E
Report of the Panel on Exoplanets, Astrobiology, and the Solar System
OVERVIEW
In the past decade, the field of exoplanet science has rapidly expanded with the discoveries of thousands of new planets and the characterization of worlds unlike those in our solar system. From the ensuing treasure trove of exoplanet demographics and characteristics, we have learned that most, if not all, stars host planets and that planets smaller than Neptune are ubiquitous. These systems and the planets that comprise them are surprisingly diverse, with few matching the solar system. We have characterized a plethora of larger worlds for density and atmospheric properties, progressing from gas giants to large terrestrial planets, and from highly irradiated planets to cooler planets, as observational sensitivity and techniques improved. Significant advances have been made in understanding solar system planetary processes and how our planetary system formed and evolved. We have expanded our understanding of formation processes and the subsequent interactions of exoplanets with their host stars and other components of their planetary systems, and identified planetary migration as a common process for exoplanet systems and our solar system. Complementing our studies of individual worlds, multiple techniques have pieced together a broad understanding of exoplanet classes, enabling a new era of comparative planetary system science as we work toward a more complete census.
Even though exciting progress has been made, significant key advances are still needed to place the solar system and our inhabited Earth in its cosmic context. Although we have discovered and characterized giant planets close to and very far from their stars, analogs of solar system giant planets have been beyond our reach. We have discovered close-in likely terrestrial1 exoplanets, but none in the habitable zone2 (HZ) of G dwarfs like our Sun. A handful of terrestrials are known to orbit in the HZ of M dwarfs, but we have not been able to probe their atmospheres to understand if they are truly Earth-like or have strongly divergent evolutionary paths. We understand that interactions within the entire planetary system are critical to
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1 A terrestrial planet has a bulk composition dominated by rock and iron, such as Mercury, Venus, Earth, and Mars. “Terrestrial” does not imply that the planet is truly “Earth-like”—that is, habitable.
2 The habitable zone is that region around a star where an Earth-like planet is considered more likely to be able to support surface liquid water.
understanding the formation, evolution, environment, and habitability of exoplanets, but interdisciplinary research is still needed to better understand planets as interacting components evolving in the context of their host star and planetary system environment.
Upcoming observations of terrestrial exoplanets will enable one of humanity’s grandest explorations—the search for habitable environments and life around a diversity of nearby stars. In the near term, the James Webb Space Telescope (JWST) and ground-based telescopes will have the sensitivity to search for and begin to characterize the atmospheres of a handful of terrestrial planets orbiting the closest M dwarf stars. Even more ambitious direct imaging missions will be needed to study HZ worlds orbiting more Sun-like stars. Exploring this exciting and unprecedented frontier will help place Earth’s sparkling oasis of life in its cosmic context. This search is now within our scientific and technological reach, and can be informed by studies of larger exoplanets and solar system analogs, as well as interdisciplinary efforts that incorporate theory and laboratory investigations. The next section outlines key discoveries in the past decade that set the stage for exciting future advances.
PROGRESS IN EXOPLANET, ASTROBIOLOGY, AND SOLAR SYSTEM SCIENCE SINCE NEW WORLDS, NEW HORIZONS
Exoplanet Detection and Planetary System Architectures
Since 2010, the number of known exoplanets has increased by an order of magnitude to more than 4,000, with large contributions from both radial velocity (RV) and transit surveys. Early on, the typical planets detected were massive Jovians orbiting within a few astronomical units (AU) of their stars. The 2009 launch of Kepler inaugurated an era of thousands of discoveries, detecting significantly smaller, but still close-in, transiting planets (1–4 Earth radii). Most are closer than Mercury is to the Sun, but, because they orbit cooler stars, several are within their star’s HZ. In parallel, microlensing surveys detected planets near the snowlines of M dwarf planetary systems, while direct imaging refined our view of the outer reaches of planetary systems orbiting more Sun-like (spectral type FGK) stars.
The Distribution and Nature of Giant Planets
Over the 25 years since the Nobel Prize–winning discovery of 51 Pegasi b, our understanding of giant planets has matured significantly. RV surveys have increased in sensitivity by orders of magnitude, and observational campaigns begun in the 1990s now have the baselines required to detect giant planets with orbital periods similar to those of Jupiter. These surveys have revealed that hot Jupiters like 51 Peg b are rare, and that close-in brown dwarfs are rarer still. While hot Jupiters are not common, their frequency increases around more metal-rich stars, indicating that present-day system architectures are partially set by the initial mass and composition of the protoplanetary disk in which they form. Mass measurements of transiting planets have revealed a large range of planetary radii at a given mass (three orders of magnitude in density), especially for planets near Neptune mass, suggesting a diversity of compositions even at fixed mass. Giant exoplanets are enriched in heavy elements compared to their parent stars, and this enrichment seems to increase with decreasing planet mass, mirroring the trend seen in solar system planets.
Although many planets have been found, our understanding of giant planets at a range of orbital distances comparable to those in our solar system is largely incomplete. RV and direct imaging surveys of young stars find that only 10 percent of solar-type stars harbor giant planets between 1 and 13 Jupiter masses inside of 100 AU, with such planets being more and less common, respectively, around higher- and lower-mass stars. Roughly a dozen planets are known with semi-major axes larger than 50 AU, and
many have poorly constrained orbits and masses. Based on all available surveys, the occurrence rate of gas giants appears to peak near a few AU and then decline at larger separations, but these estimates depend on extrapolations of power laws in mass and semi-major axis, and the ~3–10 AU region is not yet thoroughly explored. A more complete census would be needed to determine if our solar system is unusual in having a Jupiter, which has large implications for planetary evolution and the delivery of “volatiles”—water and key compounds involving C, H, N, and O that condense at lower temperatures—which can be delivered to drier inner planets by more bodies that form farther out.
The Distribution and Nature of Sub-Neptune Planets
Three of Kepler’s key discoveries were that sub-Neptunes (1–4 Earth radii) are the most abundant type of exoplanet at orbital periods <200 days, that ~50 percent of stars have small planets orbiting more closely than Mercury orbits the Sun, and that M dwarfs host close-in planets at a higher frequency than Sun-like stars. Comparing the masses and radii of exoplanets to theoretical predictions and solar system planets has started to reveal key compositional trends. At orbital periods shorter than Mercury’s, intense stellar radiation has sculpted the mass-radius diagram, inflating hot Jupiters and producing a bimodal radius distribution for small planets, likely owing to atmospheric escape. While highly irradiated planets smaller than ~1.6 REarth have bulk densities consistent with a terrestrial composition, larger planets require significant fractions of volatiles, as do solar system ice giants. Near-terrestrial masses and bulk densities have been measured for transiting planets in near-resonant configurations using transit timing variations (TTVs), and spectroscopy of white dwarfs possibly polluted by disrupted planets/planetesimals have revealed abundance ratios similar to those of the bulk Earth. However, our view of the mass-radius diagram is still dominated by planets larger and hotter than Earth. Tracking the existence and location of the planet radius gap as a function of stellar mass, stellar metallicity, and lower insolation will refine our view of the formation and evolution of low-mass planets, and will help determine whether some terrestrials are the evaporated cores of larger planets that have lost their natal volatile-rich atmospheres.
We have discovered ~20 likely terrestrial planets (R <1.6 REarth and roughly terrestrial densities) within the HZ. Although many of these planets are too distant for follow-up characterization, Kepler’s sensitivity has enabled more precise estimates of the frequency of potentially habitable planets in the Milky Way. The frequency of Earth-like planets orbiting Sun-like stars was unconstrained before the launch of Kepler, but detailed analyses of Kepler data have revealed that such planets are relatively common and occur on average around ~10 percent of Sun-like stars and ~20 percent around smaller, cooler red dwarf stars3,4,5). Ongoing ground- and space-based surveys of nearby cool dwarfs have discovered a small but growing number of HZ terrestrial planets with atmospheres accessible to JWST and potential future extremely large ground-based telescopes (ELTs).
