2

The State of the Field of Exoplanets

Although it was not the first detected exoplanet (see Box 2.1), the discovery of a planetary companion to the near solar analogue 51 Pegasi by Mayor and Queloz in 1995 launched the field of exoplanets. The discovery of 51 Peg b, which has a minimum mass of roughly 0.5 times the mass of Jupiter (M_J) but an orbital period of only about 4 days, surprised many. This is because the then-popular planet formation model predicted that such planets could not form in situ (e.g., Lin et al., 1996). Indeed, this is still the prevailing wisdom, and thus the discovery of 51 Peg b led to the realization that at least some fraction of exoplanets undergo large-scale migration from their birthplaces. The discovery of 51 Peg b heralded a general principle that has since held in the exoplanet field—namely, that planetary systems are remarkably diverse, and to “expect the unexpected.”

Since the discovery of 51 Peg b, many thousands of exoplanets have been discovered via many different techniques (see Figure 2.1). A few notable milestones include the discovery of the first transiting planet, HD 209456b (Charbonneau et al., 2000; Henry et al., 2000); the discovery of the first exoplanet via transits, OGLE-TR-56b (whose photometric signal was first identified by Udalski et al., 2002, and whose planetary nature was confirmed via radial velocities by Konacki et al., 2003); the discovery of the first exoplanet via microlensing, OGLE 2003-BLG-235/MOA 2003-BLG-53Lb (Bond et al., 2003); and the discovery of the first directly imaged planetary system around HR 8799 (Marois et al., 2008). The field took another great leap forward with the launch of the National Aeronautics and Space Administration (NASA) mission Kepler (Borucki et al., 2010), which brought the study of exoplanets into the statistical age.

METHODS OF DETECTING AND CHARACTERIZING EXOPLANETS: APPLICATIONS, BIASES, AND LIMITATIONS

By essentially every physical measure, planets are exceptionally diminutive, in particular in comparison to their host stars. This is the primary reason that it was not until nearly the end of the 20th century that the first definitive detections of exoplanets were made. It is useful to recall the relevant orders of magnitude that are involved. Considering Jupiter analogues to Sun-like stars, the size ratio is roughly 1 to 10, the mass ratio is roughly 1 to 1000,

and the visible-light flux ratio is roughly 3 parts in a billion. For Earth analogues to Sun-like stars, the size ratio is roughly 1 to 100, the mass ratio is roughly 1 to 300,000, and the visible-light flux ratio is roughly 1 part in 2 billion.

Since nearly all detection methods rely on either the indirect influence of the exoplanet on its parent star or the direct detection of the planet in the vicinity of its parent star, these ratios make the detection of planets even as large as Jupiter incredibly difficult.

In this section the committee briefly reviews the primary methods that have been used to detect and characterize exoplanets. The goals of this section are to outline the regions of exoplanet and host star parameter space in which these methods are most sensitive, and to highlight the intrinsic biases and limitations of each method, which ultimately lead to the requirement that the full arsenal of methods needs to be used to obtain as complete a picture of the demographics and characteristics of planets as possible. In Appendix C, the committee describes these methods in somewhat more physical and mathematical detail for the interested reader.

Radial Velocity

The radial velocity (RV), or Doppler technique, is an indirect method that relies on measuring the Doppler shift of the star as it orbits the center of mass of the planet/star system. The amplitude of the Doppler shift is of order 10 m/s and the period is approximately 12 years for a Jupiter analogue orbiting a Sun-like star. For an Earth analogue the amplitude is roughly 10 cm/s and the period is roughly 1 year. For a “hot Jupiter” orbiting a Sun-like star with a period of 3 days, the amplitude is typically 150 m/s. Current state-of-the-art instruments, telescopes, and data reduction methods yield Doppler precisions of roughly 1 m/s on mature, Sun-like stars. However, Earth imparts a signal of only 9 cm/s on the Sun, and thus additional instrumentation, observations, and analysis methods are needed to detect Earth analogues.

The relatively weak function of the velocity amplitude on the planet period makes the RV method able to detect planets over a broad range of parameter space. The reflex velocity of a star depends linearly on the planet mass, and decreases with increasing stellar mass and orbital period. Because of the weak dependence on orbital period, the RV technique is able to detect planets in orbits from a few days to several years, with the upper limit on the detectable period set not only by the radial velocity precision but also by the time baseline. For example, a Neptune-mass planet in a circular orbit that is viewed edge-on will induce a stellar velocity of 1.5 m/s in a 1-year orbit, and 0.95 m/s in a 4-year orbit. However, it is important to note that RV observations provide little detailed information about the detected planet itself, as RV observations alone provide only the lower limit on the mass of the planet, and a subset of the full Keplerian orbital parameters (period, eccentricity, and argument of periastron).

Finally, high-resolution spectroscopy is increasingly being used as a method for measuring exoplanet atmospheres with transmission spectroscopy and high-contrast imaging techniques. This work requires the same very stable spectrographs that precision RVs require.

Transits

Planets in nearly edge-on orbits can be detected via transits when the planet passes in front of the face of the star, resulting in a relatively brief, periodic, shallow dip in the brightness of the star. For a hot Jupiter orbiting a solar-type star, the probability that the planet will transit is 10 percent, the depth of the transit is 1 percent, and the transit lasts for a few hours. For an Earth analogue, the transit probability is roughly 0.5 percent, the depth is roughly 0.01 percent, and the duration is 10 hours, corresponding to a duty cycle of only 0.15 percent. Transits of Jupiter-size planets on relatively short orbits (roughly less than 10 days) around Sun-like stars can be detected from the ground, whereas detecting transits of Earth analogues requires a dedicated space mission, which was the primary motivation of NASA’s Kepler space telescope. Roughly Earth-size planets on shorter period orbits transiting smaller stars, such as potentially habitable planets orbiting low-mass stars (or M dwarfs), can and have been detected from the ground.

The detection of transits of a planet alone yields the period of the orbit, the radius of the planet (given a measurement of the radius of the star), and the density of the star (Seager and Mallen-Ornelas, 2003). However, when combined with the RV detection of the planet, it is also possible to determine the planetary mass and density, which is the first step in determining its basic nature.

It is also possible to detect the existence of additional, nontransiting planets in transiting systems by searching for transit timing variations (TTVs). The detection of TTVs in multiple-planet systems also enables the measurement of the masses of the planets without measuring the reflex RV they induce on their parent star. However, it is important to note that the detectability of TTV signals depends on the mass of the planetary perturber, and the proximity of the perturber to mean motion resonances. Therefore, TTVs are not a uniform or unbiased method of measuring exoplanet masses.

Transiting planets also present the opportunity to study the planetary atmosphere without the need to spatially resolve the planet from the star: atoms, molecules, and aerosols induce wavelength-dependent absorption of starlight as it passes through the planetary atmosphere during transit. Observing these effects (by comparing stellar spectra in and out of transit) allows astronomers to deduce the atmospheric composition; this method is called “transmission spectroscopy.” Similarly, observations gathered during secondary eclipse (when the planet is out of view) can be subtracted from observations when both the star and planet are in view, to deduce the hemisphere-integrated planetary emission spectrum; this technique is termed “emission spectroscopy,” or “secondary-eclipse spectroscopy.”

