

The majority of known giant exoplanets orbit much closer to their stars than the giant planets in the solar system, many within the orbit of Mercury (see Figure 15-2). Additionally, many exoplanetary systems host several terrestrial planets and no known giants (e.g., the TRAPPIST-1 system; Gillon et al. 2017). A key question is whether the apparent prevalence of close-in giant planets and the existence of systems without giant planets is primarily owing to bias associated with current detection methods, or whether it reflects formation and evolutionary processes distinct from those in the solar system

In the solar system, the dynamical properties of the trans-neptunian objects provide strong evidence for giant planet orbital migration. Multiple types of gravitational interactions are now known to be capable of driving large-scale changes in giant planet orbits, including interactions with the gas disk, interactions with a planetesimal disk, and scattering. This migration in turn may affect the dynamical properties of small body populations. Exoplanetary systems offer a unique dataset to study such processes and improve understanding of the conditions that establish the overall properties of a planetary system, including when giant planets survive versus are lost to collision with their host star or ejection.

Strategic Research for Q12.2

• Determine how the nascent planets acquired material from the protosolar disk by measuring abundances and isotopic compositions of noble gases and other key elements (e.g., H, C, O, N, and S) from major planets (e.g., Venus, Mars, and the giant planets), satellites (e.g., Titan), and small bodies via spacecraft data, in situ probes, sample return, and telescopic observations.
• Improve knowledge of the inventory, composition, and dynamical states of small bodies in the outer solar system with spacecraft flybys and telescopic observations.
• Determine noble gas abundances and isotope ratios, and stable isotope ratios (e.g., ¹²C/¹³C and ¹⁴N/¹⁵N) in multiple comets with spacecraft flybys, sample return, and telescopic observations.
• Reveal the nature of outer planet accretion in exoplanetary systems through an observational census of planetary dynamical parameters (e.g., eccentricity and inclination distributions and period ratios), dust produced by planetesimal collisions, young planets recently formed, the demographics of planets in the regions of exoplanetary systems analogous to the solar system, the composition of giant exoplanet atmospheres, and mass-radius relationships for sub-Neptunes.
• Assess how giant planets may form and migrate using theoretical and modeling studies constrained by diverse exoplanetary system properties.

Q12.3 ORIGIN OF EARTH AND INNER SOLAR SYSTEM BODIES

Understanding the origin of our inner planets benefits from observations of rocky exoplanets and studies of the mechanisms controlling the final architecture of planetary systems. Remarkably, the proportion of exoplanetary systems observed thus far that have both rocky and gas giant planets appears to be low. Several lines of exploration within the solar system and of exoplanet systems will help to answer fundamental questions about rocky planet formation.

Q12.3a How Do Disk Conditions and Stellar Metallicity Affect Rocky Planet Origin, Composition, and Volatile Content?

Large planets (≥4 R_⊕), sub-Neptunes (radii between roughly 1.8 and 4 R_⊕), and rocky planets having radii ≤1.8 R_⊕may occur more frequently around stars that are rich in elements heavier than helium, although the statistics of exoplanet populations remains an area of active research. In the solar system, the spectral composition of the Sun

matches the overall composition of elements heavier than helium in primitive meteorites. Whether stellar composition in general is a proxy for planet composition can be begun to be assessed by measuring volatile-poor and volatile-rich exoplanet bulk densities.

Earth’s composition and volatile inventories have been essential to its habitability and the development of life (Question 9, Chapter 12), and volatile content varies across the terrestrial planets. Do such differences in abundances of materials with relatively low condensation temperatures, such as those rich in S, O, and K, reflect nebular conditions as these bodies formed, or do they instead reflect stochastic events, for example, mixing induced by giant planet migration? Open issues include whether extrasolar systems without gas giants have volatile-rich inner planets, and the circumstances in which inner planets may be destroyed by giant planet migration and/or dynamical instabilities. For example, how similar are the compositions of the five planets in the TRAPPIST-1 system (see Figure 15-1, panel l), which has no known giant planets, and how do they compare with planets in the Cancri 55 system, which has both rocky and giant planets? Atmospheric emission, and/or thermal emission and reflectance spectra may constrain models of the interior compositions of rocky exoplanets—and enable comparisons between the compositions of these exoplanets and the photospheric abundances of their host stars.

Q12.3b Are Giant Impacts and Magma Oceans Common During Rocky Exoplanet Formation, and If So, What Are Their Observational Signatures?

Models of terrestrial planet accretion predict a final stage of giant impacts during the assembly of Earth-sized planets, including the impact thought to have produced the Earth-Moon system. These singular events establish initial planetary properties including rotation rate, obliquity, and the presence of an impact-generated moon(s). Giant impacts also deliver prodigious heat, and the terrestrial planets were likely melted one or more times during their formation, yielding periodic magma oceans. The complex chemical and dynamical aspects of magma ocean cooling are central elements of planetary formation and differentiation processes (Q3.5).

Observations can help further reveal the role of these processes in exoplanetary systems. Giant impacts can produce highly heated planet-disk systems with vaporized silicate that persist for ~10² years, followed by a protracted, ~10⁶-years phase in which the planet retains a hot magma ocean beneath a volatile-rich, blanketing atmosphere (Zahnle et al. 2015). Giant atmosphere-stripping impacts might produce temporary incandescence and/or atmospheric signatures that could potentially be observed (e.g., observations of Earth shortly after the Moon-forming event would reveal a hot Earth in the habitable zone). Thermal emission from such structures may be observable in surveys of young disks. Detecting exomoons around older, rocky exoplanets may become possible (see Q12.8b).

Young terrestrial planets with magma oceans and early flotation crusts may have detectable surface and/or atmospheric properties. Giant impacts during planet formation may also be inferred from anomalous rocky planet bulk densities (perhaps analogous to metal-rich Mercury or Psyche), or through detection of impact-produced debris (see Q12.4).

Q12.3c What Processes Produce Compact Exoplanet Systems, and How Do They Differ from Conditions in the Inner Solar System That Yielded No Surviving Planets Interior to Mercury?