Mapping Planetary Architectures: The Solar System in the Context of Exoplanetary Systems
Our understanding of planetary system architectures is currently in its infancy. While we have limited sensitivity to solar system–like planets, initial indications of the rarity of Jupiter analogs suggest that solar
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3 Exoplanet Exploration Program Analysis Group (ExoPAG): R. Belikov, C. Stark, N. Batalha, C. Burke, D. Angerhausen, D. Apai, E. Bendek et al., 2017, “ExoPAG SAG 13: Exoplanet Occurrence Rates and Distributions,” National Aeronautics and Space Administration (NASA), https://exoplanets.nasa.gov/system/presentations/files/67_Belikov_SAG13_ExoPAG16_draft_v4.pdf.
4 J.J. Fortney, T.D. Robinson, S. Domagal-Goldman, D.S. Amundsen, M. Brogi, M. Claire, M.S. Marley et al., 2016, “The Need for Laboratory Work to Aid in the Understanding of Exoplanetary Atmospheres,” https://arxiv.org/abs/1602.06305.
5 C. Dressing and D. Charbonneau, 2015, “The Occurrence of Potentially Habitable Planets Orbiting M Dwarfs Estimated from the Full Kepler Dataset and an Empirical Measurement of the Detection Sensitivity,” Astrophysical Journal 807:45.
system–like architectures may be rare also. Transit measurements have discovered the most multiplanet systems to date, but are biased toward finding largely co-planar, close-in, tightly packed systems that would fit within Mercury’s orbit and show signs of migration. The RV method has found more widely spaced multiplanet systems, being sensitive to gas giants at longer orbital periods.
Growing knowledge of planetesimal distributions in our own debris disk (Kuiper Belt objects [KBOs], comets, and asteroids) has modified our understanding of the solar system from an arrangement of stationary planets to a complex system of migrating planets. Models for the early migration of solar system planets, such as the Nice and Grand Tack models, reproduce many of the observed planetesimal distributions, providing a system-wide connection of solar system bodies, and predictions for exoplanet outcomes.
The distribution of material in mature debris disks can also inform the history of exoplanetary systems. Spatially resolved visible and near-infrared (NIR) images of dozens of bright debris disks, analogous to more massive versions of our Kuiper Belt, show extended halos of dust in the cold outer regions, potentially sculpted by the interstellar medium (ISM). Atacama Large Millimeter/submillimeter Array (ALMA) images of debris disks show underlying planetesimal distributions that are typically well-defined belts, indicative of sculpting by planets. Silicate emission from copious hot dust and density asymmetries in cold belts suggest possible collisional events and disks that are more dynamic than previously thought. Directly imaged variations in the AU Microscopii disk resemble material being ejected by stellar winds. Dust compositions and optical properties are varied and poorly constrained, but likely silicate and water dominated. Detection of low levels of gas in debris disks via atomic absorption and molecular CO emission suggest nonsolar compositions, with significant carbon enhancement in some systems. The two populations of imaged debris disks and systems with known planets have little current overlap, in part owing to disk imaging sensitivity limits. Yet many disks include belts and inclination warps, likely owing to exoplanets. Earth has left its imprint on the solar system’s disk by gravitationally shepherding zodiacal dust into a large, clumpy circum-solar ring; extrasolar planets should also create these telltale signposts of planets in debris disks, but so far these structures have eluded detection and may be limited to fainter disks currently below detection limits.
Exoplanet Characterization and Solar System Synergy
Efforts to characterize and model exoplanet atmospheres have focused largely on giant and Neptune-size planets; atmospheric characterization of smaller planets has just begun. Comprehensive surveys of transiting planets across a range of mass, radius, orbits, and/or insolation levels have provided key insights into interior and atmospheric composition, as well as the atmospheric circulation, chemical, and radiative properties that regulate planetary atmospheres. Observational studies have compared the atmospheric composition of dozens of planets. For directly imaged planets, spectroscopic and photometric observations have measured the abundances of multiple molecular species (H2O, CH4, CO) and revealed the presence of cloud decks, setting young giants on a continuum with more massive brown dwarfs. For transiting planets, atmospheric characterization first focused on more easily detectable atoms and molecules (Na, K, and H2O) and expanded with improved observing methodologies and capabilities.
The physical conditions in planetary atmospheres, which probe processes like global circulation and radiative energy balance, have been thoroughly studied for roughly a dozen larger planets. High-resolution spectroscopy has measured precise thermal profiles, winds, and rotation rates for a handful of giant planets. The Hubble Space Telescope (HST) and Spitzer thermal phase curves have constrained atmospheric circulation by comparison to 3D general circulation models, and HST and ground-based high-resolution spectra have detected thermal inversions arising within strongly absorbing atmospheric regions. The same techniques have detected atmospheric escape from several hot, gas-rich transiting exoplanets, confirming that escape is common and may influence the size of close-in planets. Planetary magnetic fields have been
inferred for a small number of giant planets from periodic stellar activity, or from transit light curves with evidence for bow shocks. Magnetic fields provide a window into interior processes such as convection, and likely regulate atmospheric escape. How escape scales with planetary and stellar properties is still not well understood, providing an opportunity for exoplanet/solar system synergies.
Recent discoveries of nearby terrestrial planets, including HZ worlds orbiting late-type M dwarfs, have provided some of the first terrestrial targets for characterization. However, initial characterization attempts with HST, Spitzer, and ground-based telescopes have been able to provide only atmospheric constraints via nondetections of atmospheric features. Spitzer phase curves of a hot terrestrial planet that receives 70 times Earth’s insolation indicate little or no atmosphere. HST and Spitzer observations of a handful of hot and HZ terrestrials, when combined with laboratory data and theory, make cloudless and cloudy hydrogen-dominated atmospheres less likely than denser ones.
Despite these early successes, there are many opportunities for improved atmospheric characterization. Today, chemical abundances are typically measured with a precision of only an order of magnitude, a sign of the still limited data quality of the challenging transit measurements, which preclude a detailed understanding of planetary formation and evolution. Atmospheric hazes and clouds in many exoplanets further obscure the gaseous absorbers, and narrow wavelength ranges and current approximate cloud models limit our ability to account for the effects of these atmospheric aerosols. Our reduced insight into some solar system planets (particularly Venus, Uranus, and Neptune) in turn limits our understanding of the dynamics, composition, and evolution of atmospheres, indicating the need for further study of these worlds. Transmission spectroscopy can never be sensitive to the planetary surface, supporting the need for future direct spectroscopy of potentially habitable worlds. Last, atmospheric characterization has focused almost exclusively on shorter period, larger planets, and we cannot yet systematically connect atmospheric composition to the density/bulk compositional properties of longer period planets of all sizes, which often have less well characterized masses and radii.
Astrophysics Assets and Solar System Science
The planetary science community has made valuable use of astrophysics assets such as HST, Spitzer, and Kepler to explore solar system targets, which in return advance exoplanet science and astrobiology. Planetary scientists have measured the composition and orbital dynamics of small bodies to better understand solar system formation; observed diverse planetary atmospheres to assess how planetary processes are affected by composition and incident solar radiation; probed the interiors of volatile-rich bodies and identified new potentially habitable environments through the study of plumes on Europa and Enceladus; and observed the effects of extreme tidal heating on Io’s interior composition and volcanic activity. This coordination has led to discoveries that benefit both science communities.
The Dawn of Exoplanet Astrobiology: The Search for Habitable Environments and Life
In the past 10 years, exoplanet astrobiology has transformed from a field driven by promising statistical predictions to one with targets accessible to near-term observation. Significant advances have been made in our understanding of how to identify potentially habitable worlds and how to best search for signs of life in their environments. Theory and observations now suggest that there are many evolving interactions between a planet, star, and planetary system that affect the likelihood that a planet can support a surface ocean—and that a comprehensive, systems-level approach to habitability assessment is now needed. These studies have identified systems-level challenges to habitability for M dwarf HZ planets that are less likely to be experienced by planets orbiting in the HZ of more Sun-like stars, including radiation and stellar--
wind-driven atmosphere and ocean loss, and gravitational interactions that modify orbits, rotation rate, and climate.