Direct Imaging

Detecting exoplanets via direct imaging requires resolving the light of an orbiting exoplanet from its parent star. The challenges of direct imaging are generally the large flux ratio between the planet and the star, and the small angular separation between the planet and the much brighter host star. Thus, planets with larger flux ratios and larger angular separations are the easiest to detect.

In all cases where planets have been directly imaged to date, the detections are of young (<300 million years), giant (>2 M_J), self-luminous planets whose luminosity and temperature are powered by released gravitational potential energy from formation, resulting in near-infrared planet-to-star flux ratios as large as one part in 10⁶ to 10⁴. Flux ratios of this order can be detected via combinations of large ground-based telescopes, advanced adaptive optics, coronagraphy, and sophisticated image processing. However, since young stars are almost inevitably moderately distant from Earth, with distances to the closest young star associations typically 150 parsecs (pc) (500 light-years), the necessary sensitivity is only possible for angular separations beyond a few tenths of an arcsecond from the parent star. Hence almost all the imaged planets are giant planets orbiting at separations of 20 times the distance from Earth to the Sun (also referred to as an astronomical unit, or AU), or further. As these planets are directly detected, it is also possible to obtain their spectra, which allows for inferences about their atmospheric composition.

The flux ratios of mature planets that are in thermal equilibrium with their host stars are typically much smaller. An Earth analogue orbiting a Sun-like star at a distance from Earth of 10 pc has a flux ratio in reflected light of order 10^–10, and the maximum angular separation is roughly 0.1 arcseconds. For a Jupiter analogue in the same system, the flux ratio is several times larger, although the separation is five times larger. Directly detecting such systems requires sophisticated techniques to suppress the light from the star. Two promising techniques, which are described in detail later, are internal coronagraphs and external occulters (starshades).

Detecting mature planets in thermal emission is easier in terms of flux ratio than detecting them in reflected light, as the flux ratios are generally more favorable by roughly four orders of magnitude. However, the technical challenges of imaging planets in thermal emission are greater. This is because the typically small separation of planets from their host stars, combined with the large diffraction limit of monolithic telescopes of reasonable size at thermal infrared wavelengths, prevent resolving most planets at these wavelengths with traditional telescopes. As a result, detecting and characterizing planets in the thermal infrared, except for those possibly around the Sun’s nearest neighbors, requires long baseline interferometry, which has even greater technical challenges. Thus, detecting mature planets in reflected light is currently believed to be the more straightforward path.

Direct detection allows a measurement of the planet’s spectrum, or flux as a function of wavelength. Spectra are powerful probes of a planet atmosphere’s composition (molecules) and structure (temperature-pressure profile). Transit spectroscopy (see the section “Transits,” earlier in this chapter, and Figure 2.8, later) samples a small fraction of the planet’s atmosphere at relatively low pressures, and is strongly subject to the veiling effect of hazes and clouds. Direct resolved spectroscopy, on the other hand, probes either reflected light or thermal emission coming nearly directly from the entire hemisphere of the planet, and thus probes much deeper into the atmosphere of the planet down to much higher pressures, and is relatively immune to clouds and hazes (Morley et al., 2015) and potentially able to study the planetary surface. High-resolution spectroscopy can also provide direct measurements of a planet’s spin, and enable Doppler mapping.

The ultimate goal of facilities that are designed to measure the reflected or thermal emission of Earth analogues is to search for signatures of habitability, and perhaps even find biosignatures, aspects of a planet’s atmosphere or surface that provide evidence of inhabitance. This is discussed in more detail later in this chapter, in the section “The Search for Life on Exoplanets.”

Microlensing

Microlensing is an indirect method of detecting exoplanets that is primarily useful for the statistical determination of the demographics of exoplanets over a broad range of host star and exoplanet parameter space. Microlensing surveys are primarily useful for the statistical determination of exoplanet demographics because the planetary systems detected by microlensing are typically at distances of many kiloparsecs, and thus it is generally not possible to characterize individual planets detected by microlensing surveys.

Microlensing uses the gravitational bending of light from a background star by a foreground planet and its host to magnify the background star. This magnification, which can last from a few days to hundreds of days, is called a “microlensing event.” Planetary companions to the foreground host are detected through perturbations of this microlensing event. It is important to note, however, that an isolated planet can also magnify a background star, and thus microlensing is sensitive to free-floating planets.

Microlensing is most sensitive to planets with separations of a few AU, although with a space-based mission, the sensitivity can span a much broader range of separations. In principle, microlensing is sensitive to planets with masses as low as a few lunar masses.

Because of the low probability of detecting a stellar-mass microlensing event, and the lower probability of detecting the planetary perturbation even in the case that the microlensing event is detected and assuming the planetary companion exists, microlensing surveys for exoplanets generally require continuously monitoring hundreds of millions of stars on daily time scales to detect the microlensing events, and then monitoring the known microlensing events on hourly to daily time scales to detect the planetary perturbations.

Planets detected via microlensing measure the mass ratio between the planet and the star, and the instantaneous projected separation between the planet and host star in units of the Einstein ring radius. Fortunately, as described in detail in Chapter 4, in the section “Expanding the Statistical Census of Exoplanets,” a space-based microlensing survey, such as that which will be carried out by the Wide-Field Infrared Survey Telescope (WFIRST), will enable the routine measurement of the mass of both the host star and the planet. This is because the approximately 10-fold increase in angular resolution of WFIRST compared to typical ground-based seeing will enable the measurement of several higher-order effects that will break the usual microlensing degeneracy, in which one measures only a degenerate combination of the mass and distance to the host star, and relative proper motion of the host lens and source.

Astrometry

Astrometry detects planets by measuring the reflex motion of the star in the plane of the sky. For an Earth analogue at a distance of 10 pc, the astrometric signal is roughly 0.3 microarcsecond, which is well below any realistic astrometric accuracy achievable from the ground. Furthermore, the sensitivity of astrometry to planets with periods longer than the duration of the astrometric survey drops precipitously (e.g., Casertano et al., 2008).

The European Space Agency Gaia satellite is the first space-based astrometric mission with the sensitivity to detect exoplanets. While Gaia will have the most exquisite astrometric accuracy to date (<10 microarcseconds for stars with V < 12), it will nevertheless generally be sensitive to planets with masses only several times the mass of Jupiter at separations of less than roughly 3 AU for a Sun-like host.

WHAT HAS BEEN LEARNED ABOUT EXOPLANETS IN THE PAST 30 YEARS?

The next few sections describe what has been learned about exoplanets, both their demographics and their properties, using the various detection and characterization techniques briefly mentioned above. The most important of these are as follows:

• Planetary systems are ubiquitous and surprisingly diverse, and many bear no resemblance to the Solar System.
• A significant fraction of planets appear to have undergone large-scale migration from their birthsites.
• Most stars have planets, and small planets are abundant.
• Large numbers of rocky planets have been identified and a few habitable-zone examples orbiting nearby small stars have been found.
• Massive young Jovians at large separations have been imaged.
• Molecules and clouds in the atmospheres of large exoplanets have been detected.
• The identification of potential false positives and negatives for atmospheric biosignatures has improved the biosignature observing strategy and interpretation framework.