The existence of compact systems of Earth to sub-Neptune sized planets has been one of the most intriguing discoveries of the Kepler mission. These systems (e.g., Kepler-11 or TRAPPIST-1) typically have multiple planets on low-eccentricity, low-inclination orbits, all located at distances between about 0.01 to 0.3 AU, and with orbital periods of only a few to 100 days. Compact systems are common, occurring around tens of percent of M-dwarf stars, and they differ from the solar system in two key respects. First, they include many planets with estimated masses greater than 1 and less than 10 Earth masses, which have no analog in the solar system. Second, they are remarkably compact, with multiple planets interior to Mercury’s orbit, perhaps analogous to the compact orbits of the Galilean satellites around Jupiter. Such characteristics may suggest different accretional conditions than in the solar system. Indeed, forming compact systems may require very massive compact disks and/or inward planet migration. Unraveling the conditions that led to compact systems will help us better understand the origin of the inner solar system, and perhaps why there are no planets interior to Mercury today.

Strategic Research for Q12.3

• Determine noble gas abundances and isotope ratios, and the stable isotope ratios in the atmospheres of Venus and Mars with spacecraft observations and in situ probes.
• Characterize exoplanets smaller than sub-Neptunes, including their mass–radius relations, surface or atmospheric emission, and orbital period ratios with telescopic observations and modeling studies.
• Determine how inner exoplanetary systems with and without giant planets form and evolve using compositional analyses of a wide range of planetary material and theoretical and modeling studies, including detailed predictions for young exoplanets where observational signatures of giant impacts and/or magma oceans could be detected.
• Assess how compact exoplanetary systems originated and how their origin conditions differed from accretion in the inner solar system by combining observational constraints (e.g., mass–radius relations and orbital period ratios) with theoretical models.

Q12.4 IMPACTS AND DYNAMICS

The dynamical properties of exoplanets have important implications for their overall stability and potential for habitability. Surveys of thousands of exoplanets show a wide diversity from hot-Jupiters to super-Earths. Understanding the history of these systems compared to the solar system has important implications for planetary formation, evolution, and habitability.

Q12.4a How Do Exoplanet Properties Constrain Their Collisional and Dynamical Histories, and What Can These Properties Tell Us About the Dynamical Histories of Multi-Planet Systems?

There are >700 known multi-planet systems, most more tightly packed than the solar system with multiple planets orbiting close to their parent stars, as perhaps expected given observational biases. They likely experienced different dynamical and collisional evolutionary histories with important implications for the nature of the planet’s surface, atmosphere dynamics, and the bulk composition of a planet (e.g., Bonomo et al. 2019; Q12.3). A more comprehensive picture of such processes and their influence on planets requires a census of the physical properties of exoplanets, such as size, mass, composition, orbital eccentricity, and, if possible, spin state (e.g., Gratia and Lissauer 2021). Dynamical history is also constrained by orbital properties, for example, the prevalence of mean-motion resonances among planets. Understanding such features in multi-planet systems is crucial for understanding orbital architecture, giant planet migration, planet–planet interactions, and long-term system evolution (e.g., Tamayo et al. 2020).

Q12.4b How Does Impact History Influence Planetary Bulk and Atmosphere Composition?

The surface conditions of a planet are significantly affected by its susceptibility to impacts, which can potentially strip a planet of its atmosphere or add volatile components (e.g., Hirschmann et al. 2009; Schlichting and Mukhopadhyay 2018). The heterogeneity of material in the asteroid belt points to radial mixing (e.g., DeMeo and Carry 2014), perhaps associated with giant planet orbital migration that triggered delivery of volatile-rich material from the outer to the inner solar system (e.g., Gomes et al. 2005; Walsh et al. 2011). Volatiles are a crucial factor for habitability and life.

Giant planet migration is likely a common process given the prevalence of hot Jupiters, but the extent of radial mixing and volatile delivery to the habitable zone may differ significantly from system to system. Collisions associated with giant planet migration may have produced substantial dust, which could potentially be observed using multi-wavelength analyses (e.g., Youdin and Rieke 2017). Analyses of dust around exoplanetary systems at different evolutionary stages may shed light on the influence of impacts induced by giant planet migration.

Strategic Research for Q12.4

• Determine the physical and orbital properties of exoplanets in multi-planet systems to understand their dynamical histories with telescopic observations.
• Characterize impact conditions that would produce detectable dust signatures in exoplanetary systems with numerical simulations and models.
• Determine how impacts contribute volatiles to (or, in some cases, remove volatiles from) planetary bodies via compositional analyses of a wide range of planetary materials, especially volatile-rich, primordial small bodies in the outer solar system, and by modeling.

Q12.5 SOLID BODY INTERIORS AND SURFACES

Exoplanets offer a huge sample size to provide statistical information about the factors that govern planetary evolution. An overarching goal for the next decade is to characterize first-order properties of solid exoplanet interiors and surfaces. Interdisciplinary connections between experimental, theoretical, and modeling studies will be key to developing “exogeoscience.”

Q12.5a What Are Observable Signatures of Bulk Composition and Surface Processes on Solid Exoplanets?

Characterizing exoplanetary surfaces and interiors is challenging, especially if they are shrouded in (perhaps cloudy) atmospheres. Solid bodies that are, for example, extremely close to their parent star might have bare surfaces that are amenable to direct observation. “Polluted” white dwarfs can reveal the bulk compositions of solid bodies that have been accreted by their parent star (e.g., Figure 15-3). Important open issues are how to assess exoplanet bulk and surface compositions (beyond the use of just mean density), the potential for characterization of volcanic activity and/or crustal recycling and assessing observable signs of atmosphere–surface interactions that control the cycling of volatiles through the planetary system.

Q12.5b What Are the Operative Modes of Solid-State Dynamics in the Rocky and/or Icy Interiors of Exoplanets and Their Observational Signatures?

Rocky bodies are engines that convert internal heat into interesting surface phenomena (e.g., Foley et al. 2020). Their solid components can be stagnant and transport heat mostly by thermal conduction with little, if any, melting and recycling of surface material. Alternatively, solid-state convection enables more rapid cooling and faster transport of volatiles into and out of the interior (see Q5.2). Important issues for exoplanets include the factors that control whether rocky bodies exhibit Earth-like plate tectonics or other modes of mantle convection (e.g., the stagnant-lid regime for Mars today versus the poorly understood regime of mantle dynamics seen on Venus)—and the dynamical regimes of solid bodies with large proportions of ices and/or liquid water. Modeling efforts are needed to predict the observable consequences of different dynamical regimes.

Q12.5c What Factors Control the Prevalence of Dynamos in Exoplanets Smaller Than Sub-Neptunes?

