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Exoplanet Biosignatures: Observational Prospects

Yuka Fujii, Daniel Angerhausen, Russell Deitrick, Shawn Domagal-Goldman, John Lee Grenfell, Yasunori Hori, Stephen R. Kane, Enric Palle, Heike Rauer, Nicholas Siegler, Karl Stapelfeldt, Kevin B. Stevenson

arXiv:1705.07098v3astro-ph.EP

TL;DR

The paper asks how future observations can characterize temperate terrestrial exoplanets and evaluate biosignatures using planetary context. It reviews observational methods and mission capabilities through the 2030s and beyond, concluding that near-term work will focus on a few nearby late-type-star targets while broader biosignature surveys require dedicated space missions.

  • Problem

    The paper addresses how to characterize temperate terrestrial exoplanets and interpret biosignature candidates together with broader planetary properties.

  • Method

    The paper reviews future missions, observational techniques, and their capabilities and limitations for astrophysical, chemical, climatological, and astrobiological characterization.

  • Results

    Near-term chemical and astrobiological characterization will concentrate on a few nearby late-type-star planets, while solar-type-star biosignature studies will require space-based flagship missions.

  • Takeaways & Limitations

    Initial characterization of a few nearby targets will provide a touchstone for more detailed scrutiny and larger surveys beyond 2030.

  • Takeaways & Limitations

    Atmospheric signals can be comparable to or smaller than existing noise floors, and several techniques are infeasible or strongly constrained for solar-type-star systems.

Abstract

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Exoplanet hunting efforts have revealed the prevalence of exotic worlds with diverse properties, including Earth-sized bodies, which has fueled our endeavor to search for life beyond the Solar System. Accumulating experiences in astrophysical, chemical, and climatological characterization of uninhabitable planets are paving the way to characterization of potentially habitable planets. In this paper, we review our possibilities and limitations in characterizing temperate terrestrial planets with future observational capabilities through 2030s and beyond, as a basis of a broad range of discussions on how to advance "astrobiology" with exoplanets. We discuss the observability of not only the proposed biosignature candidates themselves, but also of more general planetary properties that provide circumstantial evidence, since the evaluation of any biosignature candidate relies on their context. Characterization of temperate Earth-size planets in the coming years will focus on those around nearby late-type stars. JWST and later 30 meter-class ground-based telescopes will empower their chemical investigations. Spectroscopic studies of potentially habitable planets around solar-type stars will likely require a designated spacecraft mission for direct imaging, leveraging technologies that are already being developed and tested as part of the WFIRST mission. Successful initial characterization of a few nearby targets will be an important touchstone toward a more detailed scrutiny and a larger survey that are envisioned beyond 2030. The broad outlook this paper presents may help develop new observational techniques to detect relevant features as well as frameworks to diagnose planets based on the observables.

1. Introduction

The paper reviews how future observations can characterize potentially habitable exoplanets and assess biosignatures in planetary context. It focuses on observational capabilities, limitations, and coordinated progress toward astrobiological characterization.

  • Exoplanet science progress: Exoplanet discoveries and characterization methods have expanded rapidly, revealing diverse worlds and improving demographic analyses.Relevant approaches include radial velocity, transit, microlensing, transit spectroscopy, and direct imaging.
  • Biosignature scope: Theoretical work and observations are broadening biosignature searches from atmospheric molecules to biological surfaces and temporal variability.Candidate atmospheric molecules include O2, O3, CH4, N2O, and CH3Cl.
  • Future observations: Future facilities will be assessed for their ability to constrain planetary properties and observationally relevant biosignatures.The paper presents planned telescopes, their capabilities, expected uses, and limitations without prioritizing projects.
  • Scope and definitions: Potentially habitable exoplanets are defined as terrestrial planets in their stars’ habitable zones.The terrestrial criterion emphasizes a well-defined surface without a voluminous gaseous envelope.
  • Roadmap: The paper organizes progress from astrophysical toward chemical, climatological, and astrobiological characterization through future missions.It also places projects and future work on a timeline extending beyond 2030.

2. From Astrophysical Characterization to Astrobiological Characterization

Exoplanet characterization is progressing from discovering and measuring planetary properties toward chemical, climatological, and astrobiological investigation. Near-term work centers on chemical studies of nearby targets, while broader biosignature surveys require future space missions.

  • Characterization eras: The field is moving through astrophysical, chemical/climatological, and astrobiological eras of exoplanet characterization.The paper treats these as broad observational eras rather than a strictly monotonic sequence for individual planets.
  • Astrophysical characterization: Improved detection techniques have extended astrophysical characterization from initially biased samples toward planets with potentially habitable conditions.Kepler and subsequent ground-based surveys contributed to this expansion.
  • Chemical and climatological characterization: Transmission, eclipse, and phase-curve spectroscopy provide complementary information about atmospheric composition, dayside emission, and atmospheric heterogeneity.These methods mark the transition toward chemical and climatological characterization.
  • Chemical surveys: FINESSE and ARIEL planned chemical surveys of 500 and 1000 transiting planets, respectively, in the 2020s.Their atmospheric-composition data are intended to inform planetary-system formation histories.
  • Astrobiological characterization: JWST and ELTs are expected to initiate astrobiological characterization, but likely only for a few planets around cooler M-type stars.These stars offer larger transit depths and more favorable planet-to-star contrasts than Sun-like systems.
  • Future flagship missions: A space-based flagship mission with biosignature detection as a major design driver will likely be needed for the golden age of astrobiological characterization.Candidate concepts include HabEx, LUVOIR, and OST, with direct imaging and/or transit spectroscopy capabilities.