The Demographics of Exoplanets

The first step to understanding the complex processes involved in planet formation is the determination of a statistical census of the demographics of exoplanets over as broad a range of planet and host-star properties as possible. Understanding how planets form is not only interesting in its own right, it informs understanding of the prevalence of potentially habitable worlds—for example, by providing clues to the dominant processes of water delivery to rocky planets in the habitable zones of their parent stars. Formation models that aim to understand the demographics of mature planetary systems need to ultimately start from realistic physical conditions, and therefore be informed by observations of protoplanetary and debris disks.

Radial Velocities

Precise RV measurements benefit from the longest time baselines of exoplanet discovery methods (Mayor and Queloz, 1995). Continued improvement in Doppler precision as well as long-term stability have permitted the coherent linking of data sets across various instrumental platforms and over time. Dedicated ground-based efforts from spectrographs operating on both small telescopes and large telescopes have revealed a rich diversity of exoplanets through their mass and orbital properties (Marcy and Butler, 2000). Despite being an indirect method that targets only one star at a time, increasing Doppler sensitivity from 100 to 1 m/s has enabled the RV technique to quantify the statistical occurrence of planets at small to moderate orbital radii and to correlate the results as a function of host star properties.

The primary results from the myriad of radial velocity surveys are as follows:

• There exists a (largely unforeseen) population of short-period “hot Jupiters” and paucity of brown dwarfs located close to their parent stars (e.g., Wright et al., 2012).
• Gas giant planets have been detected with semimajor axes comparable to Jupiter and beyond. For example, HD 75784 b orbits at a = 6.6 AU (Giguere et al., 2015).
• Short-period giant planets are more commonly found orbiting stars with high metallicity (Gonzalez, 1997; Santos et al., 2004; Fischer and Valenti, 2005).

• The occurrence of giant planets tends to increase with orbital period, modulo a “pile-up” of “hot Jupiters” around 3-day orbital periods (Cumming et al., 2008). This pile-up appears to occur only for metal-rich stars, which also preferentially host high-eccentricity giants interior to 1 AU, likely implying that giant planets in close orbits have been emplaced via planet-planet interactions in systems capable of forming multiple giants (Dawson and Murray-Clay, 2013).
• Low-mass planets at short orbital periods of less than 50 days are more common than giant planets (Howard et al., 2010; Mayor et al., 2011). These results foreshadowed the Kepler result, which extended down to smaller (less massive) planets.
• Relatively short-period giant planets are less common around M dwarfs (see Figure 2.2; Johnson et al., 2010, and also Figure 4 of Wang and Fischer, 2015).
• Earth-mass planets in the habitable zones of the nearest M dwarfs have been discovered (e.g., Anglada-Escudé et al., 2016; Bonfils et al., 2018). These planets are ripe targets for atmospheric characterization with future instruments.

Transits

The transit method has become the most productive exoplanet detection technique since the last decadal survey. NASA’s Kepler mission played a key role in this revolution by performing a sensitive survey of exoplanets that spanned a wide region of parameter space. Owing to the large number of stars it monitored, the exquisite relative

photometric precision it achieved on those stars, and the fact that it monitored the same set of stars for nearly 4 years, Kepler was sensitive to planets as small as Mercury in orbits of a few days (e.g., Barclay et al., 2013), and planets just over the size of Earth for periods of slightly less than 1 year (e.g, Thompson et al., 2018).

The primary results to emerge from Kepler’s survey are as follows:

• Neptune-size planets (with radius less than four Earth radii, or R_p < 4 R_E) in close-in orbits (much less than 1 AU) are common around Sun-like stars (Howard et al., 2012), confirming the result of Howard et al. (2010) and Mayor et al. (2011) and extending it to smaller masses.
• Roughly half of all stars in the galaxy have planets intermediate in size between Earth and Neptune that orbit closer than Mercury does to the Sun (Dressing and Charbonneau, 2015; Burke et al., 2015).
• There exist two distinct populations of small planets (see Figure 2.3; Fulton et al., 2017; Van Eylen et al., 2018). A planet radius gap (sometimes colloquially referred to as the “Fulton Gap”) separates planets that are gas rich and thus have larger sizes from likely rocky planets with thin or nonexistent atmospheres that have smaller sizes. The distribution of planets in terms of size and irradiation received supports the hypothesis that mass loss from photoevaporation sculpts the population of highly irradiated planets. This result also suggests that atmospheres of mini-Neptune-size planets are predominantly composed of nebular gas accreted from the protoplanetary disk, rather than heavier elements from outgassing or sublimation (Owen and Wu, 2017).
• There is approximately one habitable-zone terrestrial planet (1-1.5 R_E) for every 4 M-dwarf stars (Dressing and Charbonneau, 2015). For Sun-like stars, there is at least one habitable-zone terrestrial planet (0.8-1.2 R_E) for every 10 stars (Burke et al., 2015), although this estimate is based on an extrapolation and significant uncertainties remain.

The constraints on the frequency of terrestrial planets in the habitable zones of Sun-like stars are less certain than the corresponding planet frequency of such planets orbiting M dwarfs. This is due to several factors, including (1) the fact that the signal of a terrestrial planet transiting an M dwarf is much larger than the signal of a terrestrial planet transiting a Sun-like star, and (2) the fact that planets in the habitable zones of low-mass stars (M dwarfs)

have shorter periods. These factors, when combined with the fact that the duration of the primary Kepler mission was only roughly 3.5 years, and the higher than expected stellar noise of the Kepler targets, made it much easier to detect terrestrial planets in the habitable zones of M stars than Sun-like stars, and thus the estimates of the frequency of such planets is much more reliable. Nevertheless, researchers have high confidence that terrestrial planets orbiting in the habitable zones of Sun-like stars are relatively commonplace (Burke et al., 2015).

Therefore, for the first time, researchers are in a position to design the next set of experiments aimed at characterizing these kinds of planets to determine their atmospheric compositions and temperatures and to search them for signs of life.

The next step to characterizing the atmospheres of potentially Earth-like planets is to find such objects around the nearest stars. The search for characterizable Earth analogues around G dwarfs remains work for the future and so is discussed later in this report. However, the search for characterizable Earth analogues around M dwarfs is already under way and yielding important successes.

