Since publication of the 2015 Astrobiology Strategy (NASA 2015), there has been significant progress in characterizing the extreme range of habitable environments on Earth and identifying potentially habitable environments elsewhere in the solar system and beyond. Habitable environments on Earth, each characterized by a specific set of processes and parameters, remain a touchstone in understanding habitability requirements. The study of the examples posed by habitable Earth ecosystems and improved understanding of the current chief astrobiological targets within the solar system—Mars, Europa, Enceladus, and Titan—have illuminated the importance of understanding these worlds as integrated systems when assessing their potential habitability. Further, the post-2015 discovery of Earth-sized planets orbiting M-dwarf stars in their habitable zones—a term that has, at times, been used to mean different things in different communities (see below)—has transformed the field of exoplanet studies from a statistical exercise predicting that such planets are common, to providing specific targets amenable to near-term telescopic study.
These advances have led to new, holistic approaches for understanding habitability. Foremost of these is the concept of habitability systems (Figure 3.1). In habitability systems thinking, the roles processes play in a system to facilitate or catalyze conditions that support life, as well as the interactions and feedbacks between these processes, supersede the specifics of any given planetary environment. For example, on Earth, plate tectonics sustain a planetary disequilibrium that generates thermal and geochemical gradients. On an ocean world, tidal heating, radiogenic heating, or interactions between circulating water and rock could play the same role.
In a similar vein, the community has begun a reevaluation of the term habitable zone. Traditionally, the habitable zone has been defined as the range of distances from a parent star in which an Earth-like exoplanet could potentially maintain liquid water on its surface (Kasting et al. 1993; Kopparapu et al. 2013). But there are different ways to estimate this range. One-dimensional (1-D) climate models yield a comparatively narrow, or conservative, range. Empirical estimates based on observations of Mars and Venus yield a somewhat broader, or optimistic, range. The optimistic habitable zone extends considerably closer to the star, which may be appropriate if clouds (which are not explicitly included in 1-D climate models) help to keep a planet cool.
More recently, systems thinking has given rise to the concept of habitability indices, which instead combine constraints on multiple parameters and processes, such as the planet’s initial composition or the host star’s evolution. Recent studies demonstrate that evolving interactions between planet, star, and planetary system impact habitability, allowing us to advance beyond the classical concept of habitable zone to provide a more comprehensive assessment of potential habitability when evaluating and ranking targets for biosignature searches. Within the solar system, multiple aspects of the environment and its history are needed for habitability assessment and
surface and subsurface site selection. In parallel with the increasingly broad identification of potentially habitable environments that stems from systems-level thinking, there has been a renewed focus on habitability assessment as a multidimensional, probabilistic process that evolves through time—multiparameter habitability assessment.
Strong synergies between solar system studies and exoplanet science are also emerging. For example, within the solar system, perspectives on volatile evolution and delivery can be investigated directly through observations of and missions to small body populations, while planetary system observations demonstrate how planetary system architecture influences small body inventories and volatile and organic delivery as it evolves from debris and protoplanetary disks. Recent discoveries of exo-Venuses—terrestrial-size planets (i.e., approximately 1.6 Earth radii) in an orbit closer to the parent star than the inner edge of the habitable zone—and habitable zone terrestrial planets have also enabled an exciting new era of comparative planetology. Comparative studies of planetary processes and outcomes in the solar system, and the recently revealed characteristics of analog exoplanets, will inform and strengthen understanding of the processes that shape planetary habitability and provide a major new opportunity for interdisciplinary research that integrates multiple research communities.
Since the release of the 2015 Astrobiology Strategy, exoplanet astrobiology has transformed from a field driven by promising statistical predictions into one with nearby targets accessible to near-term observation. The revolutionary Kepler mission began this transformation with its initial discovery of two promising planets in the habitable zones of their parent stars, Kepler-62f and Kepler-186f (1.4 and 1.1 times the size of Earth, respectively). However, both planets were too distant for follow-up observations to obtain masses that might then be used to confirm their rocky bulk densities and compositions. Instead, their likely terrestrial nature was hypothesized based on their radii being less than the 1.5 Earth radius limit below which a planet is more likely to be rocky (Rogers 2015). With Kepler’s final data set now released, observations of hundreds of thousands of distant stars have been used to project that approximately 50 percent of Sun-like stars may host Earth- and super-Earth-sized planets in their conservative habitable zones (Kopparapu et al. 2018, Table 3).
Kepler’s data set includes 4,034 planet candidates. Of these, approximately 50 are possibly rocky planets with radii less than 2.0 Earth radii and orbiting in their host star’s habitable zone (Figure 3.2). Although Kepler’s planets are many hundreds to thousands of parsecs (pc) distant, and therefore too faint for the follow-up observations needed to assess their habitability, a recent flurry of discoveries by ground-based telescopes of relatively nearby (within 20 pc of the Sun) habitable-zone planets has provided concrete targets for further study. The first of these was the discovery of Proxima Centauri b, an exoplanet with a minimum mass of 1.3 Earth masses in an 11-day orbit around the closest star to the solar system, Proxima Centauri (1.3 pc; Anglada-Escude et al. 2016). Proxima Centauri is a low-mass (0.12 solar mass), M-dwarf star, and Proxima Centauri b’s short orbital period
places it squarely within the habitable zone. The second remarkable discovery was that the late M-dwarf star named TRAPPIST-1 (0.080 solar mass) hosts at least seven Earth-sized exoplanets (12 pc; Gillon et al. 2017) that transit, or cross in front of, the star. Three (TRAPPIST-1 e, f, and g) of the seven planets orbit in the habitable zone and are within ~10 percent of Earth’s radius. Transit timing variations were used to determine that the masses of these exoplanets (0.62, 0.68, and 1.34 Earth masses, respectively) and the derived planetary densities suggest that the planets are of comparable composition to Earth, or that they have a larger fraction of interior ices (Grimm et al. 2018). Although more distant than Proxima Centauri b and orbiting a fainter star, the fact that the planets transit their star means that they will be prime targets for spectroscopic observations by the future James Webb Space Telescope (JWST). It is important to note that M-dwarf stars span a wide range of mass and luminosity. The M dwarfs observed during Kepler’s primary mission were much more massive—and, thus, more luminous—than TRAPPIST-1. This explains why Kepler did not see planetary systems like that orbiting TRAPPIST-1, despite the likely prevalence of such systems in the galaxy.