Within the solar system, observations of Mars, Europa, Enceladus, and Titan have revealed subsurface environments that potentially harbor liquid water and have greatly expanded the ocean worlds in our solar system. Comparison of the gas giant satellites provided a systems-level view of how planetary size, formation, and tidal interactions work together to impact differentiation, ocean depths, pressures, and surface activity. These efforts forged links with the oceanography community in understanding water-rock reactions, hydrothermal vents, ocean pH, circulation, and ice/ocean interactions. Observations and missions to small bodies in the solar system illuminated processes of volatile evolution and delivery to forming planets, while exoplanet science revealed planetary system architecture influences on small body inventories and organic delivery in debris and protoplanetary disks. Venus provided context for loss of habitability, with relevance for Venus-analog extrasolar planets, and studies of stellar wind/planetary atmosphere interactions at Mars discovered and informed planetary atmospheric loss processes.
The astrobiological foundation needed to guide the search for signs of life’s impact on a planet’s surface and atmosphere, so-called biosignatures, has also advanced considerably. Improved understanding of the co-evolution of life with Earth environments over the past 4 Gyr has highlighted how life has modified Earth’s atmosphere, surface, oceans, and interior. Life’s global impacts on a planet’s atmosphere, surface, and temporal behavior may therefore manifest as potentially detectable exoplanet biosignatures, or technosignatures—if that life is technologically capable. Key frontiers in biosignature science now focus on the identification of novel biosignatures beyond the canonical O2/O3 and CH4, especially those that are agnostic to life’s molecular makeup or metabolism; understanding nonlife planetary processes that may mimic, destroy, or alter potential biosignatures; and taking the first steps toward developing a comprehensive statistical framework for biosignature assessment that uses critical observables of the star, planet, and planetary system to determine the probability, and increase our confidence, that a potential biosignature is owing to life.
QUESTIONS AND DISCOVERY AREA
We now stand at a pivotal moment in our exploration of the universe, where the answers to several fundamental questions about humanity’s cosmic context are within our grasp.6 With the rapid increase of known exoplanets and the possibility of comprehensively understanding many nearby planetary systems, we can now determine if the solar system is common or a cosmic rarity. We can understand how exoplanets form, interact, and evolve within their planetary systems and work to understand how these interactions and processes might enable habitability on terrestrial worlds like our own. With the past decade of scientific and technical advances behind us, we now have the foundation to begin the search for habitable planets and life beyond the solar system in earnest—to address a question that humankind has been asking itself for millennia: Are we alone in the universe? To drive the strategy to explore strange new worlds, the panel has identified four questions that lead to a discovery area centered on the search for life in the universe (see Table E.1 at the end of this appendix). Each question is described in the sections that follow, and more detailed information on the capabilities required to address each question and the discovery area are provided in Table E.2.
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6 In addition to community inputs in the form of white papers and presentations, the congressionally mandated reports by the National Academies of Sciences, Engineering, and Medicine, Exoplanet Science Strategy and An Astrobiology Strategy for the Search for Life in the Universe, were considered as inputs to the panel.
E-Q1. WHAT IS THE RANGE OF PLANETARY SYSTEM ARCHITECTURES, AND IS THE CONFIGURATION OF THE SOLAR SYSTEM COMMON?
Until recently, our solar system seemed to be an orderly blueprint of a typical planetary system, with terrestrial planets interior to the snowline, gas and ice giants exterior to the snowline, and a remnant belt of unfinished planet formation at its edge. The discovery of evaporating hot Jupiters, sub-Neptunes, and eccentric gas giant exoplanets upended that notion. It is now an open question as to whether the solar system’s architecture is common or a rare outcome of chaotic dynamical evolution. In the coming decades, the study of exoplanets will expand from planets as individual objects, to interacting objects within planetary systems, presenting a new opportunity to understand our place in the universe and the histories of nearby planetary systems (compare the sections F-Q4 and Discovery Area in Appendix F).
E-Q1a. What Are the Demographics of Planets Beyond the Reach of Current Surveys?
Exoplanet demographics and occurrence rates form the foundation of our statistical understanding of exoplanetary systems, but we have not yet completed the planet census. Although the number of known exoplanets (~4,000) has increased by an order of magnitude since the 2010 decadal survey, very few of these are analogous to solar system planets. The Kepler mission discovered thousands of short-period exoplanets, and at longer periods, RV surveys have primarily been limited to the most massive gas giants. The Nancy Grace Roman Space Telescope microlensing survey is poised to greatly expand our knowledge to longer orbital periods and lower planet masses across a wide range of stellar spectral types, filling key gaps in the census and providing a statistical anchor for planet formation and evolution models. Much like the Kepler data set revealed a gap in planet radii indicative of atmospheric evaporation, these extended demographics should give insight into physical processes governing planetary systems—for example, by detecting an enhanced density of planets near the snowlines of systems. Although microlensing surveys generally detect only a single planet in a system, these demographics will enable investigations into the architectures of planetary systems by comparison with population synthesis models. However, population-wide statistical comparisons alone are expected to leave many degeneracies in our understanding of individual systems.
E-Q1b. What Are the Typical Architectures of Planetary Systems?
By studying individual planetary systems in detail, we can understand correlations between planet populations indicative of the dynamical histories of systems, which are not expressed in statistical demographics. However, current understanding of multiplanet systems is limited to the very innermost regions, and biased toward highly coplanar systems. To put the solar system in context, we need to study regions of multiplanet systems at stellar irradiation levels comparable to those from Mercury to Neptune in our system. In the near term, our knowledge of individual planetary systems will have to be pieced together from multiple techniques; the overlap of planetary systems detectable with Transiting Exoplanet Survey Satellite (TESS) transits and TTVs, along with RV surveys, Gaia, ALMA, and the Roman Space Telescope Coronagraphic Instrument (CGI) will likely enable a clearer picture for a small number of predominantly edge-on systems. Future ground-based ELTs may directly image dozens of additional planets. A larger sample of well-studied nearby planetary systems would require space-based direct imaging. Such a sample would inform the range of outcomes from planet formation, constrain planet formation and evolution models, enable studies of how the system architecture may relate to past stochastic events and volatile delivery, and provide solar system context.
E-Q1c. How Common Is Planetary Migration, How Does It Affect the Rest of the Planetary System, and What Are the Observable Signatures?
While there is overwhelming evidence from planetary composition and orbital parameters that planet migration occurs, we have yet to understand the details and implications of the process (see Section F-Q4 in Appendix F). Does migration commonly disrupt the rest of the system, resulting in events similar to Earth’s Late Heavy Bombardment? Is there evidence of migration reversal in other systems, as proposed by the Grand Tack model of the solar system? Migration may imprint itself in the atmospheres of planets. Atmospheric elemental ratios, particularly carbon to oxygen, may record the location of formation of the planet with respect to various snowlines. Migration may also impact the composition of planetesimals and dust. Simulations and observations of the solar system suggest that Jupiter’s migration may have radially mixed chemically separate reservoirs of material. Spatially resolved IR spectroscopy will enable general interpretation of disk composition, including identification of water ice and silicates; JWST will probe the warm regions of disks, while longer wavelengths are needed for colder regions. To fully understand migration, the chemical and dynamical conditions of planets must be studied prior to, during, and after migration. By combining detailed mm wavelength observations of protoplanetary disks with cold planet demographics and a large number of well-studied individual planetary system architectures, we can relate atmospheric properties and locations of planets in mature systems to those in protoplanetary disks.
E-Q1d. How Does the Distribution of Dust and Small Bodies in Mature Systems Connect to the Current and Past Dynamical States Within Planetary Systems?
The dynamical history of our planetary system is imprinted on the distribution of minor bodies in the solar system. Dynamical models of our system’s past are largely able to reproduce the currently known distributions under specific conditions, suggesting that our architecture is only one of many possible outcomes. The orbital distribution and total mass of debris disks can similarly probe the past dynamics of exoplanetary systems. Panchromatic imaging from the visible to mm is necessary to study both the dust and planetesimal populations, as well as break degeneracies when modeling disk composition using unresolved spectral energy distributions.
Current mm-wavelength imaging is limited to the brightest disks, typically ~1,000× the density of the Kuiper Belt. Large dust grains, observable at mm wavelengths with ALMA and next-generation radio telescopes, track the distribution of their parent planetesimals. With more debris disks resolved at mm wavelengths, via improved sensitivity to fainter disks, samples of known planet host stars and disk host stars will begin to overlap, enabling studies of dynamical interactions between exoplanets and disks (compare the Discovery Area section in Appendix F).