Ground-based transit searches focused exclusively on mid- to late M dwarfs—for example, MEarth, Transiting Planets and Planetesimals Small Telescope (TRAPPIST)/Search for Habitable Planets Eclipsing Ultra-Cool Stars—are an important complement to broad surveys like Kepler and the recently launched Transiting Exoplanet Survey Satellite (TESS) mission (see the section “Planet Discoveries Through Transits,” in Chapter 4). As an example, such surveys have already found two compelling systems of habitable-zone, Earth-size planets for followup characterization. The two systems are TRAPPIST-1 (Gillon et al., 2017) and LHS1140 (Dittmann et al., 2017). TRAPPIST-1 is a system of seven small transiting planets, with three of them in and near the habitable zone (see Figure 2.4). The TRAPPIST-1 planets show a surprising range of densities, and thus a range of volatile and gas fractions that indicate a complex history of planet formation. LHS1140 hosts two transiting planets with densities consistent with an Earth-like composition, one of which lies in the habitable zone. Both the TRAPPIST-1 and

LHS1140 systems are expected to be touchstones for exoplanet characterization studies with the James Webb Space Telescope (JWST) due to their favorable planet-to-star radius ratios and infrared bright host stars. The relative ease with which these planets were detected also suggests that many more of these systems are waiting to be discovered.

Direct Imaging

The earlier generation of young-planet direct imaging surveys was primarily sensitive to massive (>5 M_J) planets or those in very wide (>100 AU) orbits. Relatively few detections were made from these surveys, implying an occurrence rate of 0.001-0.013 planets per star with parameters in the range of 5-13 M_J and 30-300 AU (Bowler, 2016). However, detections by the current generation Gemini Planet Imager and Spectro-Polarimetric High-Contrast Exoplanet Research (SPHERE) instruments indicate that the occurrence rate of giant planets may increase somewhat at smaller separations, perhaps rising, as expected, to meet the Doppler occurrence rate at 5-10 AU.

Completion of the Gemini and SPHERE surveys should help narrow these constraints. There are several important caveats in any of these statistics. Young-planet imaging is very sensitive to the very steep mass-luminosity relationship for the planets, such that a significant population of Jovian-mass planets would be nearly undetectable. Second, the mass-luminosity relationship is very sensitive to initial entropy of formation (Marley et al., 2007; Spiegel and Burrows, 2012), which in turn is sensitive to their formation pathway; planets that accrete most of their mass through a narrow shock—which would happen in the classic Jupiter formation scenario—may have much lower initial luminosities and be nearly undetectable with current technology. The imaged giant planets with dynamical mass constraints (Beta Pictoris b and HR8799bcde) are inconsistent with low-luminosity formation. It is possible that distinct formation mechanisms operate for giant planets in wide (>10 AU) orbits (Toomre, 1964; Boss, 1998; Gammie, 2001; Rafikov, 2005; Kratter et al., 2010).

Microlensing

The primary strength of the microlensing technique is to determine the demographics of exoplanets, particularly those beyond the snow (or ice) line, the location in the protoplanetary disks where water ice is stable. Since it is generally believed that terrestrial planets that formed in the habitable zones of their parent stars largely formed without significant amounts of water, it is of great interest to understand how water (or volatiles in general) may have been delivered to terrestrial planets in the habitable zone.

To date, ground-based surveys for planets beyond the snow line have reached the following conclusions (most of which are summarized graphically in Figure 2.5):

• For cold planets orbiting M dwarfs, low-mass planets (with the mass of roughly that of Neptune or lower) are much more common than giant planets (Gould et al., 2006).
• The mass function of cold planets orbiting M dwarfs with planet/star mass ratio greater than roughly 2 × 10^–4 (similar to the mass ratio between Neptune and the Sun) is significantly steeper than the mass function of warm planets in the same mass-ratio regime orbiting solar-type stars.
• Giant planets (with mass greater than 30 M_E) are not rare among M dwarfs (Clanton and Gaudi, 2014, 2016), at least for long orbital periods, although they are rarer than giant planets in the same mass range orbiting solar-type stars.
• The frequency of planets with the mass of Jupiter or greater orbiting M dwarfs found by microlensing is roughly 3 percent, consistent with that found by radial velocity surveys of M dwarfs for planets with separations of greater than a few AU (Clanton and Gaudi, 2014).
• Among cold planets orbiting M dwarfs, Neptune-mass objects may be most common (although super-Earth-mass planets may be comparably abundant; see Figure 2.5). Even so, the absolute abundance of cold super-Earths is substantial and similar to the frequency of the best known super-Earth population, warm super-Earths orbiting solar-type stars.

Disk Properties

The last decade has seen remarkable progress in elucidating the average and range of properties of disks through both imaging and spectroscopy. The number of spatially resolved disks has increased from a handful to hundreds, thanks to a combination of scattered light observations from the Hubble Space Telescope (HST) and large, adaptively corrected ground-based telescopes, mid-infrared emission resolved with the ground-based telescopes, Spitzer, the Herschel Space Telescope, and far-infrared emission from the Atacama Large Millimeter/submillimeter Array (ALMA). Properties such as surface density and composition can now be probed within disks, and structures can be related to planet formation processes.

ALMA surveys of nearby star-forming regions confirm previously known results with much better fidelity and are teasing out the details of the physical and chemical characteristics of disks. Some important recent results that set the time scale and composition relevant to planet formation are the following:

• Disk mass correlates with stellar mass, albeit with substantial scatter at any given stellar mass (e.g., Andrews et al., 2013; Barenfeld et al., 2016; Pascucci et al., 2016; Ansdell et al., 2017).
• Disk dissipation occurs quickly, so that by roughly 3 Myr, half of stars do not have measurable dust emission (e.g., Haisch et al., 2001; Fedele et al., 2010; Ribas et al., 2014; Richert et al., 2018).

• The initial growth of solids, from the roughly 0.1 micron size characteristic of the interstellar medium to the 1 cm size and larger, occurs quickly, probably while the star is still accreting from its primordial envelope (Testi et al., 2014). Associated theoretical work has shown that planetesimals can form quickly following grain growth with turbulent dust concentration and self-gravity (see review by Johansen et al., 2014).
• Even at young ages (<3 Myr), disk masses as estimated from dust and gas tracers such as CO are lower than the Miminum Mass Solar Nebula (Ansdell et al., 2016; Eisner et al., 2018); most solids must already be in planetesimals or planets and gas must evolve rapidly.

ALMA, now in its fifth annual cycle of investigator-driven observations, has produced many novel results. In particular, ALMA observations have demonstrated that

• There exists a prevalence of gaps and other substructures at a wide range of separations in disks, as was first demonstrated by the striking observations by ALMA of HL Tau (see Figure 2.6). Whether these gaps can be attributed to planets, ice condensation fronts, dust trapping, or combinations of these and other mechanisms is not yet known.
• Observations of tracer molecules provide direct evidence for the water ice snow line (e.g., Qi et al., 2013).

• Complex organic chemical pathways are operating in disks, as inferred for the Solar System (e.g., Oberg et al., 2015).
• Disks inherit the isotopic ratios from molecular clouds in which they were born (Hily-Blant et al., 2017), and also demonstrate their changing chemistry due to stellar and interstellar irradiation (Cleeves et al., 2017).
• Outer disks have low turbulence (Flaherty et al., 2017, 2018); this is at odds with the magnetorotational instability mechanism that has been successful in explaining accretion rates of inner disks onto the star.