Other recently discovered rocky planets in the habitable zone include a planet orbiting in the outer habitable zone of the M-dwarf star Luyten Half-Second catalog 1140 (12 pc away; Dittmann et al. 2017; Kopparapu et al. 2013). Feng et al. (2017) also published a re-analysis of Doppler data on Tau Ceti, a Sun-like star just 3.7 pc distant, which supports earlier claims that the star hosts two habitable zone worlds (Tau Ceti e and f) with minimum masses of about 4 Earth masses. Finally, Bonfils et al. (2018) announced the discovery of Ross 128 b, 3.4 pc distant, with a minimum mass of 1.35 Earth masses orbiting in the habitable zone of its parent M-dwarf star. Given the ubiquity of M dwarfs in the Milky Way, these nearby discoveries (summarized in Table 3.1) suggest that the galaxy is teeming with planets orbiting in their host star’s habitable zones.
TABLE 3.1 Masses and Orbital Properties of Relatively Nearby, Potentially Habitable Worlds
|Planet Name||Distance (parsecs)||Discovery Method||Planet Mass Estimate Range (Earth masses)||Planet Radius Estimate Range (Earth radii)||Orbital Period (days)||Orbital Semimajor Axis (AU)||Host Star Type||Host Star Mass (solar masses)||Reference|
|Prox Cen b||1.3||Doppler||1.10-1.46 (min)||-||11.2||0.05||M5.5V||0.12||Anglada-Escude et al. 2016|
|TRAPPIST-1 d||12||transits and timing||0.14-0.068||0.742-0.802||4.05||0.021||M8V||0.080||Gillon et al. 2017|
|TRAPPIST-1 e||12||transits and timing||0.04-1.20||0.879-0.957||6.10||0.028||M8V||0.080||Gillon et al. 2017|
|TRAPPIST-1 f||12||transits and timing||0.50-0.86||1.007-1.083||9.21||0.037||M8V||0.080||Gillon et al. 2017|
|TRAPPIST-1 g||12||transits and timing||0.46-2.22||1.086-1.168||12.4||0.045||M8V||0.080||Gillon et al. 2017|
|LHS 1140 b||12||transits and Doppler||4.83-8.47||1.33-1.153||24.7||0.0875||M4.5V||0.146||Dittmann et al. 2017|
|Tau Ceti e||3.7||Doppler||3.29-4.76 (min)||-||163||0.538||G8.5V||0.783||Feng et al. 2017|
|Tau Ceti f||3.7||Doppler||2.56-4.98 (min)||-||636||1.334||G8.5V||0.783||Feng et al. 2017|
|Ross 128 b||3.4||Doppler||(1.30-1.45 min)||-||9.9||0.05||M4V||0.168||Bonfils et al. 2018|
NOTE: Listed in order of the date of their discovery announcement or journal publication date.
Finding: The discovery of numerous nearby exoplanets orbiting in their host star’s habitable zone, coupled with estimates of the fraction of stars with terrestrial-size, habitable-zone planets has matured the search for evidence of life beyond the solar system enough to warrant taking the next steps toward its discovery.
As these recent discoveries show, M-dwarf planets will be at the vanguard of efforts to characterize exoplanet habitability because they are abundant and relatively easy to discover and characterize. This is due to a number of key factors, including the following:
- The smaller, dimmer parent star increases the detectability of a planet in both transmitted and emitted light relative to that of a similar body in orbit about a G-dwarf star (i.e., a Sun-like star);
- The compact nature of the habitable zone of an M-dwarf star means that planets orbiting therein are more likely to be seen transiting their parent star; and
- Planets in the habitable zone of an M-dwarf star have short orbital periods, allowing multiple transits to be observed in a relatively short period of time.
M-dwarf stars are also the most common type of star in the galaxy, comprising ~70 percent of the stellar population (Henry et al. 2018). Statistical results from Kepler show that early M-dwarf stars are more likely to have smaller, terrestrial-sized planets than Sun-like, G-dwarf stars (Howard et al. 2011; Dressing and Charbonneau 2015; Mulders et al. 2015) and that compact multiplanet systems may occur for at least 50 percent of M-dwarf stars (Ballard and Johnson 2016; Muirhead et al. 2015). For example, as mentioned above, the nearby TRAPPIST-1 M-dwarf system has three planets found within the (conservative) habitable zone (Gillon et al. 2017). These results suggest that M-dwarf stars may harbor the most habitable zone terrestrial planets in the galaxy, potentially by a very large margin. It is also worth noting that Kepler/K2 and ground-based surveys are probing very different types of M-dwarf stars and, yet, both are finding that potentially habitable planets are common.