Smaller dust grains produced by planetesimals are observable with facilities like the HST Space Telescope Imaging Spectrograph (STIS), Roman CGI coronagraph instrument (CGI), and ground-based adaptive optics (AO) coronagraphy. These grains are transported inward from the cold outer regions by radiative forces, and planets can interact strongly with them to create large-scale structures. Current and near-term visible observations of cold disks will be limited to disks ~1,000× as dense as the Kuiper Belt. In these dense disks, collisions limit the types of structures that planets can create to simple ring-like belts and inclination warps; near-term observations will focus on large-scale morphology, time variability, and dust composition/optical properties. Pushing to disks ~10× as dense as the Kuiper Belt with future observations could probe a new regime of debris disk physics in which collisions subside and disks become transport-dominated. In these fainter disks, planets can imprint resonant structures that constrain planet mass and orbit, reveal the presence of otherwise undetectable planets, and help complete our picture of nearby systems (see also the Discovery Area section in Appendix F).
E-Q1e. Where Are the Nearby Potentially Habitable Planets, and What Are the Characteristics of Their Planetary Systems?
Occurrence rates derived from Kepler data suggest that HZ Earth-size planets are common around M stars and are not exceedingly rare around Sun-like (FGK) stars. Indeed, a handful of potentially habitable planets amenable to atmospheric characterization have already been identified around M dwarf stars. HST spectroscopy has ruled out the most easily characterized (cloud-free, H/He-dominated) atmospheres for some of these transiting worlds, but further characterization via transit or high-contrast reflected-light spectroscopy should be feasible with facilities like JWST and the ELTs. In the near term, transit photometry and RV surveys will find many more of these M dwarf systems, although some may be too distant for atmospheric characterization. An extended TESS mission (or the European Space Agency’s [ESA’s] Planetary Transits and Oscillations of Stars [PLATO]) could also identify new HZ transiting planets around more Sun-like stars. However, to complete the census of nearby systems, detection of nontransiting potentially habitable planets around Sun-like stars will require improved RV sensitivity and space-based direct imaging. Solar observations and collaboration with heliophysicists may be fruitful for understanding and modeling the effects of stellar variability on RV observations (compare to Section G-Q3 in Appendix G).
In addition to finding these nearby planets, we must understand the systems within which they reside. Studying their planetary systems could reveal correlations and key processes that impact habitability. For example, the specifics of Jupiter’s past migration may have largely determined the architecture of and volatile delivery in our solar system. Accordingly, looking at systems with Jupiter analogs might be a way to find systems with architectures broadly similar to that of the solar system.
E-Q2. WHAT ARE THE PROPERTIES OF INDIVIDUAL PLANETS, AND WHICH PROCESSES LEAD TO PLANETARY DIVERSITY?
We know from decades of solar system exploration that planets are individually complex and collectively diverse. A complete understanding of planets requires appreciation of not only their physical properties but also the underlying processes that shape them. Thus, the goal of exoplanet characterization is to measure their atmospheric, surface, and internal compositions and to infer their radiative, chemical, dynamical, and magnetic field processes. Comparisons between multiple planets within single planetary systems are particularly helpful for illuminating such processes. While exoplanet science has only scratched the surface of our understanding of individual planets and their properties, the next decade will allow more detailed and meaningful characterization of a diversity of planets. Important goals for the coming decade include addressing the following questions.
E-Q2a. Which Physical Processes Govern a Planet’s Interior Structure?
Exoplanet science has been guided by the mass-radius relationship, which helps constrain planetary bulk density. While general trends are apparent, important questions remain, including why planets with similar masses have different densities and how their interior composition can be inferred. While Uranus and Neptune have notably different densities and internal heat flows despite their similar mass, the diversity of exoplanet densities in this mass range, albeit at higher stellar irradiation than Neptune and Uranus, is particularly large, suggesting an even larger diversity of interior structures and compositions. Placing solar system planets within the context of the intrinsic diversity of all exoplanets, including the sub-Neptune and super-Earth-size planets that have no solar system analogs, will illuminate how bulk planetary properties, and formation and thermal histories, affect planetary interior structure and magnetic fields. These studies
will be informed by larger samples of planet radii and masses at higher precision (particularly for cooler planets than yet characterized), improved theoretical approaches, and new laboratory measurements of the equations of state and chemical properties of planetary materials at high temperatures and pressures.
E-Q2b. How Does a Planet’s Interior Structure and Composition Connect to Its Surface and Atmosphere?
The atmosphere of a planet is not necessarily a tracer of its bulk composition. A planet may have discrete compositional layers of which the atmosphere is only the outermost. Furthermore, models suggest that the observable atmospheres of transiting hot Jupiters are separated from deeper atmospheric layers by a radiative layer that may inhibit mixing, disconnecting the atmosphere from the deeper interior. Condensation, circulation patterns, and various sources of chemical disequilibrium likely also affect the composition of the remotely detectable atmosphere. For terrestrial planets, surface/atmosphere exchange mechanisms mediate atmospheric composition, and planetary magnetic fields can illuminate processes occurring deep in a planet’s interior, while providing critical insights into how the planet’s atmosphere interacts with the space environment. Meeting the goal of determining the bulk composition of a planet thus entails connecting the observable atmosphere, as sculpted by such processes, to deep atmospheric or surface processes and chemical composition. Theory and laboratory studies inform our understanding of the deep atmospheric process. Statistical surveys of atmospheric composition, such as by ESA’s Atmospheric Remote-sensing Infrared Exoplanet Large-survey mission, and bulk planetary properties will illuminate the diversity and trends. For example, do lower-mass gas giant planets exhibit consistently higher atmospheric enrichment in heavy elements than do higher-mass planets? High-resolution spectroscopy will certainly be a major capability of the proposed ELTs, which would provide particularly robust measurements of molecular species and thermal structure in both short-period and directly imaged planets. These data will be complemented by transmission and emission spectra of transiting planets spanning nearly the entire range of exoplanet masses, sizes, and temperatures, especially with the expanded spectral range and enhanced sensitivity of JWST. More precise high-frequency radio observations can be used to increase the sample of planets known to have magnetic fields, and lower-frequency observations can detect the weaker magnetic fields that are more likely to be present in ice giant and smaller gas giants. With larger observational samples, more sensitive observing facilities, and better theoretical and laboratory insights, we can hope to meaningfully connect observed planetary characteristics to formation and evolutionary history. A better understanding of structure and composition of solar system planets—especially Venus and the ice giants—will inform these exoplanet studies. Including exoplanet scientists as team members and science investigators on future missions to Venus, Uranus, and Neptune would advance efforts to understand analog exoplanets.
E-Q2c. What Fundamental Planetary Parameters and Processes Determine the Complexity of Planetary Atmospheres?
The current state of a planet’s atmosphere depends not only on its formation environment and coevolution with its planetary interior through such processes as interior outgassing and magnetic field generation but also on ongoing processes such as photochemistry, cloud formation, and atmospheric dynamics. A clearer understanding of a planet’s evolutionary pathway on all time scales demands substantial efforts in both observations and theoretical modeling. Phase-resolved observations from facilities such as JWST, and high-dispersion spectroscopy conducted from the ground, ideally in tandem with climate, photochemistry, and 3D atmospheric dynamical models, provides estimates of the atmospheric composition and dynamics, and insights into planetary rotation state. The microphysical and dynamical processes that govern the morphology
and transport of clouds have yet to be untangled for giant exoplanets, and detailed models of these processes will be imperative for understanding formation and transport of clouds on all types of terrestrial planets with atmospheres. Polarization observations could lend further insights. Photochemistry may produce observable features in transmission or reflected spectra, and are likely to be sensitive to stellar UV output, atmospheric composition, and dynamical processes. Panchromatic stellar characterization (see Sections G-Q1 and G-Q3 in Appendix G), laboratory experiments, and studies of solar system analogs will support studies in these areas.
E-Q2d. How Does a Planet’s Interaction with Its Host Star and Planetary System Influence Its Atmospheric Properties over All Time Scales?