Magnetic fields are unique windows into planetary interiors. There are no unambiguous patterns in the solar system governing which solid bodies host long-lived dynamos and the magnetic histories of some worlds, especially Venus, are poorly known (Laneuville et al. 2020). Are there correlations between the occurrence rates of expected exoplanetary dynamos with, for example, planet–star distance, stellar composition, system age, planetary mass, planetary rotation rate, or planetary atmospheric properties? Metallic cores are considered the usual location of dynamos, but liquid silicates have sufficient electrical conductivity under high pressure/temperature conditions, suggesting that dynamos could also be generated in magma oceans of, for example, super-Earths. Currently, brown dwarfs and gas giants serve as testing grounds for observations of, for example, radio emission and star-planet interactions related to magnetic fields. Eventually, these techniques can be refined and extended to search for magnetic fields around smaller exoplanets.

Q12.5d How Do Thermophysical Properties of Planetary Materials Affect the Interior and Surface Evolution of Solid Bodies?

Solid bodies in the solar system vary considerably in bulk compositions and internal pressures and temperatures. Exoplanets span an even broader range of conditions (Unterborn et al. 2020). Properties of planetary interiors are typically quite different from those observed at ambient conditions on Earth’s surface. Some key quantities (e.g., the viscosity of solid silicates at extreme pressures) are uncertain by many orders of magnitude. To build realistic models of solid-body exoplanet evolution, accurate mineral physics data relevant to exoplanet conditions are needed. Ideally, data-collection efforts would be integrated in close collaboration with modeling and observational studies to build a framework for interpreting observations and generating new, testable predictions.

Strategic Research for Q12.5

• Characterize the surfaces and interiors of solid body exoplanets by constraining key properties (e.g., bulk composition, rotation rate, and atmospheric signatures of surface and interior conditions) with telescopic observations.
• Search for magnetospheric activity at exoplanets with remote sensing of phenomena associated with magnetic fields, potentially including radio emission, far-ultraviolet auroral emission, infrared H₃⁺ auroral emission, variations in transit light curves, and star–planet interactions.

• Determine how solid bodies over a broad range of conditions relevant to exoplanets evolve through geologic time with modeling and theoretical research on processes including, for example, solidification of magma oceans; solid-state mantle convection; thermally and chemically driven dynamics in metallic cores; and dynamos generated in liquid metals and silicates.
• Determine thermophysical properties of ices, silicates, and metal alloys under the ranges of pressure and temperature conditions and compositions relevant to exoplanets with first-principles simulations and laboratory experiments.

Q12.6 ATMOSPHERE AND CLIMATE EVOLUTION ON SOLID BODIES

The present-day atmospheres of solid bodies in the solar system are snapshots in time of continuously evolving systems. While continued research in the solar system is essential to understand this evolution (see Q.6, Chapter 9), exoplanets will extend our understanding of the diversity of planetary atmospheres across space as well as time. Today, observational constraints on the atmospheres of small exoplanets are limited, but over the next few years major progress is expected. A coordinated program of solar system and exoplanet atmospheric evolution research would maximize advance in understanding.

Q12.6a What Determines the Division Between Gas-Rich and Solid-Body Planets?

In the solar system, there is a clear distinction between the four giant planets with puffy hydrogen-dominated atmospheres and all other solid planetary bodies. However, observations of exoplanet radii for planets intermediate in mass between Neptune and Earth indicate that this boundary is not as clear in many cases (Fulton et al. 2017 and see Figure 12-1k). The gaseous component of planets is thought to be established both by early nebular capture and longer-term loss processes, and continued research is required to understand this interplay. Observations of systems of different ages are particularly important to discriminate between different evolutionary possibilities. These issues are also relevant to understanding the early evolution of rocky planets in the solar system (see Q6.1, Chapter 9).

Q12.6b Which Rocky Exoplanets Have Retained Atmospheres? Can We Use This Information to Constrain Atmospheric Loss History on Solar System Objects?

Once an exoplanet has been identified as solid, based on its mass and radius, perhaps the next most basic question is whether it possesses an atmosphere. Initial constraints on rocky exoplanet atmospheres have been achieved using existing observatories (e.g., Diamond-Lowe et al. 2018; Kreidberg et al. 2019), but tests for the presence of atmospheres on a much wider range of objects in the next decade are expected as new space- and ground-based observatories come online. Such observations will test and constrain theories of solid-body atmospheric loss, in turn helping us better understand the evolution of solar system objects with thin atmospheres today, such as Mars.

Q12.6c For Exoplanets That Have Retained Atmospheres, What Are Trends in Atmospheric Composition with Planet Mass, Orbital Distance, and Host Star Type?

In the next decade, spectroscopic characterization of solid body exoplanets that have atmospheres will allow us to begin to identify trends with key parameters such as orbital distance and planet mass. In the solar system, the divergent evolution of Venus, Earth, and Mars is clearly a result of their differing bulk properties, but with such a small sample size, understanding which properties are most crucial is extremely difficult. While the data on any individual exoplanet will be extremely limited initially, the ability to observe a large number of targets would provide completely new constraints on atmospheric evolution.

Q12.6d How Does the Evolution of the Host Star Affect Planetary Atmospheres, Including Photochemistry and Escape Processes?

The Sun’s changing output has had a dramatic impact on atmospheric evolution in the solar system, and the effect of host stars on exoplanets is likely to be similarly important. Indeed, for exoplanets orbiting low-mass M stars (the most observable type in the next decade), atmospheric loss is likely to be much more severe than in the solar system, owing to the high extreme ultraviolet output, frequent coronal mass ejection, and long pre-main sequence stage of these stars (Baraffe et al. 1998; Tarter et al. 2007). High-energy stellar emissions are also important drivers of atmospheric chemistry, both in the steady state and episodically owing to transient events. Characterizing host star properties and exoplanet atmospheric composition simultaneously would provide important new data on the coupling between stellar radiation and atmospheric chemistry in a range of different contexts, strengthening our understanding of processes in the solar system.

Q12.6e What Processes Impact the Evolution of Atmospheric Chemistry, Cloud, and Haze Formation in Diverse Planetary Atmospheres?