Finding: The availability of near-term data on the atmospheres of terrestrial exoplanets orbiting M-dwarf stars will enable the first observational tests of their potential habitability.
Consequently, whether M-dwarf habitable-zone planets are indeed habitable is a key question with important implications for understanding of the distribution of life in the galaxy. However, M-dwarf stars undergo a very different luminosity evolution track compared to more Sun-like, G-dwarf stars, as the young M-dwarf star contracts more slowly to reach its main sequence size. During this pre-main sequence phase, an M-dwarf star’s luminosity can be significantly higher than it will be on the main sequence, subjecting planets that inhabit orbits consistent with the main sequence habitable zone to much larger amounts of stellar radiation early on. For the smaller M-dwarf stars, this super-luminous pre-main-sequence phase can last for as long as a billion years (Baraffe et al. 2015) before the star contracts, dims, and joins the main sequence. During this phase, M-dwarf stars could strip planetary atmospheres and evaporating oceans via stellar X-ray ultraviolet (XUV) and extreme ultraviolet radiation and the stellar wind (Dong et al. 2017; Garcia-Sage et al. 2017). Even failure to lose a dense primordial atmosphere may inhibit or preclude their habitability (Owen and Mohanty 2016). The maintenance of a planetary atmosphere occurs through an interplay between atmospheric loss and atmospheric replenishment from outgassing and volatile delivery. Whether M-dwarf planets, after initially losing atmosphere and ocean, can regain them remains uncertain and remains a valuable line of research. Recent work has also shown that an increased understanding of stellar variability is crucial for optimizing exoplanet characterization (Morris et al. 2018) and that parameters such as metallicity and star age need to be constrained to inform planetary formation and evolutionary models and infer terrestrial exoplanet composition (e.g., Dorn et al. 2018). The evolution of stellar luminosity and XUV radiation are key to understanding planetary atmospheric retention, composition, and evolution (e.g., Luger and Barnes 2015). Additionally, characterizing the stellar energy distribution, especially in the ultraviolet wavelengths, is needed to understand the photochemistry that can enhance or destroy biosignatures (e.g., Segura et al. 2005; Rugheimer et al. 2015). In the near term, observations with Atmospheric Remote-sensing Infrared Exoplanet
Large-survey (Ariel) spacecraft,1 JWST (e.g., for the TRAPPIST-1 system and GJ 1132b; Meadows et al. 2018), and large ground-based telescopes (e.g., Proxima Centauri b; e.g., Lovis et al. 2017; Snellen et al. 2015) may be able to address such questions about the coevolution of exoplanet atmospheres and surfaces with their host stars.
Finding: Because of the coevolution of host star and exoplanet, stellar activity and evolution are critically important for understanding the dynamic habitability of exoplanets.
Assessing habitability of exoplanets requires observations of the parent star and, in particular, its spectral energy distribution and flare frequency and energy. Additionally, observations of the planet that probe the planetary atmosphere, and ideally the planetary surface, are also necessary. Using transmission observations, the stratospheres, and perhaps upper tropospheres of transiting planets like TRAPPIST-1, are anticipated to be accessible. These observations could hold information about atmospheric composition, the presence or absence of water vapor in the stratosphere (Meadows et al. 2018; Lincowski et al. 2018), the presence of clouds or aerosols (Arney et al. 2017), including volcanic products (Misra et al. 2015), and day-night temperature differences (Kreidberg and Loeb 2016; Meadows et al. 2018). These observations may be possible in the near term with JWST and ground-based telescopes.
In the longer term, direct imaging observations of planetary atmospheres and surfaces will be needed to build upon these initial transmission assays of planetary upper atmospheres, by probing near-surface atmospheres and constraining planetary surface conditions. Transmission observations are not an option for more Sun-like F-, G-, and K-dwarf stars, due to the larger stellar size and more distant habitable zone, which diminishes the planet signal and the probability that it transits its star. Direct imaging observations of F-, G-, and K-dwarf stars will complement the near-term transmission observations of M-dwarf stars. These direct-imaging observatios can also be used to search for water and biosignatures, which may be more detectable in the near-surface environment than in the transmission-probed upper atmosphere and to map the planetary surface using light curves and spectra to look for continents and oceans (Cowan et al. 2009). The initial assay of M-dwarf habitability using transmission observations with JWST and ground-based, high-resolution spectroscopy (Snellen et al. 2015; Lovis et al. 2017) will therefore be complemented and succeeded by direct imaging undertaken by more capable space telescope missions currently under study and by future 30 m and 40 m ground-based telescopes (Quanz et al. 2015). These future capabilities will complement transmission observations of M-dwarf stars to provide a more complete census of the habitability of terrestrial planets orbiting a wide range of stellar hosts, and expanding the context of Earth’s habitability.
For exoplanet studies, the first-order assessment of habitability has been the habitable zone (Figure 3.3). As energy from the star and the essential elements for life are presumed to be common at the surface of rocky planets, surface water has traditionally been seen as the limiting resource for exoplanet habitability. Because exoplanets can only be probed by remote sensing, astronomers focus on exoplanets that possess surface water, which is in contact with the atmosphere, and which can support a surface biosphere that is more likely to be detectable. The criterion of focusing on surface water has historically been used to define the habitable zone (Shapley 1953; Strughold 1955; Huang 1959, 1960; Hart 1978, 1979; Kasting et al. 1993; Kopparapu et al. 2013). The value of such a definition is that interactions between the hydrosphere, rocky surface, and atmosphere of these planets might make signatures of their surface habitability and biospheres remotely detectable.