A planet’s external environment—the star (see Section G-Q4 in Appendix G) and other bodies in the system—also plays a critical role in shaping the evolution of its atmosphere. The sub-Neptune exoplanet radius gap has been interpreted as owing to strong hydrodynamic escape processes driven by the star. A growing number of exoplanets exhibit evidence for active escape processes driven by stellar photon fluxes and stellar winds, and it is anticipated that stellar activity in the form of flares and coronal mass ejections can dominate atmospheric processes for some systems. The stellar ultraviolet (UV) spectrum influences planetary atmospheric photochemistry, which can modify atmospheric composition and atmospheric loss, with subsequent impacts on planetary climate. Obtaining both transit and directly imaged observations of planets, particularly (but not exclusively) at UV wavelengths that are sensitive to upper atmospheric processes, will illuminate escape processes and time scales and provide crucial inputs for modeling. Measurements of escape and photochemistry for solar system planets reveal the variety of atmospheric escape processes, and validate models for atmospheric escape from exoplanets. Other known “star-planet” interactions include aurora (observed on brown dwarfs but not yet exoplanets) and direct magnetic connection similar to that between Jupiter and its satellites (reported for multiple systems). Beyond stellar output, planetary impactors, perhaps mediated by other planets in the system, deliver and remove volatiles; and star-planet-planetary system gravitational interactions can induce tidal heating and enhance outgassing.
E-Q2e. How Do Giant Planets Fit Within a Continuum of Our Understanding of All Substellar Objects?
The thousands of known brown dwarfs span the gulf in mass and temperature from the smallest main sequence stars to cold gas giant planets like Jupiter and Saturn. Brown dwarfs are easier to study in detail than planets, offering the opportunity to rigorously test models of thermal evolution, atmospheric dynamics, chemistry, cloud formation, and magnetic dynamos that are also applicable to extrasolar giant planets. Characterizing the differences and similarities of the two classes of objects will elucidate their formation mechanisms, informing the limits of both planet and star formation (compare to Section G-Q1 in Appendix G).
Young, low-mass brown dwarfs serve as particularly valuable laboratories for refining models of young directly imaged planets because they have similar gravities and temperatures. Roman CGI will obtain optical wavelength thermal emission spectra of young companion objects as a complement to JWST and ground-based, longer-wavelength observations, and perhaps reflected light spectra of a few cool giants. Both types of observations will inform the properties of giant planets, helping to place them in context with low-mass brown dwarfs. Additional surveys for substellar companions to stars that probe to higher contrasts and smaller separations will find younger planetary-mass companions.
While many individual and binary brown dwarf systems are known, the relatively rare companions to main sequence stars allow comparison of the companion’s composition to that of its primary star. Future observations of the masses and atmospheric and magnetic properties of brown dwarfs and giant planets
will reveal how substellar companions to stars form (compare to Section G-Q2 in Appendix G) and evolve, and whether the processes are similar for giant planets and brown dwarfs over the observable ranges of mass and orbital separation. Progress here requires improved spectral data, including higher resolution and greater wavelength coverage in polarized and unpolarized light, astrometric and/or RV masses for many more substellar companions, and improved theoretical modeling approaches and laboratory data.
E-Q3. HOW DO HABITABLE ENVIRONMENTS ARISE AND EVOLVE WITHIN THE CONTEXT OF THEIR PLANETARY SYSTEMS?
The habitability of a planet is governed by a complex interplay of planet, star, and planetary system architecture and the mutual evolution of these components over time. Consequently, the context provided by the host star and planetary system architecture, including the distribution of small bodies and their potential for volatile delivery, is important for determining whether a habitable planet can form and maintain its habitability over time. An improved understanding of the factors and processes influencing habitability are needed to support exoplanetary exploration and target selection. To identify habitable environments and connect them to the planetary systems in which they reside, foundational research on exoplanet properties and processes through observations of planets, disks, and planetary systems and theoretical models, laboratory studies, and comparisons with solar system analogs is needed.
E-Q3a. How Are Potentially Habitable Environments Formed?
A planet acquires volatiles and organics essential for a habitable environment either during formation and migration or via subsequent impacts of volatile and organic-rich bodies. Factors that affect the type and amount of volatiles acquired include the type of star, its metallicity, the composition of the disk that formed around the star during planet formation, and migration of planets within the system after bodies form. Answering this question requires an improved history of volatiles in our solar system, characterization of volatiles in exoplanet systems, and modeling how volatiles are acquired and lost by potentially habitable planets. Completing the volatile and organic inventory of planets within the solar system, including the dwarf planets, asteroids, KBOs, and comets, and determining the dynamical interactions that formed these populations, provides the tightest constraints on the properties of the protoplanetary disk from which our system formed. These studies would also help determine how volatiles were distributed during and after planet formation and migration and could reveal how volatiles are incorporated in forming and evolving planets. Complementary measurements of volatile content across planet forming regions in exoplanetary systems could provide context for how and when major dynamical events took place within the solar system, and reveal the photochemical processes that gave rise to known small-body compositions. These studies could constrain theoretical models of dynamical evolution, and volatile accretion and delivery, informing how architecture, composition, and timing interact to determine which planets acquire volatiles.
E-Q3b. What Processes Influence the Habitability of Environments?
Once a volatile- and organic-rich planet is formed, acquiring and maintaining an atmosphere and surface ocean relies on a suite of planetary, stellar, and planetary system properties and interactions. These include star-planet interactions that govern the loss or maintenance of a primordial planetary atmosphere (see Section G-Q4 in Appendix G), and the planetary interior/atmosphere exchanges that can generate and replenish a secondary atmosphere and ocean. Taking a systems science approach to habitability will strengthen our understanding of these different processes for planets of different compositions. Within the
solar system, processes like tidal heating, asteroid bombardment, and the loss and evolution of planetary atmospheres through escape and photochemistry can be studied to better understand their impact on solar system terrestrial planets and ocean worlds. Additionally, Earth’s interior, surface, and atmosphere have evolved significantly over its history owing to a wide range of geological, photochemical, and biological processes, providing a range of different habitable environments over time. Combination of measurements and theory of the nature and processes that drove Earth’s early habitability and the loss of habitability on Venus and Mars can inform our understanding of exoplanet habitability. The subsurface ocean worlds of Europa, Enceladus, and Titan likely also harbor habitable environments that are governed by processes, such as tidal heating, that may also be relevant to habitable exoplanets. To support the insights provided by the systems science approach and solar system analogs, strongly interdisciplinary work is needed between planetary science, astronomy, Earth science, and heliophysics/stellar astronomy, including laboratory and theory. In the longer term, studying Earth-size planets near the HZ of other stars (see Section E-Q3d, below) will provide observational insights into the characteristics and processes of a broader range of habitable planets.
E-Q3c. What Is the Range of Potentially Habitable Environments Around Different Types of Stars?
Earth, orbiting a G dwarf, is the only habitable planet known to support life. However, exoplanets coevolve with their host stars, just as Earth coevolved with the Sun. The host star impacts planetary atmospheric loss, composition, and climate, and the host star’s spectrum (including X-ray/extreme UV flux), activity, and long-term luminosity evolution are critically important for understanding the dynamic habitability of exoplanets. Exoplanet surveys have shown that terrestrial planets can exist around a range of stellar types, but observations have yet to confirm if habitable environments can exist around all types of stars. Although M dwarf planets will be the first accessible to near-term observation, they are far more likely than Sun-like stars (FGK dwarfs) to drive planetary atmosphere and ocean loss. The close-in HZ makes M dwarf HZ planets potentially more vulnerable to atmospheric loss, coronal mass ejection events, tidal heating, and orbital evolution. To better understand the distribution of habitable environments in the local solar neighborhood, it is important to understand how the star’s properties and evolution influence the evolution and habitability of terrestrial planets (compare to Sections G-Q2, G-Q3, and G-Q4 in Appendix G). To improve our understanding of these processes, stellar energetic output for a range of spectral types and over multiple temporal scales is needed, combined with theoretical models of both magnetized and unmagnetized planets to understand the impact of the host star on atmospheres and oceans for a large sample of systems and spectral types.
E-Q3d. What Are the Key Observable Characteristics of Habitable Planets?