The number of planetary atmospheres in the solar system is limited, but even so great diversity in composition and chemistry is seen. By observing exoplanets, we can begin to build and test generalized models of atmospheric chemical pathways important in maintaining any atmospheric composition. Besides the stellar effects mentioned above, key questions include how surface processes and trace gas species affect disequilibrium chemistry and how cloud and photochemical haze formation proceed in diverse situations and impact the atmospheric energy budget.

Q12.6f What Is the Diversity of Atmospheric Circulation Patterns Among Solid Planets, and How Does Circulation Vary with Atmospheric and Planetary Properties?

Atmospheric circulation is known to vary strongly with atmospheric mass and composition, planetary rotation rate, received solar flux, and other factors. While constraining atmospheric circulation on solid-body exoplanets observationally is extremely challenging, future progress may be possible via techniques such as thermal phase curve analysis and high-resolution Doppler spectroscopy (Snellen et al. 2010; Showman et al. 2015; Kreidberg et al. 2019; see Figures 15-1h and 15-1i). Many characterizable solid-body exoplanets are in close orbits around low-mass stars, which means they may have low rotation rates and permanent day and night sides owing to tidal locking. The circulation on such planets is predicted to be very different from that of Earth, Venus, Mars, and Titan, and so studying them in parallel with solar system objects is likely to lead to rich insights into atmospheric circulation generally.

Strategic Research for Q12.6

• Characterize the atmospheres of solid-body exoplanets by conducting transit spectroscopy, high-dispersion spectroscopy, and thermal phase curve observations, and compare them with atmospheres of solid bodies in the solar system.
• Determine past atmospheric mass and composition in the solar system by measuring and/or collecting noble gas abundance and isotopic fractionation from solid- body atmospheres within the solar system (i.e., for Venus, Mars, and Titan).
• Determine the properties of the atmospheres of terrestrial planets (i.e., Earth, Venus, and Mars) that would be observable on exoplanets to build a foundation for atmospheric characterization of analogue exoplanets through coordinating in situ/remote sensing measurements and theoretical studies of wind velocities, radiative balance, cloud dynamics, and atmospheric compositing as function of orbital phase, local time, and solar conditions.

• Determine the connection between exoplanet observables and atmospheric properties and dynamics by conducting theoretical and modeling studies to include the following: simulations (1D and 3D) with hazes and clouds; radiative-microphysical feedbacks; volatile transfer between atmospheres and surfaces; and interactions with the solar wind including the influence of magnetic fields on atmospheric escape processes.
• Determine key radiative properties, gas absorption, and other quantities of interest to understand feedbacks on planetary atmospheres for the solar system and exoplanets through targeted laboratory studies, including of atmospheres with different primary constituents (e.g., N₂, CO₂, and CH₄) and temperatures.

Q12.7 GIANT PLANET STRUCTURE AND EVOLUTION

The interior structure of a giant planet is the result of both formation and evolution processes. For many giant exoplanets, both mass and radius have been measured, providing estimates of the density of these planets. In some cases, molecules have been observed in the atmosphere, providing hints to the planet’s bulk composition. This information enables new modeling of interior structure and evolution, leveraging what is understood about the giant planets in the solar system. Although extrasolar Neptune-mass planets could have very different bulk compositions and interior profiles from Neptune and Uranus, our current limited knowledge of them strongly limits our ability to even begin to understand one of the most commonly detected classes of exoplanets: sub-Neptunes (see Figure 15-1k). Additionally, current understanding is limited by the lack of solar system analogs for some types of exoplanets (e.g., hot Jupiters and super-Earths). Dedicated missions to answer fundamental questions about Uranus and Neptune will provide a needed basis to significantly advance our understanding of this class of exoplanets.

Q12.7a What Does the Discovery of Dilute Cores Inside Saturn and Jupiter Mean for the Interior Structures of Giant Exoplanets? Can We Expect the Cores of Giant Planets Closer in Size to Uranus and Neptune to Be Compact or Dilute?

The internal structures of Jupiter and Saturn show the cores are diffuse—that is, they are partially diluted into the outer layers (Wahl et al. 2017; Mankovich and Fuller 2021). However, modeling of giant exoplanets to date has assumed compact cores. Furthermore, we do not know whether Uranus and Neptune also are partially or fully differentiated (see Q7.1, Q7.2, and Q7.4, Chapter 10). Future studies should address what the interior structures of the solar system’s giant planets can tell us about the internal structure, formation, and evolution of giant exoplanets.

Q12.7b How Did Thermal Evolution of the Giant Planets in the Solar System Progress Over Time and What Can This Tell Us About the Current State and Future of Directly Imaged Young Exoplanets?

Recently formed giant exoplanets radiate their heat of formation, making them the easiest planets to directly image thanks to the planet’s own emitted infrared radiation. Nevertheless, one needs to separate the radiation from the planet from that of the much brighter star, as well as suppress the diffracted light from the star. The thermal evolution of the giant planets in the solar system is important for interpreting such observations and their connections to future direct imaging campaigns (Berardo et al. 2017). Key open issues include initial planet entropy (e.g., hot versus cold start models), how quickly a young giant planet cools, differences in the thermal evolution of gas versus ice giants, and whether the difference in intrinsic heat between Uranus and Neptune is common among exoplanets of this size, and if so, what this implies for understanding the intrinsic heat flux of hydrogen-dominated planets (see Q7.5).

Q12.7c What Can Uranus and Neptune Teach Us About Magnetic Field Generation and Configuration in Neptune-Size Exoplanets?

The magnetic fields of Uranus and Neptune are radically different from those of Jupiter and Saturn and from each other, suggesting that planets of this size may display a wide diversity of interior structures and magnetic fields (e.g., Soderlund and Stanley 2020; see also Q7.2 and Q7.4). Where the magnetic field is generated within

Uranus and Neptune and what this can tell us about sub-Neptune magnetic field generation remains uncertain, owing to very incomplete knowledge of the Uranus and Neptune field configurations and properties.

Q12.7d What Factors in the Formation and Evolution of Planets Define the Crossover Regime Where a Planet Becomes Either a Super-Earth or a Sub-Neptune?

As noted in Q12.2, the most commonly detected exoplanets are super-Earths and sub-Neptunes (see Figure 15-1k and Fulton et al. 2017). Is the observed “gap” in intermediate planet radii for close-in exoplanets the result of formation processes, evolution, or a combination of both? One possibility to explain this gap is that super-Earths have lost their primary atmospheres (or perhaps never accreted one) while sub-Neptunes retained theirs. Better understanding of the formation and evolution of Uranus and Neptune are critical for answering this question (see Q7.1, Q7.2, and Q7.5). An overall issue is over what range of planetary masses, stellar insolation, and formation timescales are planets able to retain hydrogen-rich atmospheres, accounting for competing processes such as outgassing, accretion, and atmospheric escape.