The habitable zone has become the first-order assessment method for newly discovered exoplanets, and it has guided the development of large space-based telescopes that will search for potential habitable exoplanets. A planet in the habitable zone is not necessarily habitable, and its habitability cannot be inferred, but only observationally confirmed. However, based on current knowledge of terrestrial planetary processes, the habitable zone is useful
to identify that region around a star where an Earth-like exoplanet is most likely to be able to support surface liquid water, compared to elsewhere in the planetary system. The surface liquid water in turn increases the probability that the exoplanet can host a surface biosphere, which would be more accessible to remote-sensing observations. Identifying the habitable zone requires two readily observable characteristics—the planet’s distance from its star (semi-major axis and eccentricity) and the type of star that it orbits. These parameters are combined with climate model-derived limits on the distance an Earth-like planet can be from its star and maintain water on its surface.
Previous work has attempted to significantly widen the limits of the habitable zone by invoking hydrogen greenhouses that might extend the habitable zone to an orbit equivalent to Saturn’s in the solar system (Seager 2013). That would imply that nearly every star has a potentially habitable planet, however, and could lead us to underdesign future space telescope missions to search for them. Alternatively, a near-desiccated planet on which oceans are confined to the planet’s polar regions has been invoked (Abe et al. 2011). This model allows the habitable zone to be moved in toward the star. Setting aside the question of how likely such planets are to exist, this possibility is dealt with in the “optimistic” habitable zone definition of Kopparapu et al. (2013), which extends well inside the inner boundary of the “conservative” habitable zone. Other work on redefining the limits of the habitable zone has looked at the habitable zone limits for terrestrial planets larger than Earth (Kopparapu et al. 2014) and used sophisticated general circulation models to understand climate limits for planets with different rotation rates orbiting M-dwarf stars (Yang et al. 2014; Kopparapu et al. 2017). Energy-balance climate models have been used to show that the outer regions of the habitable zone may be less temperate than previously thought because of limit-cycling behavior in which the planet’s climate alternates between frozen and nonfrozen states (Menou 2015; Haqq-Misra et al. 2016).
As useful as the habitable zone’s first order assessment of potential habitability can be, many factors impact a planet’s habitability. Thus, the habitable zone can be thought of as a two-dimensional slice through a far
more complex, interdisciplinary, and multidimensional parameter space. Consequently, a planet’s position in the habitable zone does not guarantee habitability, because aspects of its formation or evolution may preclude habitability. The field of habitability assessment is now advancing to study the parameters and processes that affect an exoplanet’s potential to host life. Such assessments seek to identify which of the relevant planetary, stellar, and galactic parameters and processes are accessible to observation and study. This new multiparameter habitability assessment synthesizes knowledge and observations from many different fields to provide a more comprehensive and powerful assessment of the likelihood of exoplanet habitability, thereby improving our ability to pick the best targets to search for life.
The most obvious example of a nonhabitable habitable-zone planet is one that formed with little or no water (Raymond et al. 2004, 2007; Lissauer 2007), or that lost oceans of water during its M-dwarf host star’s super-luminous pre-main-sequence phase (Luger and Barnes 2015; Meadows et al. 2018). Even if the initial conditions were favorable for life, the planet interacts with its host star and planetary system, and habitability can be enhanced or lost over time. A planet will interact and evolve with the spectrum, luminosity, and activity of its host star. The spectrum of the host star, interacting with the atmosphere, will define the surface radiation environment and drive compositional changes to the planet’s atmosphere through atmospheric loss processes and photochemistry. The planet’s initial composition, the delivery of volatiles, its orbital evolution, and the subsequent interior and atmospheric evolution are also influenced by interactions with other planets in the system, including giant planets, and asteroid and Kuiper belts. The masses, orbits, and migration history of jovian planets in particular are critical to understanding the potential habitability of rocky planets in a planetary system (Raymond et al. 2012), as jovians can affect volatile delivery to forming terrestrial planets (e.g., eccentric jovian planets can inhibit water delivery to forming rocky planets; Raymond et al. 2004, 2007).
Consequently, the habitability of a planet is governed by a complex interplay between planet, star, planetary system architecture, and the mutual evolution of these components over time. Additional examples include the importance of plate tectonics and the role of outgassing in counteracting atmospheric loss and generating the secondary atmosphere that we may see in our observations. Plate tectonics recycles volatile forms of elements such as carbon and sulfur efficiently on modern Earth. However, volatile recycling may occur even on stagnant-lid planets (Foley and Smye 2018), so this habitability requirement may not be absolute. Habitability may also require continents and relatively shallow oceans (relative to the planet’s radius) that allow for recycling of other biogenic elements like phosphorus. Phosphorus is supplied to the modern oceans almost entirely by weathering of continental rocks (Tyrell 1999); hence, it could conceivably be scarce on waterworlds with few or no emergent continents.