Modern Earth provides the only observable example of a habitable surface environment. To expand our understanding of the observational discriminants for habitable environments, we need to study Earth’s environments through time and relevant solar system environments, and both model the observable characteristics of, and ultimately observe, potentially habitable environments under the influence of different types of stars. For exoplanets, initial observational assessment of habitability requires determining the presence and nature of an atmosphere and searching for atmospheric or surface signs of the presence of an ocean. In the near term, observations of M dwarf HZ planets with JWST and ELTs could identify the presence of atmospheres and detect key molecules that could make habitability more or less likely. In the longer term, direct imaging mapping of phase-dependent ocean glint could directly show the presence of an ocean, although the likelihood of habitability could also be inferred from observations and theory
constraining the surface conditions. Strongly interdisciplinary efforts, combining observations, laboratory, and theoretical studies, are needed to study and identify signs of habitability prior to future observations.
E-Q4. HOW CAN SIGNS OF LIFE BE IDENTIFIED AND INTERPRETED IN THE CONTEXT OF THEIR PLANETARY ENVIRONMENTS?
Over the next 10 years, JWST and upcoming ground-based telescopes will have the opportunity to conduct the first searches for signs of life on terrestrial planets orbiting a handful of nearby M dwarf stars. However, detecting potential signs of life with upcoming technology is only one component of our search for life. To support these efforts, strongly interdisciplinary science is also required to identify which biosignatures to look for and to understand how to assess whether a potential biosignature is more or less likely to be owing to life, given the context of the planetary environment.
E-Q4a. What Biosignatures Should We Look for?
Astrobiologists in solar system and exoplanetary science have worked together to identify a short list of proposed atmospheric, surface, and temporal exoplanet biosignatures—based largely on our modern Earth and the past environments and dominant metabolisms of early Earth. An ideal biosignature must satisfy three major criteria—it must be reliably produced by life, must survive or be preserved in its environment, and must be detectable with anticipated technology. Under these criteria, the broad global impacts of the harnessing of abundant sunlight by oxygenic photosynthesis remain a key set of biosignatures. However, to increase the probability of finding and recognizing life elsewhere, we need to continue identifying alternative biological pathways that could produce detectable biosignatures and explore the potentially detectable impacts on a planetary environment by “life as we don’t know it.” The latter goal would be met by developing the new frontier of “agnostic biosignatures” that are not associated with a specific metabolism but may take the form of unanticipated complexity in a planetary environment, as revealed by atmospheric chemical networks or disequilibria. To propose potential biosignatures that are more likely to be detectable, we also need to observe and model how processes in the interiors, surfaces, and atmospheres of planets can work to enhance or destroy a biosignature, or how abiotic processes might mimic a biosignature and complicate its interpretation. Coordinated work by heliophysicists/stellar astronomers (compare to Sections G-Q3 and G-Q4 in Appendix G), biologists, and planetary and Earth scientists is needed to combine observations of Earth and solar system planets, laboratory work, and theory to identify novel atmospheric, surface, temporal, and agnostic biosignatures accessible by upcoming missions.
E-Q4b. How Will We Interpret the Biosignatures That We See?
Recent advances in astrobiology research have shown that it is likely that all biosignatures, including abundant O2, O3, and CH4, will need to be interpreted in the context of their planetary environment to rule out false positives—planetary processes that could mimic the biosignature. Modeling of star-planet interactions suggests that O2 may have abiotic production mechanisms, including photochemistry and ocean loss, which are especially likely for planets orbiting M dwarfs. Consequently, any potential biosignature (or technosignature) will need additional assessment to determine whether it is more likely to have a biological origin, by using environmental context to rule out false positives, and to search for secondary confirmation of the biosignature hypothesis. Over the next decade, we will need to develop a comprehensive framework for probabilistic biosignature assessment to determine whether the observed phenomenon is more or less likely to be owing to life. For each biosignature considered, such a framework would need to consider the context
of the stellar and planetary environment, and include an understanding of the false negatives, false positives, and observational discriminants. To support this framework, observations of a host star’s UV spectrum and activity (an overlap with Section G-Q4 in Appendix G) and a wide range of planetary types, from gas giants to uninhabitable terrestrials, will need to be combined with theoretical modeling and insights from solar system planets to improve our understanding of the physical and chemical processes that modify planetary environments. In particular, an empirical census of atmospheres on terrestrial worlds under a wide range of conditions, both in and outside the HZ, will be needed to validate or adjust current ideas about atmospheric signatures produced through abiotic and biotic processes. Additionally, we can use solar system spacecraft data to identify if the surfaces or atmospheres of solar system bodies show evidence for chemistry and organic products and processes, and whether these suggest biogenic or prebiotic potential, or constitute false positives.
E-Q4c. Do Any Nearby Planets Exhibit Biosignatures?
The next decade will present several opportunities to characterize terrestrial exoplanets and undertake the very first search for biosignatures on a handful of planets orbiting nearby M dwarfs. Owing to their host stars’ super-luminous pre-main sequence phase, activity, and the proximity of the HZ to the star, M dwarf planets likely undergo a very different evolutionary history—which may include atmosphere and ocean loss—than planets orbiting more Sun-like stars and may allow us to expand our understanding of biospheres for different stellar hosts. JWST can likely detect CO2 and CH4 on transiting HZ planets orbiting a few late-type M dwarfs and so will search for biologically induced disequilibrium conditions that may have been prevalent on the early Earth. TRAPPIST-1 d, e, f, and g are likely to be the most promising targets for such searches. ELTs will complement JWST’s initial assay by accessing a larger sample (~10) of nearby earlier-type M dwarf planets using AO techniques and/or high-resolution spectroscopy to search for O2, which is unlikely to be detected with JWST. The detection of O3 in the atmospheres of HZ M dwarf planets is unlikely with either JWST or the ELTs, but more precise transmission spectroscopy (~5 ppm sensitivity) could detect it in the mid-IR. However, transmission spectroscopy cannot probe the near-surface atmosphere and planetary surface and may miss more UV-labile biosignature molecules, such as volatile organic compounds, that are better preserved in the near-surface environment.
DISCOVERY AREA: THE SEARCH FOR LIFE ON EXOPLANETS
Is there life elsewhere in the universe? This profound question has echoed down through the millennia, and the answer is now within our scientific and technical grasp. Ground-based surveys have transformed the search for life from a philosophical question to a near-term scientific observable by providing a handful of high-priority M dwarf terrestrial planets amenable to spectroscopic atmospheric characterization with JWST and the ELTs. However, these host stars may present challenging environments for life and be more likely to generate false positives for biosignatures, and interpretation of their planets may need substantial extrapolation of solar system–informed knowledge of habitability. A robust search for life therefore requires surveying the HZs of Sun-like stars, where we know that life can arise. This is only possible with a large, high-contrast, direct-imaging space telescope.
Directly imaging and obtaining spectra of objects 10 billion times fainter than their host stars is a remarkable challenge. However, the past decade has seen significant reductions in the two largest sources of astrophysical uncertainty for these observations. First, the Kepler mission has shown that roughly Earth-size planets in the HZ of Sun-like stars are not exceedingly rare, with an estimated occurrence rate of ~0.1. Second, the Large Binocular Telescope Interferometer (LBTI) Hunt for Observable Signatures of Terrestrial Systems (HOSTS) debris disk survey indicates that warm exozodiacal dust is not prohibitively bright, being
typically just a few times that of the zodiacal cloud; observations with the Roman CGI may also improve exozodiacal dust constraints. Terrestrial exoplanets now appear to be common enough to detect in substantial numbers with sufficient resources.
Maximizing Our Chances of Finding, Recognizing, and Quantifying Life
There are multiple ways to maximize our chances of finding life, including searching the HZ of a wide variety of stellar spectral types; being sensitive to biosignatures over a wide range of evolutionary history; probing the deepest levels of a planet’s atmosphere, where a larger range of biosignatures may persist; and increasing the chances of observing molecules that reveal biosignatures and environmental context. The following suite of parameters and capabilities will maximize the probability for life detection along these different axes.