Q12.7e How Are Heat and Chemicals Transported in the Atmospheres of Uranus and Neptune, and What Does This Mean for Interpreting Future Spectra of Spatially Unresolved Exoplanets with Hydrogen Atmospheres?

Future observations of giant exoplanets will seek to constrain the bulk composition of the atmosphere (see Figures 15-1g and 15-1j), and to understand atmospheric processes at work (e.g., Madhusudhan 2019). Both are critical for determining how these planets formed and evolved and for understanding processes currently at work in their atmospheres. Most of our understanding of hydrogen-dominated atmospheres comes from Jupiter and Saturn. However, Uranus and Neptune are very different, and many questions remain about their atmospheric processes (see Q7.2, Q7.3, and Q7.4). A better understanding of the atmospheres of Uranus and Neptune is required to interpret future spectral observations of sub-Neptunes.

Strategic Research for Q12.7

• Determine the most likely formation mechanism and interior structure of Saturn, Uranus, and Neptune by measuring noble gas abundances and isotope ratios, stable isotope ratios (e.g., ¹²C/¹³C, ¹⁴N/¹⁵N, and the noble gas isotopes) in their envelopes, and the bulk composition and interior structure of Uranus and Neptune with spacecraft observations and in situ probes.
• Determine the current thermal state and variability of Jupiter, Saturn, Uranus, and Neptune by conducting long-time duration thermal infrared observations of these four planets with spacecraft and telescope observations.
• Determine the contribution of solid materials to the giant planets by measuring cometary and interstellar object noble gas abundances and isotope ratios, and stable isotope ratios (e.g., ¹²C/¹³C, ¹⁴N/¹⁵N, and the noble gas isotopes) in multiple comets by spacecraft flybys and telescopic observations.
• Create a census of a large population of young planets recently formed, of the composition of giant exoplanet atmospheres, of magnetospheric activity in exoplanets, and of mass-radius relations of sub-Neptunes by radio and telescopic observations.
• Observe planets forming within a disk by telescopic observations.

Q12.8 CIRCUMPLANETARY SYSTEMS

The solar system displays a myriad of circumplanetary systems around both gas and solid planets. The giant planet systems demonstrate that even when planets themselves are not habitable, their moons may be. Moons may not only feed material into their ring systems, but may also “shepherd” rings, clear gaps, and/or gravitationally confine ring material. Detection and characterization of exomoons and circumplanetary disks would enhance our understanding of the formation, evolution, and lifetime of circumplanetary systems, as well as the potential for habitable exomoons.

Q12.8a What Can Observations of Circumplanetary Material Reveal About the Formation and Evolution of Circumplanetary Systems?

The composition of rings and moons hinges on conditions in their precursor circumplanetary disks, as well as their subsequent evolution. In the solar system, protosatellite disks are thought to be produced by two general processes: giant impacts (for solid planets) and gas co-accretion (for giant planets). Uranus may represent a case in which both processes were involved in the origin of the current satellite system, and Neptune likely provides an example of an original satellite system that was largely destroyed by the intact capture of a large and retrograde orbiting moon, Triton. Characterization of circumplanetary disks in exoplanetary systems and detection of exomoons would greatly expand our knowledge of ring and moon origin and evolution, including both an improved understanding of circumplanetary disk properties (e.g., radial scale, density, and lifetime) and of the balance of moon formation versus loss. A recent breakthrough has been the clear detection of a circumplanetary disk orbiting a gas giant in a 10-million-year-old stellar system, with a disk diameter of roughly 1 AU (Benisty et al. 2021).

Q12.8b How Common Are Exomoons and What Are Their Properties?

Exomoon detection is extremely technically challenging, requiring, for example, very accurate photometric light curves and long-baseline observations in combination with numerical data analyses. To date, the primary results have been upper limits on exomoon-to-planet mass ratios, in addition to some inconclusive data (e.g., for Kepler-1625b; Kreidberg et al. 2019; Teachey et al. 2020). Microlensing techniques to be used by the Roman Space Telescope could provide detections of exomoons (Liebig and Wambsganss 2010). The presence of volcanically active exomoons might be inferred from enhancements of elements such as sodium and potassium in the spectra of the giant planets that they orbit (Oza et al. 2019), while the presence of cryovolcanially active exomoons could be detected from enhancements in hydrogen, oxygen, and/or water vapor, as suggested by cryovolcanically active moons in the solar system (e.g., Quick et al. 2020). While similar moon formation mechanisms to those envisioned for the solar system are expected in exoplanetary systems, there may be notable differences too—for example, where moon loss may be more important for exoplanets in compact orbits and/or that have undergone large-scale orbital migration.

Strategic Research for Q12.8

• Detect and constrain the properties of circumplanetary disks and exomoons using telescopic observations and theoretical modeling.
• Constrain the formation and lifetime of exoplanet ring and moon systems via telescopic observations, and by comparing these observations with spacecraft and long-term telescopic observations of the ring and moon systems of Jupiter, Saturn, Uranus, and Neptune, and by theoretical modeling for ring and satellite formation and evolution.

Q12.9 INSIGHTS FROM TERRESTRIAL LIFE

As the only known habitable and inhabited world, Earth offers invaluable insights into life’s requirements, how life coevolves with its environment, and the remotely observable signatures of a global biosphere. But Earth would have looked quite different at different times in its history. Atmospheric O₂, for example, only reached its present high level (21 percent by volume) sometime within the past 500 million years. And prior to ~2.4 Ga, O₂ would have been nearly absent from the lower atmosphere, despite Earth having been inhabited for at least a billion years before then. Early Earth is thus our best example of a planet that supported life but that would have borne little resemblance spectroscopically to the world we inhabit today. Studying the history of life on Earth is fundamentally important for determining whether exoplanets provide habitable environments and potentially support life similar to what once existed or now exists on Earth.

Q12.9a Which Among the Necessary Ingredients and Conditions Required by Life on Earth Can Be Inferred or Detected on Exoplanets?