Understanding the factors that affect habitability will enable identification of those exoplanets that are most likely to be habitable and inform our interpretation of upcoming exoplanet data to be used to search for life beyond Earth. The likely impact of relevant interactions, relationships, and evolution of these parameters on planetary habitability has also been studied more extensively in the past 5 years. For future observation target selection of habitable exoplanets, it will be important to develop a means of moving beyond habitability assessment based upon the traditional habitable zone alone. Rather, habitability potential will need to be assessed using as many of the characteristics and processes outlined in Figure 3.1 and those yet to be discovered, as can be observationally accessed or theoretically constrained. Preliminary steps in this direction have been taken by proposing habitability “indices” that are observations of an exoplanet’s semi-major axis and a parameter sweep through the radiative and climatic impact of possible orbital eccentricity and planetary albedo to determine the probability of habitability within the habitable-zone limits, e.g., the “habitability index for transiting exoplanets (HITE)” (Barnes et al. 2015). Although these are steps toward a multiparameter assessment for habitability, none of these initial attempts comprehensively models interactions between planet-star and planetary system, as constrained by observations, and so the concept of multiparameter habitability assessment is still a very fruitful avenue for future research.
Finding: The context of solar and planetary system architecture and evolution is important for determining a planet’s history of habitability and limits on habitability, and is important to inform target selection and exploration.
Another promising area currently being developed is that of intermodal complementarity and comparison for
habitability assessment. Multi-model approaches are used in Earth science to constrain uncertainties in climate change (Taylor et al. 2012) and this approach is also relevant to other bodies in the solar system with atmospheres and to exoplanets where the range of stellar insolation and the spread of model results could be even larger (e.g., Popp et al. 2016). One-dimensional (1-D) and three-dimensional (3-D) climate models have complementary strengths, applicability and computational expense, and can support each other for modeling the habitability of exoplanet environments by passing environmental variables that are best calculated by each model. One-dimensional climate codes generally do a more careful calculation of radiative transfer and gaseous absorption than 3-D general circulation models (GCMs) and can model a broader range of exoplanet atmospheric conditions and condensates (Robinson and Crisp 2018), but are most applicable to rapidly rotating planets. 3-D GCMs, in contrast, are excellent for modeling synchronously rotating planets orbiting M dwarfs where there is a strong day-night difference, and these models can self-consistently calculate the effects of water vapor, ice albedo and cloud feedback on climate, and simulate spatially nonuniform phenomena that impact observations such as the formation of dayside clouds and day-night temperature. However, 3-D GCMs are computationally expensive and difficult to couple to chemistry models, so there are still applications where 1-D comparisons with the 3-D GCMs can be informative. Similarly, it is important to compare results from different 3-D models to each other to search for consensus in modeled phenomena, and increase confidence in the simulations. For example, the inner edge of the habitable zone is best determined by 3-D models because these models can account for changes in relative humidity and clouds. But the model predictions differ in their specifics: Leconte et al. (2013) predict that a runaway greenhouse would occur for a 10 percent increase in solar flux relative to present-day Earth, whereas Wolf and Toon (2015), using a different GCM, find that runaway would require a flux increase of 21 percent. In distance units, this puts the inner edge at 0.95 AU and 0.91 AU, respectively. While these differences may not seem large, they highlight the fact that not all climate models are the same and that simulating a wide range of planetary climates is a complex task.
Finding: Continued theoretical modeling of planetary environments, including model intercomparisons, is required to explore processes, interactions, and environmental outcomes and to understand habitability and biosignatures in the context of their environment.
The discovery of over 3,700 confirmed planets beyond the solar system opens up an exciting opportunity to understand a diversity of planetary characteristics and processes. Many of these new worlds have no analog in the solar system and can enrich our understanding of the characteristics of the planetary system, star, and planet that contribute to, or detract from, planetary habitability. Other exoplanet discoveries can be likened to planets within the solar system—for example, planets GJ 1132b (Berta-Thompson et al. 2015) and TRAPPIST-1b (Gillon et al. 2016), which may have characteristics and processes in common with Venus and are thus labeled “exo-Venuses.” Such comparisons provide impetus for the solar system and exoplanetary science communities to share expertise and collaborate on understanding planetary processes and evolution. On the one hand, the discovery of rocky exoplanets in a habitable zone offers the exciting possibility to discover habitable environments that would place Earth in a cosmic context. If signs of life were detected on these planets, a new era of comparative astrobiology would begin. On the other hand, comparisons among the terrestrial planets within the solar system illuminate the divergent paths that terrestrial planet evolution can take both toward and away from habitability (Figure 2.1), or entirely different models of planetary formation can be proposed, as with the ocean worlds. This is an increasingly rich area for future development.
Comparative Planetology of Solar System Planets
Presumably Venus, Earth, and Mars formed from the same initial inventory of the solar nebula material. Isotopic and geological evidence suggest the presence of surface liquid water on all these planets in the past, along with plentiful solar and chemical energy and minerals for nutrients. Arguably, the right conditions existed on all three planets for them to be habitable in the past. Both Venus and Mars have masses and internal structures that
differ from Earth’s, and they occupy different distances from the Sun within the solar system’s habitable zone. Their climate evolution in the past 4 billion years has led them on dramatically divergent paths, however. Today, only Earth is undeniably habitable and inhabited, while Venus is hot and dry with a dense atmosphere, and Mars is cold and dry with a thin atmosphere. For these reasons, Mars and Venus inform the search for life by improving understanding of the diversity of terrestrial planet evolutionary outcomes and illuminating how these outcomes influence habitability. Such diversity of habitable planets in the solar system may also exist in other stellar systems with multiple “habitable” planets as well. Their comparative planetology is going to be just as important to reveal whether life exists on any of them.