There are likely many more advances to come from ALMA spectroscopy of gas and imaging of dust continuum. Observations of both dust and gas indicate that pressure bumps in disks can be created and influence the migration of solids; the buildup of icy or dry pebbles may determine if a planet is a water world or a rocky terrestrial planet. Scattered light observations of disks, from HST and large ground-based telescopes, provide complementary information on the disk structure by tracing small grains, so that greater constraints may be placed on the dynamics induced by any young planets (e.g., Debes et al., 2017; Avenhaus et al., 2018).

From Disks to Planets

Population synthesis models try to tie what researchers know about the initial conditions of planet formation, as deduced from studies of circumstellar disks described in more detail below, with the observed exoplanet demographics. There have been many attempts at such synthesis, and they generally serve to highlight the major unknowns. Examples include the following:

• The recent idea of “pebble accretion” enables rapid growth of gas giant cores (e.g., Ormel and Klahr, 2010; Lambrechts and Johansen, 2012) but also makes the growth rates subject to many unknown disk properties such as the dominant sizes of solids as a function of distance from the star, turbulence, evolving gas-to-dust ratio, and feedback in the disk pressure structure between the forming planets and the disk gas.
• The rate of migration of gap-opening planets (Type II migration) is uncertain due to the uncertainty in how the local disk mass changes self-consistently around a migrating planet (Lin and Papaloizou, 1986; Durmann and Kley, 2017).
• The rate of disk dispersal and the radial profile of disk dispersal change the migration rates of planets (Alexander and Pascucci, 2012; Ercolano and Rosotti, 2015; Wise and Dodson-Robinson, 2018; Jennings et al., 2018).
• The physics of mass accretion onto a giant planet determines how fast the planet can build up mass and feeds back into migration rate (Alessi and Pudritz, 2018).
• Dynamical instabilities late in planet formation may change the final distribution of planets (Carrera et al., 2018).

Debris disk observations reveal structures such as rings, warps, and asymmetries that highlight the complicated dynamics of young planetary systems and the importance of collisions between large planetesimals in the first 100 Myr of planetary evolution (Hughes et al., 2018). Many more debris disks have been found with gas; some planetesimals that survive to the debris disk phase are extremely volatile rich (e.g., Moór et al., 2017; Kral et al., 2017). While debris disks around M stars are rare or have very low surface densities, the complex variation of the AU Mic disk suggests that stellar winds are very important in driving disk dynamics (Boccaletti et al., 2015); such winds could also have profound impacts on close-in planets.

Disks around mature stars cannot be primordial but rather need to be generated by the evaporation and collisions of planetesimals. Their presence signals that planet formation proceeded at least to the point of making planetesimal belts.

• The dust mass in debris disks decays with time, such that detectable disks are much more common around

• stars with ages <100 Myr than around stars of solar age, but roughly 20 percent of field stars appear to host cold disks more massive than that of the solar system (Montesinos et al., 2016).
• Most debris disks are composed of cold rings, and therefore some dynamical process removed planetesimals from some portions of the circumstellar environment but left or shepherded belts in others.
• There is no evidence for a strong correlation between the presence of a certain type of disk and the type or location of inner planets (Moro-Martín et al., 2015; Wittenmyer and Marshall, 2015; Meshkat et al., 2017), although there are some exceptions (e.g Dawson et al., 2011; Wilner et al., 2018).
• It appears that most debris disks with warm inner dust also have cold outer dust, but the physical connection between the two is unclear (Ertel et al., 2018).

At the time of the last decadal survey in astronomy, uncertainty in the amount and distribution of exozodiacal dust was considered a key issue in planning a future direct imaging mission. Much progress has been made on this topic (see review by Roberge et al., 2012) from Spitzer, the Wide-Field Infrared Survey Explorer, the Keck Interferometer, and, most recently and sensitively, the Large Binocular Telescope Interferometer. Sun-like stars with no known cold dust rarely have dust in their habitable zones, with an upper limit of ~26 zodis (Ertel et al., 2018).

Exoplanet Atmospheres and Interiors

Polluted White Dwarfs

One of the only constraints on the compositions of extrasolar planetesimals comes from observations of white dwarfs with metal lines created by the accretion of planetary material that survived the post-main sequence evolution of the star. A major conclusion of many studies is that extrasolar planetesimals have compositions similar to the Solar System’s terrestrial planets (see the review of Zuckerman and Young, 2017).

Bulk Composition and Interior Structure from Transits

Exoplanet bulk compositions can be probed by comparing measurements of planetary masses and radii to theoretical models. Figure 2.7 shows the data for the 418 transiting exoplanets with fractional measurement errors less than 20 percent in both mass and radius, the vast majority of which are on close-in orbits (a < 0.1 AU) and orbit mature stars. Key results from this data set are as follows:

• The observed diversity of planetary radii for a given mass generally implies significant variations in bulk composition.
• Most close-in Jovian-mass planets have radii larger than Jupiter, in some cases twice that of Jupiter. These so-called inflated hot Jupiters need to be hydrogen-dominated objects like Jupiter and Saturn. The mechanism by which these planets have been “inflated” has eluded a definitive explanation (Thorngren and Fortney, 2018), although it is clear that the high levels of irradiation that most of these planets receive from their host stars need to play a role.
• The existence of Jovian-mass objects that lie below the hydrogen-helium model curve implies that some giant exoplanets need to have enhancements of heavy elements relative to Jupiter and Saturn.
• The variations in the sizes of transiting Jovians with more moderate levels of irradiation than the majority of the sample indicates that the planetary heavy element abundances are correlated with the metallicities of the stellar hosts (Thorngren et al., 2016).

The tendency of lower mass planets to have smaller sizes indicates that the trend of increasing heavy element abundance for lower mass planets seen in the Solar System also roughly holds for exoplanets. These intermediate-size planets are likely composed of some combination of iron, rock, ice, and gas, with the precise ratios unknown due to the existence of inherent degeneracies in interior structure models when only the mass and radius are known (Adams et al., 2008). It is thought that observational constraints on atmospheric composition can be used to break

these degeneracies (Miller-Ricci et al., 2009), but attempts to date have been stymied by the difficulty of getting precise atmospheric metallicities for planets in this regime. The committee anticipates that JWST will yield a transformative breakthrough in this area.

Definitively low-mass (mass less than 10 M_E) planets have been identified at increasing rates in recent years. From the systems discovered to date, it is known that

• Truly terrestrial (“rocky”) planets have compositions that are remarkably consistent with a precisely Earthlike composition (e.g., Dressing et al., 2015), although it remains to be seen if this is an artifact of small sample sizes and large uncertainties.
• There is an unexpected apparent overlap between terrestrial super-Earths and volatile- or gas-rich mini-Neptunes in the mass range 1-10 M_E (Hadden and Lithwick, 2017); this discovery suggests that there is not a single threshold mass at which planets can efficiently accrete gas, and that low-mass planets are not guaranteed to be rocky.
• There appears to be a critical size below which planets are likely mostly rocky. Mass-radius data indicate that this turning point is around 1.6 R_E (Rogers, 2015), which is consistent with the location of the radius gap described above, although there is significant evidence that this is not a sharp transition (Wolfgang et al., 2016). The frequency of small planet sizes and the distribution of mass-radius values together provide a path for homing in on rocky planets, using the measurement of planetary radii (Lopez and Fortney, 2014).