All known life requires the presence of liquid water as a solvent in which oxidation and reduction reactions can occur. It also requires the presence of carbon-based macromolecules, metals, and trace elements, along with energy to drive metabolisms (e.g., Baross et al. 2020). Even more fundamentally, life requires a stable pressure-temperature environment in which it might originate. Imaginative researchers have proposed that organisms could exist at certain heights within giant planet atmospheres if they had mechanisms, like air bladders in certain fish, to maintain their altitude (Sagan 1995). But such organisms would have to be highly evolved, multicellular organisms. If life originated as single-celled organisms, as is widely believed, it would not have been able to do this; instead, any such organisms would have been wafted up into the cold upper reaches of the planet’s atmosphere or down into the hot interior. Hence, the most basic requirement for a habitable planet is the presence of a solid (or liquid) surface. Gas (or ice) giants can be effectively ruled out.

While in situ searches for life in the solar system may be able to seek evidence of all these properties in detail and on local scales, observations of exoplanets will be limited to globally observable planetary properties. In the solar system, liquid water and thus habitable environments are found in the subsurface of icy bodies in the outer solar system (e.g., Europa and Enceladus). For exoplanets, the boundaries of the habitable zone are generally premised on a requirement of surface liquid water (Kasting et al. 1993, 2014). Planets within that zone can maintain an active photosynthetically based biosphere that is capable of altering the planet’s atmosphere in a way that is remotely detectable. More generally, habitability depends on a complex web of planetary, stellar, and planetary system parameters (e.g., Meadows and Barnes 2018; Q10.1 and Q12.10), and the environmental limits of life on Earth are still under investigation (Question 9). Understanding these properties, and which can be remotely observed or inferred, is imperative for defining the search space for potentially habitable exoplanets. While the molecular building blocks and required chemicals for life may themselves be unobservable on exoplanets, other observable phenomena related to life’s requirements might be sought. For example, ultraviolet radiation is thought to play an important role in synthesis of molecules related to the origin of life (e.g., amino acids and ribonucleotides), and insights from synthesis of terrestrial biomolecules and prebiotic molecules under varied ultraviolet irradiation may provide insight for exoplanets under varied stellar spectra (e.g., Ranjan and Sasselov 2016, 2017). As another example, dry-wet cycles on Earth are one method known to aid polymerization of organic molecules, and while such molecules are unlikely to be observed on an exoplanet, land masses on which such cycles could occur may be observable through diurnal light/color curves (Cowan et al. 2009). Detections of such properties would not prove that a planet is habitable (and, conversely, their nondetection would not prove that it is uninhabitable), but they would provide additional context upon which to interpret potential biosignatures.

Q.12.9b What Biosignatures Can Be Sought on Exoplanets Analogous to Earth, Including Past Phases of Earth History?

The metabolisms powering life on Earth may produce detectable byproducts, which could be sought as biosignatures in exoplanet atmospheres with future telescopes. Additionally, life on Earth can produce surface reflectance biosignatures (e.g., the sharp increase in reflectivity produced by plants known as the “red edge”), and analogous features might be observable in exoplanet spectra (e.g., Schwieterman et al. 2018). A thorough understanding of the biosignatures on Earth through time can guide our understanding of the types of features that could be observed on habitable worlds—and also the observational requirements to detect these features. For instance, oxygenic photosynthesis is the dominant metabolism on modern Earth. Oxygen and its photochemical by-product ozone (O₃) produce prominent spectral features that could be sought at ultraviolet, visible, and infrared wavelengths in the spectra of modern Earth analog planets (Meadows et al. 2018). However, over geological history, Earth’s observable biosignatures have varied considerably. For instance, Archean Earth (~4–2.5 Ga) had an anoxic atmosphere, but other biosignatures could still be sought, (e.g., methane) (Kaltenegger et al. 2007).

Q12.9c What Can We Learn About False Positive and False Negative Detection of Life on Exoplanets from Earth History?

Environmental contextual information will be critical to distinguish true biosignatures from abiotic “false positive” mimics. For example, while most of Earth’s methane is biological, abiotic methane is also produced (e.g., Etiope and Sherwood Lollar 2013), so methods of distinguishing true biological methane from abiotic methane on exoplanets are needed. More broadly, this type of analysis is needed for all biosignatures, including for planets with different environmental contexts than Earth. For instance, while oxygen has no significant abiotic sources on Earth, theoretical research has discovered several paths to abiotic oxygen formation that might occur on exoplanets (e.g., Meadows 2017). However, debate continues as to which of these might actually operate (Harman et al. 2018).

Earth history also presents examples of possible “false negatives”—that is, periods when life may be difficult to detect remotely. For example, the Mid-Proterozoic eon, 1.8–0.8 Ga, may have been a time when atmospheric O₂, O₃, and CH₄ may all have been too low to be detected by existing or planned space telescopes (Reinhard et al. 2017). For these periods, it is important to determine what biosignatures—if any—could be detected and the observing requirements to sense them so that similar exoplanets are not incorrectly excluded in the search for life.

Q12.9d What Are “Novel” Biosignatures Not Expressed in Earth’s Spectrum Over Geologic Time That Might Be Detected on Exoplanets?

The biosignatures expressed by Earth through time are a function of not just what life produces, but also are dependent on the planetary and stellar environment. Different biosignatures might be more or less prominent for planets with different dominant metabolisms, orbiting different types of stars (which drive different types of photochemistry), with, for example, different levels of background atmospheric gases. To evaluate which of the diverse suite of molecules produced by life (e.g., Seager et al. 2016) could serve as biosignatures for exoplanets, laboratory, field, and theoretical work is needed to evaluate the plausibility of these biosignatures to survive and be detected in varied exoplanet environments. Ideally, one would wish to identify so-called agnostic biosignatures, that is, combinations of gases—Earthlike or not—that could only be produced by a biosphere (NASEM 2019). This remains an outstanding challenge for exoplanet astrobiologists.