Mars: Another Habitable World?
Interest in Mars stems from the fact that among the planets in the Sun’s habitable zone, the surface of Mars exhibits characteristics suggesting that it may have been more Earth-like in the past. That leads to the question of whether microbial life ever existed on Mars in the past or survives today. Mars meets the minimum criteria for the existence of life—plentiful solar and chemical energy, surface water in the past (and possibly in the subsurface today), and nutrients, including carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur (CHNOPS; see, e.g., Grotzinger et al. 2014; Mahaffy et al. 2015; Ehlmann et al. 2016). The detection and variability of methane in the atmosphere of Mars (Webster et al. 2015, 2018) and the detection of organic molecules (e.g., chlorobenzene and dichloropropane) in the martian surface (Freissinet et al. 2015; Eigenbrode et al. 2018) indicate processes of carbon geochemistry not previously anticipated; however, their origin could well be geologic or exogenous. Current techniques have not been able to discriminate between the biogenic and abiotic origin of the martian molecules reported to date. The European Space Agency (ESA) Trace Gas Orbiter promises to determine the carbon-isotopic ratio in methane and the abundances of the heavier hydrocarbons such as ethane, which would help with this question but is unlikely to resolve it beyond doubt (Olsen et al. 2017). The Mars Organic Molecular Analyzer (MOMA; Goesmann et al. 2017) on the ExoMars 2020 rover may also address this question by looking for organics from depths of up to 2 m. As the early Mars cooled, it evolved from a period of active plate tectonics to the current epoch of a single, global plate forming a stagnant lid over a convecting mantle (e.g., Bruer and Spohn 2003). Thus, one question the planet poses is: For how long could a one-plate planet host life? NASA’s InSight mission, launched in May 2018, is anticipated to reveal answers to questions concerning Mars’ internal structure and level of geodynamic activity.
Venus: The End State of Habitable Planet Evolution?
Despite its location on the inner edge of the solar system’s classically defined habitable zone, and receiving twice the insolation as does Earth, Venus is thought to have hosted a global ocean early in its evolution and may have been as habitable as Earth was at that time. An enhanced D/H ratio in the venusian atmosphere, however, suggests that the planet suffered a runaway greenhouse effect (Watson et al. 1981; Donahue et al. 1982) resulting in the loss of its ocean (de Bergh et al.1991). Furthermore, desiccation of the atmosphere and surface may have inhibited subduction, fused the crustal plates, and extinguished the interior dynamo (Nimmo et al. 2002). Such a chain of events would have exposed the planet’s atmosphere to predation from the solar wind. Moreover, the processes Venus underwent to reach its current state may define the inner edge of a star’s habitable zone.
The heating of Venus and the loss of its ocean allow the study of processes that reduce planetary habitability and may even represent the trajectory of Earth’s own, continuing evolution. Furthermore, current methods in exoplanet detection favor detection of these bodies on the inner edge of their star’s habitable zones—much closer to Venus’s position than to Earth’s (Kane et al. 2018). Thus, the exploration of Venus is important, not only for understanding the evolution of planetary habitability, but also as an analog to habitable-zone exoplanets.
In the last 5 years, major discoveries by the ESA Venus Express and Japan Aerospace Exploration Agency (JAXA) Venus Climate Orbiter Akatsuki have revealed key processes in the venusian atmosphere related to atmospheric and water loss, ozone formation, temperature structure, and magnetoprotection (see, e.g., Markiewicz et al. 2007). Venus’s atmospheric chemistry shows many intriguing features, including unexpectedly efficient recom-
bination of photolyzed carbon dioxide, and thus little formation of abiotic oxygen. Further, Venus’s slow surface rotation and atmospheric superrotation provide clues to processes that may drive other high-irradiation worlds or enhance habitability on tidally locked worlds. Looking into the future, there is much to be learned not only about Earth, but also about exoplanets, from Venus.
Ocean Worlds: Comparing Surface and Subsurface Oceans
Some of the most exciting recent work in the ocean worlds community has come from comparing these bodies to each other. In addition to their surface differences, the available Galileo and Cassini data sets allow for comparisons to be made between ocean worlds based upon their internal structure. Despite similar sizes, comparisons place the large, icy, ocean moons along continua defined by ocean depth and pressure effects (low-density Enceladus to high-pressure Ganymede), activity (from fully resurfaced Europa to heavily cratered Callisto), or differentiation (from fully differentiated Europa to partially differentiated Titan) (Schubert et al. 2010; Iess et al.
The discovery of potential plumes on Europa has driven comparisons with Enceladus, a moon for which the ocean composition is better known because of its regular plume expression (Figure 3.4). Studies of both of these potentially habitable environments complement each other. While the differences between these bodies are significant, confirmation of modern serpentinizing conditions on Enceladus, despite its lower tidal and radiogenic heating, suggests that Europa could host similar processes. Moreover, Cassini demonstrated that the ocean composition and putative habitability of Enceladus could be measured using a combination of mass and dust spectroscopy of particles emitted from the planet (e.g., Postberg et al. 2006; Waite et al. 2017). This has informed the exploration strategy for Europa and driven improvements to the instruments selected for Europa Clipper, including the ability to measure the europan atmosphere and sample potential plume ejecta in the search for longer-chain carbon molecules that may provide insight into biological versus abiotic processes (Pappalardo et al. 2018). In addition, follow-on missions to Enceladus have proposed using the improved Europa Clipper approaches to attempt life detection at Enceladus using high-resolution mass spectrometry of plume samples (Lunine et al. 2015; Lunine 2018).