It should be emphasized that the mass-radius diagram in the small planet regime is highly biased toward planets on very short period orbits and the more massive versions of planets for a given size because these are the easiest planets to detect, and the data set is highly heterogeneous in terms of relative precision on the measurements, host star type, semimajor axis, and so on. Further exploration of the mass-radius relationship for small planets that accounts for these complexities using discoveries from NASA’s recently launched TESS mission discoveries is highly anticipated.

Exoplanet Atmospheres from Time-Series Techniques

The geometry of transiting planets also permits observations of their atmospheres (see Figures 2.8 and 2.9). During planetary transits the so-called transmission spectrum is measured by determining how the apparent size of the planet varies with wavelength. The combined light from a planetary system also includes the reflected (typically dominant in the optical) and emitted light (typically dominant in the infrared) from a planet. This can be probed just around secondary eclipse to measure a planet’s dayside spectrum as it disappears behind its host star, or during larger fractions of the planet’s orbit to measure its phase curve. Combined light measurements of planetary spectra can be obtained for nontransiting planets using the same time-series techniques, although the absolute flux is unknown without observations of a secondary eclipse. Transmission spectra can only be obtained for transiting planets.

Exoplanet atmosphere observations with time-series techniques have been very successful over the last two decades, despite the fact that none of the instruments that were used was designed specifically for these measurements. Such observations are challenging because planetary signals are 10^–3 that of their host stars or smaller. Systematics typically dominate the raw data at this level, and thus stringent control of these systematics via careful calibration is an essential requirement in extracting reliable inferences. Signals down to the 10^–4 level have been

detected, and signals at the 10^–5 level will be sought moving forward. The current state of the field is that it is strong and poised for major discoveries with upcoming facilities.

Successful exoplanet atmosphere observations using time-series techniques have been made with a wide range of instruments, both ground- and space-based observations, and over wavelengths ranging from X-rays to the mid-infrared. These observations have led to the detection of many gas-phase chemical species (e.g., H₂O, CO, Na, K, H, and He) and constraints on temperatures as a function of pressure, longitude, and latitude (for a recent review, see Deming and Seager, 2017). These results have led to a wide range of findings about the composition, chemistry, and physics of exoplanet atmospheres. Some highlights include the following:

• Determination of water abundances as a tracer of the underlying elemental abundances in numerous exoplanets, and the connection of this marker to planet formation (e.g., Kreidberg et al., 2014);
• The discovery that aerosols are prevalent in exoplanet atmospheres (e.g., Sing et al., 2016; see Figure 2.7, earlier);
• The mapping of exoplanet temperatures with phase curve observations and the resulting constraints on energy transport in highly irradiated atmospheres (e.g., Knutson et al., 2007);
• Observations of planetary evaporation (e.g., Vidal-Madjar et al., 2003); and
• The advance of the high-dispersion spectroscopy technique as a unique ground-based tool for detecting exoplanet atmospheres and probing their compositions and dynamics (e.g., Snellen et al., 2010).

Similar to the overarching picture that has emerged from the mass-radius diagram of exoplanets, the overarching picture of exoplanet atmospheres is that they display a surprising variety of chemical compositions, thermal structures, energy budgets, and heat transport efficiencies. Therefore, it has been challenging to discern the low-order trends in atmospheric properties as a function of planetary properties (e.g., atmospheric metallicity versus planet mass) that are expected from theory. It is difficult to say at this point how much intrinsic planetary variation, limited measurement accuracy and precision, modeling degeneracy, and the inherent complexities of atmospheres each play a role in this lack of clarity. What is clear is that the expanded capabilities of upcoming facilities like JWST and the giant segmented mirror telescopes (GSMTs; see the section “Ground-Based Studies,” in Chapter 4) will bring transformative data to bear on these issues.

Exoplanet Atmospheres from Direct Imaging

As discussed above, current direct imaging techniques are limited to young, self-luminous, giant planets. However, since they are spatially resolved from their parent star, it is relatively straightforward to obtain near-infrared spectra of these planets. These spectra have led to the following results and conclusions:

• The spectra of these young giant planets resemble similar but higher-mass brown dwarfs. Indeed, they show the same overall features—water and carbon monoxide at higher temperatures (Konopacky et al., 2013) and methane features at lower temperatures (Macintosh et al., 2015).
• Interpretation of these spectra, as with transit spectroscopy, is model dependent; in particular, the emergent spectrum is very sensitive to properties of the thermally emitting cloud layer in the planet’s atmosphere (Figure 2.10).
• These clouds have distinct properties from the high-mass brown dwarfs, in that they persist to lower (<700 K) temperatures.
• The probed photospheres are significantly out of chemical equilibrium, likely due to vertical circulation. In turn this makes extraction of elemental abundances challenging.
• Planets are consistent with Sun-like to somewhat enhanced carbon-to-oxygen ratios (Barman et al., 2015) and solar to somewhat enhanced overall metallicity (Rajan et al., 2017), although no clear trend with mass or separation can be detected.
• Planetary rotation periods have been determined for several planets, and are similar to that of Jupiter (Snellen et al., 2014; Bryan et al., 2018).

Higher-resolution spectra and improved models should allow researchers to determine if giant planet composition varies from the inner solar systems probed by transit techniques to the outer wide-orbit planets seen in imaging.

Moderate-resolution (R = 50-100) spectroscopy of mature giant planets, combined with atmospheric-retrieval modeling, can determine atmospheric molecular abundances and provide insight into formation processes (Lupu et al., 2016). For example, planets that formed through global disk instabilities may have stellar-like abundances, although it should be noted that post-formation accretion of planetesimals may substantially change the elemental abundance ratios even for these objects (Thorngren et al., 2016). Planets that have migrated may retain element ratios indicative of their original formation zone (e.g., Oberg et al., 2011). Validating these techniques by practicing first on giant planets will also increase confidence in their application to Earth-like worlds.

The Search for Life on Exoplanets

Impact of Host Star Properties

The vast majority of the photons observed in exoplanetary systems originate from the host star, and not from the planet. In fact, the most fruitful planet detection techniques to date, and for the immediate future, are the transit and RV methods, both of which are indirect detections of exoplanets as observed through the influence of the planet on the host star. Thus, the ultimate precision and accuracy with which planet properties can be measured is governed by the precision and accuracy with which the properties of host stars can be measured.