Strategic Research for Q12.9

• Determine the environmental requirements of life on Earth to inform the limits of habitability on exoplanets through field and laboratory investigations, and studies of “extreme” forms of Earthly organisms (e.g., organisms that survive at very low or high temperatures or in high radiation environments).
• Study the observable properties of Earth’s modern biosphere through field and laboratory studies that measure the features of Earth’s biota accessible to remote sensing (e.g., reflectance spectra of pigments).
• Observe and characterize modern Earth as an exoplanet analog through remote sensing observations across a wide wavelength range, and via observing techniques that emulate exoplanet observing methods (e.g., direct imaging and/or transit observations, time resolved to examine temporal variability).
• Assess the remotely observable properties (e.g., spectral indications of habitability and biosignatures) of an Earth-like planet across varied planet ages, and environmental and stellar conditions through theoretical modeling and laboratory studies that emulate possible exoplanet conditions, informed by remote sensing and field work data.
• Study the coevolution of life on Earth and its environment, including biosignatures and biosignature false positives and false negatives through time, by conducting geological field studies that examine past epochs of Earth history.

Q12.10 DYNAMIC HABITABILITY

The geologic records of Earth, Mars, and Venus show us that planetary habitability is highly variable in space and time. Exoplanet observations soon will offer us the opportunity to address fundamental questions about how a planet’s habitability depends on orbital distance, mass, stellar type and other factors. In turn, more detailed study of the dynamic habitability of solar system objects is essential to ensure that insights we gain from exoplanets are well-grounded. Because the presence or absence of an atmosphere is an important constraint on surface habitability, the questions in Q12.6 are highly relevant to this section. Airless exoplanets or exomoons may have subsurface habitable regions (e.g., Europa), but subsurface biospheres are unlikely to be remotely detectable, and so the current focus for exoplanets is on surface habitability.

Q12.10a How Can Solar System Objects Be Used to Determine the Boundaries of Exoplanet Habitability as a Function of Orbital Distance, Planetary Mass, and System Age?

Exoplanet orbital distance, mass, and age can be determined (with varying accuracy), and so assessing the likelihood of habitability as a function of these parameters is important for planning future observations of potentially habitable exoplanets. This will require generalization of insights from solar system planets, particularly Earth, Mars, and Venus. Earth and Venus have similar masses, but Earth’s surface is habitable while Venus’s is not. Does the inner edge of the habitable zone generally lie between the orbits of these two planets, or were there unique aspects to Venus’s evolution (e.g., very early loss of water; Gillmann et al. 2009; Hamano et al. 2013) that were more important? Smaller Mars has a rich geologic record that indicates past surface habitability and atmospheric loss, illustrating the potential for dynamic and intermittent habitability in the early evolution of a planetary environment (e.g., Ehlmann et al. 2016).

Q12.10b What Constraints Can Be Placed on the Presence of Liquid Water on Exoplanets?

The composition of a planet’s atmosphere determines surface temperatures via the greenhouse effect, which in turn determines whether surface liquid water can exist. After atmospheric characterization (Q12.6), the next step in assessing exoplanet habitability is to search for surface (or, if possible, subsurface) liquid water. Determining if oceans are present on exoplanets in the next decade will be difficult through transit spectroscopy that probes typically dry stratospheric altitudes, but indirect methods such as testing for the absence of water-soluble species such as sulfur dioxide in the upper atmosphere will allow constraints to be placed (e.g., Loftus et al. 2019). Glint may also be detectable from the reflection of starlight off exoplanetary oceans (e.g., Robinson et al. 2010). Understanding how many exoplanets possess water will allow us to better assess the evolution of oceans, lakes, and rivers on the solar system planets across geologic time, in a direct complement to the aims of Q12.10a. Last, techniques to search for other surface liquids (e.g., hydrocarbon lakes) would also be valuable to explore, in order to keep a broad view of the potential for habitability with solvents other than H₂O.

Q12.10c What External Factors Influence the Loss or Maintenance of Surface Habitability Over Time on Rocky-Type Exoplanets?

Besides direct observations, studying exoplanet habitability will also require observations and modeling of the factors that drive climate evolution on long timescales. A priority in this area is stellar emission: characterizing the high-energy radiation emitted from host stars (particularly extreme ultraviolet radiation and stellar winds) is vital to determining how rapidly exoplanet atmospheres are lost to space (Q12.6b), and what drives their chemistry (Shields et al. 2016). Other poorly understood factors of key importance for habitability include the dynamical and orbital history of a system, the flux of meteoroid impacts on planetary surfaces with time, and exchange of surface volatiles with the interior. Constraining all of these factors for a given exoplanet system based on observations alone will be difficult or impossible, so detailed study in the solar system to provide ground-truth validation for limited cases is important.

Q12.10d How Does Atmospheric Chemical Evolution Affect Habitability?

Atmospheric chemistry is critical to habitability and the emergence of life. The composition of a planet’s atmosphere determines surface temperatures via the greenhouse effect, but it also determines the extent to which prebiotic chemistry can develop. Rich organic chemistry is possible in reducing atmospheres that are no longer H₂-dominated, while oxidizing atmospheres are hostile to the emergence and survival of primitive Earth-like life. Oxidation via hydrogen loss to space is extremely important to early atmospheric chemical evolution, as is exchange with the planet’s interior. Studying the chemical state of exoplanet atmospheres will allow us to determine how common reducing, or weakly reducing, atmospheres are on young rocky planets. (A weakly reducing atmosphere is one dominated by N₂ and CO₂ but containing smaller amounts of reduced gases such as H₂ and CO.) This will have implications for understanding how life emerged on Earth, as well as whether it could also have emerged on early Mars and/or Venus. Last, the chemical characterization of exoplanets does not need to rely on direct analogies to Earth’s history. Such an agnostic approach may ultimately allow insights into the possibility of life emerging under conditions very different from those of the early Earth.

Strategic Research for Q12.10

• Determine the presence or absence of atmospheres on potentially habitable rocky exoplanets via telescopic observations, including transit spectroscopy, thermal phase curve analyses, and reflected light spectroscopy (NASEM 2023), and via comparisons to solar system planets.
• Determine the abundances of water-soluble species (e.g., SO₂ or NH₃) and aerosols in the atmospheres of potentially habitable exoplanets to constrain the presence of surface liquid water via a combination of space- and ground-based transit spectroscopy.
• Assess the frequency of planetary conditions conducive to prebiotic chemistry by determining stellar activity, atmospheric chemistry (i.e., reducing versus oxidizing), and impact/dynamical history on rocky exoplanets via space- and ground-based transit spectroscopy and modeling.
• Constrain the inner edge of the solar system’s habitable zone by studying the surface geomorphology and geochemistry of Venus to assess whether it ever possessed oceans via orbital radar observations, spectroscopy, in situ analysis, and associated chemical modeling.
• Improve exoplanet habitability predictions for cold, low-mass planets by determining the key factors that made Mars habitable 3–4 Ga, via a combination of in situ geological and atmospheric analysis and sample return, orbital observations, and climate modeling.
• Determine if subsurface exoplanet biospheres could ever be detected remotely via in situ and/or remote sensing study of potentially habitable “ocean worlds” and accompanying theory and modeling.