The recent emphasis on comparing moons has resulted in community prioritization of a mission to the neptunian rather than the uranian system, as Triton has more indications of geologic activity and potential habitability than any of the uranian moons (OPAG 2018). Furthermore, comparative moon studies have enabled stronger bridges between the planetary and Earth oceanography communities, as progress in understanding the habitability of ocean worlds hinges on comparisons with terrestrial processes such as serpentinization (e.g., MacDonald and Fyfe 1985; Mayhew et al. 2013; Vance et al. 2016), hydrothermal vents and ocean pH (e.g., Hsu et al. 2015; Glein et al. 2015), and circulation and ice-ocean interactions (e.g., Craven et al. 2009; Vance and Goodman 2009; Soderlund et al. 2014). This has enabled parallel technology development for instrumentation and strategies for measuring and interpreting signals from these worlds.
Accessing the subsurface of the ocean worlds is a challenge unlike any yet encountered on another celestial body. Today, this challenge is being explored in Earth’s analog polar regions. Development of underwater and through-ice capability on a wide range of platforms, including for Earth’s poles, is enabling scientific advancement and driving instrument and sampling development while bridging technological gaps in communication, navigation, and autonomous integration. The development path relies heavily on integrating science and engineering seamlessly to optimize the handling of challenging environments while achieving critical science goals. Looking into the future, however, similar expeditions could explore deep within the icy ocean shell, within a water cavity, or in the ocean below.
Comparative Planetology within the Exoplanet Populations
The wealth of newly discovered exoplanets has allowed statistically significant comparisons between populations of exoplanets, which has challenged existing models and expanded understanding of planet formation and evolution beyond the planets in our solar system. These new data have revealed unexpected processes, such as migration, that may have been key mechanisms for volatile delivery during Earth’s formation. Comparisons between size, density, and distance from the parent star have informed atmospheric escape processes (Lopez and Fortney 2014; Owen and Mohanty 2016) and improved understanding of which planets are more likely to be rocky (Rogers 2015; Weiss and Marcy 2014). Statistical studies of Kepler’s M-dwarf population suggest that multiplanet systems may be very common, with many small planets in the habitable zone (Ballard and Johnson 2016; Gillon et al. 2017).
While early studies of exoplanets focused on detection, and the recent plethora of planets has informed demographics, exoplanet science is moving into an era in which rocky exoplanets are being initially characterized. Theoretical models of rocky planet formation and evolution, including the maintenance or loss of habitability and volatile transport within forming planetary systems, will soon be confronted with new data on the atmospheric compositions of a number of potentially habitable worlds, leading to theoretical refinements incorporating those observations.
The Kepler mission revolutionized knowledge of exoplanet demographics and has helped to determine the frequency of potentially habitable worlds around Sun-like and low-mass stars (G-, K-, and M-dwarf stars).
Kepler’s data and recent discoveries of nearby habitable-zone worlds using ground-based observations (Muirhead et al. 2015; Anglada-Escudé et al. 2016; Gillon et al. 2016, 2017; Dittmann et al. 2016) have ushered in a new era of comparative planetology for habitable-zone planets. Radii, masses, and densities are now being measured for habitable-zone planets around nearby M-dwarf stars (Gillon et al. 2017; Grimm et al. 2018; Dittmann et al. 2016). These planets are likely rocky, although the densities (Grimm et al. 2018) and corroborating evidence of migration (Luger et al. 2017) suggest that these planets may be more volatile rich than rocky planets in the solar system. These planets also likely underwent significant atmospheric evolution due to the long, early brightness of M-dwarf stars, a phase that is much shorter for G-dwarf stars like the Sun. As a result, they may have lost oceans (Luger and Barnes 2015) and acquired oxygen- or carbon dioxide-dominated atmospheres (Meadows et al. 2018). The first attempts at probing the atmospheric composition of these small habitable-zone planets—with Hubble Space Telescope, Spitzer, and ground-based telescopes (Delrez et al. 2018; deWit et al. 2018; Southworth et al. 2017)—have provided broad constraints that can only rule out hydrogen-dominated atmospheres, although future technologies will have the ability to do more.
Additionally, with increasing technological capacity, exomoons and exorings provide constraints on exoplanet formation mechanisms as well as the bombardment history of planetary systems and the potential delivery of volatiles (Heller 2017). Current transit timing and duration techniques have provided potential methods for exomoon and exoring detections (e.g., Kipping et al. 2012; Arnold and Schneider 2004; Teachey et al. 2018) that will be employable with upcoming space- and ground-based transit programs.
Solar System and Exoplanet Synergies in Comparative Planetology
Recent acquisition of observational constraints on the atmospheres of habitable-zone exoplanets opens a new interdisciplinary field of comparative planetology for habitable-zone planets. The potential for growth is large and requires expanded collaboration between scientists studying bodies in the solar system and those who observe and model exoplanets. Interdisciplinary, crossdivisional collaborations between NASA-supported planetary scientists and exoplanet astronomers can be facilitated by research coordination networks (see the section “Programmatic Challenges and Opportunities Related to the Search for Life in the Universe” in Chapter 5).