The RV and transit exoplanet detection methods rely on measuring or inferring the mass or radius of the host star in order to estimate the mass and radius of the planet. Up until quite recently, measuring the mass or radius of an (effectively) isolated star has been either difficult or impossible. However, with the launch of Gaia, which provides both exquisite parallaxes of bright stars (roughly 10 microarcsecond precision for V < 12) along with all-sky surveys that have obtained broadband absolute photometry of stars over a broad wavelength range covering the majority of their spectral energy distributions (SEDs) from the near ultraviolet to the near infrared, it has now become possible to estimate the mass and radius of relatively bright (V < 12) stars with low-mass transiting companions (e.g., single-lined spectroscopic binaries) with excellent precision (Stevens et al., 2018). The broadband photometry, combined with stellar atmosphere models, can be used to estimate the (unextincted) bolometric flux. Combined with an estimate of the stellar effective temperature from the SED or high-resolution spectra, it is possible to measure the radius of the stars essentially empirically to sub-1 percent levels (Stevens et al., 2018). The transit constrains the density of the star to exquisite precision (Seager and Mallen-Ornelas, 2003), and thus both the mass and radius of the host star and their transiting planets can be directly constrained (Stassun et al., 2017; Stevens et al., 2018).

Intrinsic stellar variability, including the effects of stellar flaring and modulated surface heterogeneities, can have detrimental effects on measuring the mass of a planet with RVs and its radius if it transits its star. The same fact is true across the entire wavelength range, although the impact is generally greater at shorter wavelengths, ranging from a 50 percent overestimate in radii of giant planets orbiting Sun-like stars in the X-ray (Llama and Shkolnik, 2015), to a few percent in the infrared for small planets orbing M dwarfs (Rackham et al., 2018).

As discussed, there is great emphasis in detecting the planet’s atmosphere and measuring its composition through transmission and emission spectroscopy. As these techniques are applied to smaller and smaller planets around more and more active stars, such as M dwarfs, imperfect knowledge of the central star grows in importance, as it often becomes the limiting factor in the detection of small planets and, if detected, in the precision with which the planet’s mass, radius, and, thus, its density and spectrum can be measured.

In addition to the challenges that stellar variability and heterogeneities produce for the accurate measurement of planet spectra, the high-energy radiation (10-300 nm) from the upper stellar atmosphere—namely, the chromosphere, transition region, and corona—directly alters the temperature and chemistry of the planet’s atmosphere of all types of exoplanets, from Earth’s to Jupiter’s (Figure 2.11). For terrestrial planets, including those in the habitable zones of low-mass stars, this radiation has the potential to strip the planet atmosphere completely, or to generate hazes, two scenarios that result in a flat transmission spectrum of a rocky planet. Should the planet

atmosphere survive and be haze free, the ultraviolet stellar light can alter the planet’s photochemistry in several ways, potentially creating false positive and even false negative biosignatures.

Astrobiology

In the last 10 years, exoplanet astrobiology has transformed from a field driven by promising statistical predictions into one with nearby targets accessible to near-term observation. Over this time, astrobiology research that supports the search for life on exoplanets has advanced in two major areas: (1) the development of exoplanet habitability assessment as an interdisciplinary multiparameter concept including planetary and stellar properties and planet-star-planetary system interactions, and (2) in enhancing confidence in biosignature assessment for more reliable biosignatures like O₂, while proposing new exoplanet biosignatures that may broaden the search. A biosignature is a global impact of life on its planetary environment that may be detectable at interstellar distances, and so likely requires a surface ocean supporting a surface biosphere (Des Marais et al., 2002; Schwieterman et al., 2018). This detectability requirement also drives the definition of a habitable zone for exoplanets to be that region around a star where a planet with an Earth-like atmosphere can maintain liquid water on its surface (Kasting et al., 1993; Barnes et al., 2009; Kopparapu et al., 2013).

Biosignature science has recently advanced three major new areas of research:

• An in-depth evaluation of O₂ as a potential biosignature that identified additional gases and other environmental context to strengthen researchers’ ability to discriminate between abiotic and biological production of O₂. This has greatly increased the ability to identify O₂ a reliable indicator of life.
• Building on the detailed treatment of O₂, the community has taken the first steps toward the development

• of a comprehensive framework that can be used to interpret other potential biosignatures in the context of their environment, and similarly increase confidence that they are indeed due to life.
• The search for and identification of new potential biosignatures, especially those that may permit detection of non-Earth-like metabolisms.

From Demographics to Habitable-Zone Targets

The Kepler mission has revolutionized knowledge of the frequency of potentially habitable worlds, particularly around low-mass stars. Recent discoveries of nearby potentially habitable planets using ground-based observations (Anglada-Escudé et al., 2016; Gillon et al., 2016, 2017; Dittmann et al., 2017) have ushered in a new era of comparative planetology for potentially habitable-zone (HZ) planets. A bevy of resources are being trained on these planets in order to measure their detailed properties, including radii and masses (Gillon et al., 2017; Dittman et al., 2017; Grimm et al., 2018), to determine if they are rocky.

Whether M dwarfs can host habitable planets is an interesting question that is still open for debate (Shields et al., 2016; Kaltenegger, 2017). In particular, the long superluminous pre-main-sequence phase of M dwarf hosts may drive initial strong atmospheric and ocean loss (Ramirez and Kaltenegger, 2014; Luger and Barnes, 2015), potentially resulting in oxygen- or carbon dioxide-dominated atmospheres for planets in the habitable zone (Luger and Barnes, 2015; Tian, 2015; Meadows et al., 2018a). Initial attempts to probe the atmospheric composition of small habitable-zone planets with the Hubble Space Telescope, the Spitzer Space Telescope, and ground-based telescopes (Southworth et al., 2017; Delrez et al., 2018; deWit et al., 2018) have provided only broad constraints that rule out hydrogen-dominated atmospheres. In the next 5-10 years, TESS will find more nearby transiting HZ planets (Sullivan et al., 2015; Barclay et al., 2018), and JWST and the GSMTs will search for and characterize high-molecular-weight atmospheres, providing the first glimpses into the atmospheres of potentially habitable planets. These new observations will enable the first, albeit challenging, search for signs of life on nearby worlds, and test researchers’ understanding and models of habitable planetary environments and processes.

What Makes a Planet Habitable?

In parallel with the advances in observations, the exoplanet, Solar System, and astrobiology communities have generated a more comprehensive picture of planetary habitability. Unlike Solar System worlds where subsurface and sparsely inhabited environments (e.g., those that only have low levels of microbial life) could be probed by in situ spacecraft, for exoplanets this report considers only global or large-scale surface habitability that is potentially accessible to telescopic remote sensing.

Many factors and interactions are now expected to impact planetary habitability. These include the following:

• The presence and distribution of liquid water oceans on the planetary surface, which depends on initial volatile delivery, outgassing, and retention against ocean loss.
• The presence of a stable secondary atmosphere. It is believed that potentially habitable planets need to lose most or all of their primary H₂-dominated atmospheres (Pierrehumbert and Gaidos, 2011; Owen and Mohanty, 2016).
• The presence of tectonic or volcanic activity and weathering processes to replenish atmospheric loss (Lenardic et al., 2016), and buffer climate (Walker et al., 1981).
• The internal energy budget of a planet, which can be derived from the energy of formation and radionuclides (Frank et al., 2014; Unterborn et al., 2015; Dorn et al., 2018; Young et al., 2018), or from tidal energy deposition from gravitational interaction with the host stars or other planets (Jackson et al., 2008; Driscoll and Barnes, 2015).
• The presence and strength of a global-scale magnetic field, which depends on interior composition and thermal evolution (Driscoll and Bercovici, 2013).