Large Sample Size
The sample size of HZ terrestrial planets increases our chances of observing life and improves our ability to quantitatively answer whether there is life elsewhere in the universe. A large sample size provides robustness against remotely detectable life being an unlikely outcome—for example, if remotely detectable life arises on 10 percent of HZ terrestrials, 30 such planets must be surveyed to detect such life with 95 percent confidence. For a null result, a larger sample size places a stricter constraint on how often remotely detectable life could arise—for example, if 30 HZ terrestrials are surveyed and none exhibit signs of life, we can conclude with 95 percent confidence that remotely detectable life arises on fewer than 10 percent of HZ terrestrials. Broadly speaking, dozens of habitable planets are required to provide an informative null result. Both the odds of “yes” and the scientific impact of “no” are increased.
Diverse Stellar Sample
Although JWST and ground-based telescopes will soon allow a tantalizing first attempt at the search for life on ~10 planets around M dwarfs, these stars present many challenges to habitability and biosignature interpretation. Earth and our G dwarf Sun are the only known planet-star combination to host life. A more definitive and informed answer to the question of whether we are alone will require searching stars spanning a broad range of spectral types, including the more Sun-like FGK stars. By using a larger sample size that includes a range of FGKM stars, we improve our chances of finding inhabited planets, and understanding how the stellar environment impacts them.
Direct Imaging
While transmission observations will likely work well for M dwarf planets, direct imaging is needed to study the atmospheres of planets orbiting Sun-like FGK stars. Importantly, direct imaging probes the lower atmosphere near the surface, the most useful region for biosignature and ocean detection, the latter phase-dependent observations of ocean glint. In the longer term, mid-IR interferometric imaging would provide a valuable complement to visible-NIR imaging observations.
UV Capability
O2 reveals the presence of a photosynthetic biosphere on our planet, but it was likely present at directly detectable levels for only the past 1–2 Gyr of Earth’s history. Prior to that, the presence of low levels of O2
could have been inferred from the strong UV absorption feature of O3. UV observations therefore critically enhance sensitivity to signs of photosynthesis over a larger range of a planet’s lifetime.
Multiple Spectroscopic Bands and Species
We can increase our chances of detecting life by having the capability to detect multiple potential biosignatures (e.g., O2, O3, CH4), including those produced by a range of metabolisms other than oxygenic photosynthesis. Detection of a putative biosignature gas is more robust if multiple spectral bands are detected. Similarly, interpretation of biosignature gases to assess false positive scenarios requires context on the planetary environment, including atmospheric and surface characterization. A broad wavelength range enables the detection of multiple key species, and potentially multiple bands of those species, as well as providing better constraints on any atmospheric aerosols, all of which increase the robustness of biosignature detection and interpretation.
The capabilities needed to study the environments and possible biospheres of habitable planets orbiting more Sun-like stars are not met by any existing or currently selected facilities, but developing the scientific community and technological capabilities required to do so would enable huge advances in multiple aspects of exoplanet science and astrophysics. This Discovery Area would benefit significantly from collaboration across disciplinary boundaries and ongoing support for enabling observations, theory and laboratory work. The search for life on exoplanets will provide a bold and unifying vision for exoplanets, astrobiology, and solar system science.
TABLE E.1 Science Questions and Discovery Area
| Questions | Subquestions |
|---|---|
| E-Q1: What is the range of planetary system architectures, and is the configuration of the solar system common? | E-Q1a: What are the demographics of planets beyond the reach of current surveys? E-Q1b: What are the typical architectures of planetary systems? E-Q1c: How common is planetary migration, how does it affect the rest of the planetary system, and what are the observable signatures? E-Q1d: How does the distribution of dust and small bodies in mature systems connect to the current and past dynamical states within planetary systems? E-Q1e: Where are the nearby potentially habitable planets, and what are the characteristics of their planetary systems? |
| E-Q2: What are the properties of individual planets, and which processes lead to planetary diversity? | E-Q2a: Which physical processes govern a planet’s interior structure? E-Q2b: How does a planet’s interior structure and composition connect to its surface and atmosphere? E-Q2c: What fundamental planetary parameters and processes determine the complexity of planetary atmospheres? E-Q2d: How does a planet’s interaction with its host star and planetary system influence its atmospheric properties over all time scales? E-Q2e: How do giant planets fit within a continuum of our understanding of all substellar objects? |
| E-Q3: How do habitable environments arise and evolve within the context of their planetary systems? | E-Q3a: How are potentially habitable environments formed? E-Q3b: What processes influence the habitability of environments? E-Q3c: What is the range of potentially habitable environments around different types of stars? E-Q3d: What are the key observable characteristics of habitable planets? |
| E-Q4: How can signs of life be identified and interpreted in the context of their planetary environments? | E-Q4a: What biosignatures should we look for? E-Q4b: How will we interpret the biosignatures that we see? E-Q4c: Do any nearby planets exhibit biosignatures? |
| E-DA Discovery Area: The search for life on exoplanets. |
TABLE E.2 Capabilities, Facilities, and Future Needs to Address the Science Questions and Discovery Area
| Capability | Science Enabled | Current/Expected Facilities | Future Needs |
|---|---|---|---|
| Large-aperture, space-based direct imaging | E-Q2a, E-Q2b, E-Q2c, E-Q2e, E-Q3b, E-Q3d, E-DA |
Large-aperture space-based UV-NIR imaging/spectroscopy (0.3–1.8 microns, contrast ~1e-10, IWA <~60 mas, OWA ~500 mas, R ~150 spectroscopya for dozens of potential Earth analogs in the HZs of Sun-like stars | |
| Radial velocity observations | E-Q1, E-Q2a, E-Q2b, E-Q2e, E-Q3d, E-DA |
Ground-based PRV facilities (e.g., NEID): cold gas giants in TESS systems, masses of planets with known radii including potentially habitable planets orbiting M dwarfs; detection of long-period planets (P = 1–100 yr); using solar observations to understand effect of stellar variability on exoplanet observations | EPRV (ground and/or space; 10 cm/s semi-amplitude sensitivity for P = 50–400 days): masses and orbits of habitable planets orbiting FGKM stars |
| High-contrast imaging | E-Q1, E-Q2a, E-Q2c, E-Q3b, E-Q3c, E-Q3d, E-Q4c, E-DA |
GPI, SPHERE, GRAVITY, Roman CGI: few gas giants, dozens of exozodiacal/debris disks ELTs: detection of habitable zone Earth-size planets around M stars (0.5–1.8 microns, contrast ~1e-8, IWA <~30 mas, OWA ~200 mas, ~dozen targets) |
Space-based high-contrast imaging: full planetary systems including faint debris disks (visible wavelengths, contrast ~1e-10, IWA <~60 mas, OWA >~1”, spatial resolution <~0.01 mas, ~100 planetary systems) |
| High-contrast spectroscopy | E-Q1e, E-Q2, E-Q3b, E-Q3c, E-Q3d, E-Q4c, E-DA |
ELTs: characterization of habitable zone Earth-size planets around M stars (0.5–1.8 microns, contrast ~1e-8, IWA <~30 mas, OWA ~200 mas, R >1e-5, dozens of targets) to search for biosignatures | Space-based UV-NIR spectroscopy: characterization of HZ Earth-size planets around FGK stars (0.3–1.8 microns, contrast ~1e-10, IWA <~60 mas, OWA ~500 mas, ~100s of stars, R ~150 spectroscopy for potentially dozens of Earth analogs) |
| Astrometry | E-Q1, E-Q2a, E-Q2b, E-Q2c, E-Q2e, E-Q3d, E-DA |