Observations of the distant Earth from interplanetary spacecraft have informed understanding of remotely detectable signs of habitability and life and observing strategies for future exoplanet characterization telescopes (Lustig-Yaeger et al. 2017). Additionally, exoplanet science has greatly expanded the known diversity of planet types and planetary architectures and has provided insights into planet formation, migration, and evolution that have directly impacted understanding of the solar system’s early evolution in dynamics and composition (e.g., Walsh et al. 2011). In turn, the solar system provides information on planetary processes for nearby targets that can be studied in more detail than will be possible for exoplanets. In the near term, such processes include tidal forces on Jupiter’s moons as analogs for tidal heating in exoplanet systems (Jackson et al. 2008; Heller et al. 2014) and analog geometries that inform exoplanet observing techniques, such as Titan solar occultations as analogs for exoplanet transmission spectroscopy (Robinson et al. 2014).
Shared themes between solar system small bodies and exoplanet studies are also providing synergistic links between planetary and exoplanet scientists. Studies comparing water and its isotopes, gases, and organic material between comets, asteroids, the Kuiper belt, and the scattered disk reveal how such materials incorporate into planets during formation or are delivered later. Achievements made by ESA’s Rosetta mission at comet 67P/Churyumov-Gerasimenko informed the structure and evolution of comets and their dust and ice materials. Further, the mission encountered black, radiation-resistant organic polymers on the surface of the comet and the presence of a large variety of molecules, including amino acids and even a sugar-precursor (Goesmann et al. 2015).
The Dawn mission to asteroid 1 Ceres detected ice in surface exposures (Combe et al. 2016), in the global crust (Prettyman et al. 2016), and within specific geological features, including landslides and impact ejecta (Schmidt et al. 2017). Moreover, relatively high albedo features within the floor of Occator crater (e.g., Krohn et al. 2016; Scully et al. 2018; Bowling et al. 2018) were suggested to be consistent with the presence of hydrated salts (De Sanctis et al. 2016). While it is unlikely there is a modern ocean, a volatile-rich crust and deeper mantle could have undergone active water-rock reactions early in its history or during recent impact events.
Together, these findings suggest that ongoing small-body processes may have also impacted the veneer of materials, including volatiles and organics, delivered to Earth and other planets in the solar system. The influence of solar system architecture on small-body inventories and how it might impact volatile and organic delivery has bearing on studies of planetary systems. Together with observations of debris disks and protoplanetary disks, which represent the evolution between scattered planetesimals and evolving planetary systems, such observations help constrain which exoplanets may be volatile-rich and how those volatile inventories have evolved.
The solar system planetary community can also provide atmospheric, surface, and interior models as well as a systems science approach to interpreting data from terrestrial exoplanets (Meadows et al. 2018). Earth can continue to serve as an analog for habitable exoplanets, and understanding the environments of Earth through time provides a series of examples of alien habitable environments for which there are geochemical and biological constraints. Venus also has a role to play in understanding of biosignature interpretation, because its carbon dioxide and oxygen catalytic chemistry can inform our understanding and models of the likelihood of false positive biosignatures on worlds that experience high irradiation. Meanwhile, Titan may serve as an analog for exoplanets with hazy atmospheres (Trainer et al. 2018). Understanding the evolution of the solar system, with its G-dwarf host star, will serve as a key comparison for what will be learned about M-dwarf planet evolution from upcoming exoplanet observations.
Finding: Comparative planetology between the solar system and exoplanetary systems is
- a powerful approach to understanding the processes and properties that impact planetary habitability;
- essential to inform experiments, modeling, and mission planning in astrobiology; and
- fundamentally crossdivisional and requires collaboration between astrobiologists, planetary scientists, and exoplanet astronomers and is therefore ideally suited to a research coordination network.
Statistical Methods for Comparative Planetology
A wide range of problems in astrobiology have benefited from recent progress in statistical methods. Planetary evolution is a complex set of nested processes, operating on different timescales, that combine to produce the end state of the planet. Statistical methods for comparative planetology allow information to be extracted from large or complex data sets, and probabilistic approaches can be utilized to assess how the likely scenarios may unfold, and to evaluate likely end points (Bean et al. 2017). Such approaches are providing a common theme across planetary science, astrophysics, heliophysics, biosignatures, Earth science, and stellar astronomy. This interdisciplinarity is contributing to astrobiology moving more effectively toward understanding complex systems, be they life itself, an ecosystem of multiple interdependent species and environmental conditions, or a planet with complex geological and atmospheric cycles with input from the host star. Statistical methods provide a fundamental capability that allows astrobiologists to compare and collate diverse data into a framework that enable testing of hypotheses and model development within and across disciplines (e.g., Clanton and Gaudi 2017; Schwartz and Cowan 2015). Such tools include but are not limited to Monte Carlo methods that sweep through solution space; cluster analysis that assesses the relationship of data points to each other; nearest neighbor assessments, which weight the importance of close versus distant data values; Bayesian frameworks, which describe the conditional dependencies of various inputs upon each other; and network theory, which represents nodes and relationships in maps of data. For habitability, these approaches may offer the chance to integrate wide-ranging types of data into a single model as well as to assess the relative importance of various elements into the end result. For studies of biosignatures (see Chapter 4), these approaches can produce information about associations between genes or organisms. For biomarkers, such techniques can assess the relative value of various biomarkers within a given system or determine whether a measurement of nonterran materials contains information consistent with life—for example, by determining if a methane lake on Titan contains evidence for nonwater-based life (Box 3.1).
Finding: Techniques based on statistical methods, scaling laws, information theory, and probabilistic approaches are useful in other branches of science and are increasingly being applied in the search for life.
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