There are important feedbacks identified between the processes listed above, which have been studied in limited cases. For example, the persistence of a secondary atmosphere over billion-year time scales requires low atmospheric loss rates, which in turn can be aided by the presence of a planetary magnetic field (Driscoll and Bercovici, 2013; Garcia-Sage et al., 2017; Dong et al., 2018).

Multiparameter Habitability Assessment

To support upcoming observations, and target selection for searches for life in particular, the astrobiology and planetary science communities have worked to identify characteristics and processes that enable planetary habitability and embrace a more interdisciplinary assessment framework. As useful as the habitable zone’s first-order assessment of potential habitability has been for identifying that region around a star where a planet is more likely to be habitable (Kasting et al., 1993; Kopparapu et al., 2013), it is now understood that many factors, including those listed above, impact a planet’s habitability. Consequently, efforts have begun to synthesize knowledge and observations from many different fields to provide a more comprehensive and powerful assessment of the likelihood of exoplanet habitability, thereby improving the ability to pick the best targets to search for life.

For example, whether M-dwarf habitable-zone planets are indeed habitable is a key current question in astrobiology, with strong ramifications for the distribution of life in the galaxy, including in the Solar System neighborhood, due to the ubiquity of M dwarfs. M-dwarf planets may face significant impediments to habitability due to their energetic host stars, with perhaps the most significant challenge being the potential early loss of ocean and atmosphere due to stellar X-ray and extreme ultraviolet radiation (Ribas et al., 2016; Luger and Barnes, 2015; Schaefer et al., 2016), and the stellar wind (Dong et al., 2017; Garcia-Sage et al., 2017). Conversely, even failure to lose a dense primordial (H₂-dominated) atmosphere may inhibit or preclude their habitability (Owen and Mohanty, 2016). Stellar UV and proton flux drive chemistry that continues to modify the planetary atmospheric composition (Segura et al., 2003, 2005; Rugheimer et al., 2015), which in turn impacts the planetary climate and surface UV protection (Segura et al., 2010; Rugheimer and Kaltenegger, 2018; Arney et al., 2017; Tilley et al., 2018), two important aspects of habitability.

Toward a Comprehensive Framework for Biosignature Assessment

Researchers’ ability to search for life is constrained by the ability to recognize life’s impact on its global environment, a biosignature. The presence of abundant O₂, or the simultaneous observed presence of O₂ (or O₃) and CH₄ (or N₂O) have traditionally been considered the highest priority biosignatures to search for, for reasons of both reliability and (relative) observability (see Appendix D for a summary of known biosignatures, their spectral features, and supporting observations for interpretation). These pairs of gases imply an atmosphere out of balance from chemical equilibrium, and they can be strong absorbers across the visible or infrared wavelengths.

However, the last 5 years have brought a rapid evolution in researchers’ understanding of the complexity of biosignature interpretation and an impetus toward more rigorous standards of proof. Rather than being isolated phenomena, biosignatures are now understood to be highly influenced by the local planetary system environment, in which geological, atmospheric, and stellar processes interact and evolve to enhance, suppress, or sometimes mimic biosignatures. Consequently, the interpretation of the significance of the potential biosignature, rather than its measurement, is the most important process in life detection.

The field has so far focused on increasing the ability to interpret O₂ as a biosignature, and used geological constraints from the early Earth as well as photochemical and climate models to identify both false negatives (environmental processes that suppress the biological signal; e.g., Reinhard et al., 2017) and false positives (abiotic planetary processes that can mimic biosignatures) for O₂. These include several mechanisms for abiotic production of O₂ and O₃, for planets within the habitable zone (see Meadows, 2017, for a review). A summary of key components of this information is presented in Figure 2.12. An understanding of potential false negatives can inform the choice of environments for life searches that are least likely to suppress the biological signal, and knowledge of false positives can guide a comprehensive measurement strategy to rule them out and increase the credibility of the biosignature interpretation.

Theoretical research into false positive mechanisms has also helped identify the potentially observable signals of abiotic production of O₂ or O₃ that might leave a detectable impact on the environment, or that result from characteristics of the parent star and the planetary environment. False positive discriminants so far identified include the presence of collisionally induced absorption from O₂ molecules that collide more frequently in dense, O₂-rich post-ocean-loss atmospheres (Schwieterman et al., 2016; Meadows et al., 2018b); CO from the photolysis of CO₂ (Schwieterman et al., 2016); lack of water vapor (Gao et al., 2015); lack of collisionally induced absorption from N₂ (Schwieterman et al., 2015); and the absence of reducing gases like CH₄ (Domagal-Goldman et al., 2014; Meadows et al., 2018b). Biosignature interpretation therefore requires a suite of observations, rather than detection of a single molecule. Similarly, other characteristics that may enhance the interpretation of an atmospheric gas as a biosignature are also being considered and revisited, including additional biologically driven disequilibria (Krissansen-Totton et al., 2016), and secondary characteristics of a photosynthetic biosphere, such as surface reflectivity (e.g., the “red edge” of vegetation; Gates, 1965; Seager et al., 2005) or seasonal variations (Schwieterman et al., 2018; Olson et al., 2018).

Using the treatment of O₂ as a template, the community has started the development of a comprehensive framework that can be used to interpret other potential biosignatures in the context of their environment, and similarly increase confidence that they are indeed due to life, and not abiotic planetary processes. A comprehensive series of papers that reviewed the current state of the field and outlined this new assessment framework was published as part of a Nexus for Exoplanet Systems Science community-wide biosignatures workshop activity (Kiang et al., 2018; Schwieterman et al., 2018; Meadows et al., 2018a; Catling et al., 2018; Walker et al., 2018; Fujii et al., 2018).

Identifying Novel Biosignatures

While traditional biosignatures are the highest priority for biosignature searches, recent research continues to identify new ideas for remote-sensing biosignatures, to potentially maximize the chances of recognizing alien biospheres. These newly proposed biosignatures include new atmospheric, surface, and seasonal biosignatures (see Schwieterman et al., 2018, for a recent review), and advances in a new area of research called “agnostic biosignatures.” Research into the relative detectability of these newly proposed biosignatures is ongoing. Agnostic biosignatures are particularly exciting, as they are not identified as tied to a particular metabolism, but manifest as unexpected complexity in a system-wide alteration of a planetary environment, such as in its atmospheric chemistry (Walker et al., 2018). They may be the best chance of searching for non-Earth-like life. Novel promising potential biosignatures include the following:

• Seasonality in gas abundances, which may produce large features in the UV (Olson et al., 2018);
• The formation of hazes in anoxic environments, due to methanogenic production of CH₄ well above that expected from geological processes (Arney et al., 2018);
• The presence of ammonia on hydrogen-dominated worlds (Seager and Bains, 2015);
• The presence of an ocean, which may be detected in direct imaging, for a habitable world with an N₂/O₂ atmosphere (Krissansen-Totton et al., 2016); and
• The simultaneous presence of N₂, CH₄, CO₂, and liquid water for an early-Earth-type environment (Krissansen-Totton et al., 2018).

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