Q12.11 SEARCH FOR LIFE ELSEWHERE

Given limitations of exoplanet data, it is crucial to consider how the worlds of the solar system, including Earth (Q12.9) can help guide our search for life on exoplanets. Three major criteria important when searching for biosignatures are reliability (i.e., the likelihood of a potential biosignature being produced by life), survivability (i.e., the likelihood of a potential biosignature surviving long enough to be detected in the context of its environment), and detectability (i.e., the likelihood of actually being able to detect a given biosignature with a given technology) (e.g., NASEM 2019; Questions 9 and 11, Chapter 14). Relevant issues have also been discussed above in Q12.9 and Q12.10.

Q12.11a Can Formal Frameworks Be Devised for Interpreting Biosignatures on Exoplanets, Given Their Unique Challenges?

Efforts are under way to devise formal frameworks for evaluating and interpreting biosignatures (e.g., Neveu et al. 2018; Question 9). Such frameworks are needed not only in the solar system—where claims of biosignatures

have generated considerable discussion in the literature in recent decades (e.g., McKay et al. 1996)—but also for exoplanets, given the inherent limitations and challenges to observing worlds light years away.

Evaluating potential solar system biosignatures is a useful template to develop lessons for evaluating possible biosignatures in exoplanet atmospheres. As a highly irradiated planet, Venus may help us better understand false positive O₂ produced by water loss (that Venus may have experienced in the past) or through CO₂ photolysis (that Venus experiences today; NASEM 2019). Trace quantities of methane on Mars (Mumma et al. 2009; Webster et al. 2018), and phosphine on Venus (Greaves et al. 2020) have been claimed and suggested as possible biosignatures. However, continuing vigorous discourse (e.g., Zahnle et al. 2011; Snellen et al. 2020) on the presence and potential sources of these gases underscores the significant challenge we will face when evaluating possible biosignatures on distant planets. Differentiating true biosignatures from abiotic false positives critically hinges on understanding the context of their environments so that the plausibility of abiotic explanations can be evaluated, and their environments’ habitability assessed.

Q12.11b Are Biosignatures Observable on Exoplanets in the Near Future?

Just as comparative planetology is the study of natural phenomena and processes across and between multiple worlds, the discovery of one or more inhabited planets beyond Earth would create the new science of comparative astrobiology. The launch of the James Webb Space Telescope (JWST) will provide a new window into terrestrial exoplanet atmospheres, but JWST’s targets will be limited to transiting planets orbiting the lowest mass stars, M dwarfs. These stars often exhibit extreme levels of stellar activity, which may adversely impact the habitability of orbiting planets. However, M dwarfs comprise 75 percent of all stars in the galaxy, so understanding whether life can persist around them is critical to understanding the distribution of possible life in the galaxy, and they provide examples of planets with significantly different star-planet evolutionary histories compared to the worlds of the solar system. Beyond JWST, understanding whether there are biosignatures on planets orbiting around more massive “Sun-like stars” will require new types of facilities capable of suppressing their stars’ light so that their orbiting planets can be observed directly, allowing us to study worlds (and possibly biospheres) in the context of systems with planet–star evolutionary histories more akin to our own. There are important synergies with the 6m, space-based ultraviolet/optical/near-infrared telescope capable of directly detecting and characterizing planets in reflected light recommended in NASEM (2021).

Strategic Research for Q12.11

• Study methods to discriminate past and present false positive biosignatures on solar system bodies (e.g., abiotic O₂ on Venus and Mars) from true biosignatures to inform false positive discrimination methods for exoplanets through in situ, remote sensing, theoretical/modeling studies, analog field research, and laboratory studies that characterize remotely observable properties of these features.
• Study whether methods exist to remotely observe and correctly interpret biosignatures from subsurface biospheres (e.g., as on icy moons) or other potential habitable environments that might present false negative detections of life for exoplanets through in situ, remote sensing, theoretical/modeling studies, analog field research, and laboratory studies that characterize remotely observable properties of these features.
• Obtain an inventory of properties of solid body exoplanets (i.e., mass, composition, bulk atmospheric chemistry and abundance of clouds and hazes, potential biosignatures, rotation rates, relative distance from host star, and type of host star) through telescopic observations including radial velocity measurements, transit spectroscopy, high-dispersion spectroscopy, thermal phase curve observations, secondary eclipse analyses, and, eventually, direct image spectroscopy.
• Devise metrics and frameworks to establish confidence in interpretation of biosignatures in the solar system and exoplanetary systems, informed by a synthesis of relevant in situ, remote sensing, theoretical modeling, field, and laboratory data.

SUPPORTIVE ACTIVITIES FOR QUESTION 12

• Observations of solar system planets and moons through transit spectroscopy and direct-imaging as analogs to exoplanet observations, including hemispherically averaged fluxes as a function of orbital phase and time; observations of particle and gas opacity in the giant planets and Venus as a function of phase angle to help determine the dependence of reflectivity and scattering on particles and clouds in exoplanet atmospheres; and ultraviolet-/near-infrared-scattered light observations from the poles of the giant planets for comparison with future direct imaging of giant exoplanets.
• A census of protoplanetary disks, young planets, and mature planetary systems across a wide range of planet–star separations to determine how the initial composition and conditions in a protoplanetary disk influence the diversity of resulting planets.
• Improved spatial resolution of telescopic techniques to determine variations in the structure and composition of circumstellar disks, as well as the next-generation telescopes recommended by NASEM (2021) that will allow for observations of circumplanetary disks, detection of exomoons and ring systems, and characterization of exoplanets around sun-like stars.
• Laboratory studies to understand the relationship between the bulk composition of a planet and its atmosphere, and to determine the optical properties of clouds and hazes relevant to exoplanet atmospheres.
• Increased interactions between the astronomy and planetary science and astrobiology communities (supported under, e.g., NASA’s Planetary Science and Astrophysics divisions) are needed to maximize advances in exoplanetary science and to address the questions identified in this chapter. This point was emphasized in multiple white papers received by the committee.

REFERENCES