Gaia, Roman WFI supplement: population studies overlapping with Kepler, cold gas giants in TESS and nearby systems | Near-IR astrometry to measure substellar object masses/orbits; masses and orbits of temperate planets orbiting FGKM stars |
| Polarization | E-Q1d, E-Q1e, E-Q2c, E-Q2e, E-Q3c, E-Q3d, E-Q4, E-DA |
Roman: polarization of disks Ground-based instruments, including on ELTs: polarization signatures of disks and giant planets |
Direct imaging to probe polarized ocean glint on terrestrial planets |
| Microlensing | E-Q1a | Roman population studies | |
| Transit observations | E-Q1b, E-Q1c, E-Q1e, E-Q2a |
TESS: discover and measure radii of inner planets, evaporated cores, and migrated planets orbiting bright stars and potentially habitable planets orbiting KM stars PLATO: find planets orbiting bright stars; precisely determine planet and star properties; asteroseismology |
Large collecting area: detection of extremely small (terrestrial, <1.6 Earth radius) transiting objects, exomoons, and planetary ring systems |
| Capability | Science Enabled | Current/Expected Facilities | Future Needs |
|---|---|---|---|
| Transit spectroscopy—O/NIR/MIR | E-Q1c, E-Q1e, E-Q2, E-Q3d, E-Q4c |
JWST: characterize atmospheres of a few potentially habitable planets orbiting M dwarfs JWST, ARIEL: NIR-MIR spectra of a few mature Jovians and dozens of Jupiter to >2 R_Earth close-in planets (TESS planets) HST: atmospheric composition of warm/hot gas giants |
Large collecting area ground or space telescopes: extremely high SNR transit observations to study atmospheric dynamics and variability; wavelength coverage out to 20 micron; spectrophotometric stability in transit of <10 ppm below 10.5 microns, and <25 ppm above 10.5 microns |
| Stellar characterization | E-Q2d, E-Q3b, E-Q3c, E-Q3d |
HST, Chandra, XMM: EUV/NUV/O/IR spectra of known planet host star; important to have multiple lines and multiple bandpasses (e.g., Lyman alpha and Mg II in UV, and X-ray for CMEs) UV: atmospheric escape | X ray: More sensitive observations (~50× greater than Chandra) with R >5,000 and high spatial resolution UV: unresolved UV monitoring of nearby FGKM stars and time-resolved stellar spectra IR: high spectral resolution (R >20,000) Radio observations >10 GHz and high-resolution Lyman alpha (>~30,000) for photoevaporation and inferring stellar mass loss rates |
| UV observations of planets and host stars | E-Q2a, E-Q2c, E-Q2d, E-Q3c |
HST limited UV transit capability | UV space telescope: R >1,000 spectroscopy; monitor atmospheric escape; high-contrast imaging of planets to detect UV absorbers; time-resolved UV stellar flux |
| High-resolution O/IR spectroscopy | E-Q2a, E-Q2.c, E-Q2d, E-Q2e, E-Q4c |
R >~1e5 O/IR spectroscopy (8–10 m telescopes): giant planet characterization (~dozens) HST, Keck: UV-VIS-NIR transit spectroscopy for atmospheric characterization/escape ELTs: VIS-NIR spectroscopy (transmission and reflected light) to detect O2 in M dwarf terrestrial exoplanet atmospheres (0.5–1.8 microns, contrast ~1e-8, R >1e5, dozens of targets) |
High-contrast vis-NIR reflected light spectroscopy of mature planets Coupling of AO and high-dispersion spectrographs on ELTs |
| Emission photometry and spectroscopy | E-Q2 | Large ground-based telescopes; JWST; ARIEL Eclipse, phase curve spectra (JWST): near- to mid-IR spectra of dozens of >2 R_Earth transiting planets. | R ~100 MIR eclipse spectroscopy of temperate/cooler Neptune and larger planets |
| Long-baseline mm interferometry | E-Q1a, E-Q1c, E-Q1d, E-Q2a |
ALMA: ~dozen planetesimal belts around known massive disks to measure belt locations/geometries/masses, many detailed characterizations of bright protoplanetary disks | High-sensitivity mm interferometry: ~10–100× improved sensitivity to image dozens of SS-like planetesimal belts (1 mm wavelength, resolution <~1”, sensitivity ~0.1 microjansky [μJy]/beam) |
| Long-baseline, long-wavelength interferometry | E-Q2d | VLA | Low-frequency radio arrays with several mJy sensitivity from ~50 MHz: detecting radio emission from magnetic fields of exoplanets |
| Mid-IR direct imaging | E-Q2a, E-Q2b, E-Q2c, E-Q2e, E-DA |
ELTs: 10 micron high-contrast imaging from ground for a few stars to measure radii and temperatures of few planets | Mid-IR interferometry: measure temperature, radii, and atmospheric features of planets around FGKM stars, including Earth-size HZ planets (5–18 microns) |
| Solar system small-body characterization | E-Q1b, E-Q1c, E-Q1d |
HST, ground-based telescopes | Large time allocations and/or improved detection algorithms; continued detection and spectroscopic characterization of small bodies in UV/IR; rotation rates and orbital characteristics; small KBO binary fraction |
| Capability | Science Enabled | Current/Expected Facilities | Future Needs |
|---|---|---|---|
| Characterization of solar system planets | E-Q2, E-Q3b, E-Q3d, E-Q4b |
Venus atmospheric composition Atmospheric escape Mars |
Ice giants: atmospheric and interior structure and composition Venus: atmosphere entry probes |
| Habitability relevant solar system environments | E-Q3b, E-Q4a |
Dragonfly: Titan; Europa Clipper: Europa JUICE: Galilean moons Mars2020: Mars Ground-based observations: Venus |
Venus: atmospheric chemistry assays Enceladus: future missions Earth: detectable characteristics of Earth environments through time |
| Interdisciplinary theory, laboratory, field | E-Q3, E-Q4a, E-Q4b, E-DA |
Identification of novel biosignatures, comprehensive multifactorial framework for habitability assessment, identification of biosignature false positives and negatives and their observational discriminants. Probabilistic framework for biosignatures assessment. Always needed. | |
| Laboratory studies | E-Q2, E-Q4b, E-DA |
Planetary interiors: volatile solubilities in planetary materials, equations of state, high-pressure melting curves, viscosities, thermal conductivities. Planetary atmospheres: composition, UV-MIR opacities, and other properties of gases/aerosols/particles for atmospheres and disks.b Photochemical and ion rate reactions. Always needed. |
|
| Theory | E-Q1c, E-Q2, E-Q3b, E-Q4a, E-Q4b, E-DA |
Simulations of orbital evolution, migration, and interactions of planets with small bodies. Modeling of planetary interiors and interior-surface-atmosphere-magnetosphere exchange. 1D to 3D atmospheric models (including star-planet interactions) evaluating photochemistry and haze formation, clouds, dynamics, climate, and escape; modification of planetary atmospheres over daily, seasonal, stellar cycle, and evolutionary time scales. Computational molecular opacities and line profiles. Always needed. |
|
| Cross-division data analysis programs | E-Q1b, E-Q1c, E-Q1d, E-Q2d, E-Q3a, E-Q3b, E-Q3c, E-DA |
Programs that enable and expand opportunities for synergistic exoplanet/solar system/Earth science/heliophysics research and interactions. Always needed. | |
| Cross-division mission participating scientist programs | E-Q2, E-Q3b, E-Q3c, E-DA |
Opportunities for participation by exoplanet scientists in heliophysics, Earth science and solar system exploration missions, and participation by planetary scientists, Earth scientists, and heliophysicists in exoplanet relevant astrophysics missions. Not currently provided by NASA R&A program structure. Always needed. |
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a T.D. Brandt and D.S. Spiegel, 2014, “Prospects for Detecting Oxygen, Water, and Chlorophyll on an Exo-Earth,” Proceedings of the National Academy of Sciences U.S.A. 111(37):13278–13283; Y.K. Feng, T.D. Robinson, J.J. Fortney, R.E. Lupu, M.S. Marley, N.K. Lewis, B. McIntosh, and M.R. Line, 2018, “Characterizing Earth Analogs in Reflected Light: Atmospheric Retrieval Studies for Future Space Telescopes,” Astronomical Journal 155(5):200.
b For example, J.J. Fortney, T.D. Robinson, S. Domagal-Goldman, D.S. Amundsen, M. Brogi, M. Claire, M.S. Marley et al., 2016, “The Need for Laboratory Work to Aid in the Understanding of Exoplanetary Atmospheres,” arXiv preprint arXiv:1602.06305.
NOTE: IWA/OWA: inner and outer working angles for optimum starlight suppression in direct imaging systems; R: spectral resolution; P: planetary orbital period; CGI: coronagraphic instrument; E/PRV: extreme/precision radial velocity; NEID: NN-EXPLORE exoplanet investigations with Doppler spectroscopy; GPI: giant planet imager; SPHERE: Spectro-Polarimetric High-Contrast Exoplanet Research; Roman WFI: Roman Wide-Field Imager; CME: coronal mass ejection.