3. Characterizing Transiting Planets

For transiting potentially habitable planets, the paper reviews astrophysical, chemical, and climatological measurements and their sensitivity considerations. These methods include basic planetary properties and spectroscopic probes of atmospheres.

  • Measurement framework: Transiting-planet characterization covers radius, mass, and orbital elements as astrophysical properties.Chemical and climatological characterization is treated through transmission and eclipse spectroscopy.
  • Spectroscopic methods: Transmission spectroscopy probes atmospheric composition through wavelength-dependent transit depths caused by opacity and scattering.Eclipse spectroscopy measures planetary dayside emissions, while phase curves probe atmospheric heterogeneity.
  • Sensitivity considerations: The paper evaluates each method together with its observational sensitivity and practical constraints.The discussion is specifically framed around potentially habitable transiting planets.

3.1. Astrophysical Characterization

Astrophysical characterization of transiting planets establishes their sizes, masses, orbits, and approximate interior properties. The available measurements depend strongly on the observing method, host star, and instrumental precision.

  • Radius: Transit depth primarily measures planetary radius relative to the host star, so stellar radius must be well constrained.Accurate planetary radii therefore depend on accurate stellar characterization.
  • Mass: Transiting-planet masses are measured mainly with radial velocity or transit-timing variations.RV benefits from the near-90° inclination of transiting systems, while TTV uses mass-dependent dynamical perturbations.
  • Mass sensitivity: Earth twins around G-type stars induce RV variations of about 10 cm/s, which are challenged by stellar jitter and instrumental noise.HZ Earth-sized planets around late-type stars produce larger RV amplitudes because their stars are less massive and their orbital distances are smaller.
  • Interior composition: Radius and mass together constrain whether planets are rocky, water-rich, or surrounded by thick atmospheric envelopes, although intermediate densities remain degenerate.Stellar composition can further inform rocky-material composition.
  • Orbital elements: Semi-major axis and eccentricity determine incident flux and its variation, and are constrained using transit timing, RV, TTV, or dynamical stability.Precise orbital ephemerides are important for efficient follow-up observations.
  • Survey and instrument development: Transit surveys and improved spectrographs expand the sample of small, nearby, and potentially habitable planets available for follow-up.TESS and CHEOPS target nearby transiting planets, while CHEOPS improves radii and densities for spectroscopic targets; PLATO extends surveys to a broader parameter space.

3.2 Chemical/Climatological Characterization: Transmission Spectroscopy

Transmission spectroscopy compares in-transit and out-of-transit spectra to probe atmospheric absorption and scattering, but detectability is limited by faint signals, observing time, and systematic noise. JWST and high-resolution ground-based spectroscopy will extend atmospheric studies of nearby temperate planets.

  • Method: Transmission spectroscopy compares out-of-transit and in-transit spectra to reveal absorption and scattering by planetary atmospheres.Earth’s transmission spectrum exhibits features from H2O, O2, O3, CO2, and CH4 on a Rayleigh-scattering slope.
  • Sensitivity: The signal depends on atmospheric scale height, mean molecular mass, surface gravity, and the relative sizes of the planet and host star.Hydrogen-rich atmospheres can amplify signals through increased scale heights, while stellar radius strongly affects the signal relative to stellar flux.
  • Limitations: Systematic noise can remain at tens of ppm, challenging atmospheric features expected at approximately 10 ppm or less and making bright nearby targets especially valuable.The cited noise floor is not reduced by co-adding HST and Spitzer observations.
  • Retrieval: Atmospheric abundance retrievals use molecular absorption depths to constrain spectrally active molecules and the Rayleigh slope to constrain inactive components.Higher-order absorption-band structure can further constrain mixing ratios when measured.
  • Atmospheric structure: Clouds, haze, and refraction can weaken or limit the atmospheric layers and molecular features accessible to transmission spectroscopy.Even tenuous clouds or haze can produce substantial optical depth along transit chords, while refraction imposes a lower-altitude sensitivity boundary.
  • Facilities: JWST will provide the first opportunity to characterize temperate terrestrial atmospheres, but requires tens of transits or tens to hundreds of hours of integration.For some nearby late-type systems, initial habitability signs or inconclusive biomarkers may be possible if systematic noise is sufficiently low.

3.3 Chemical/Climatological Characterization: Eclipse Spectroscopy

Eclipse spectroscopy measures the difference between out-of-eclipse and in-eclipse spectra to probe planetary thermal emission. Its interpretation depends on wavelength-dependent contrast, atmospheric temperature structure, clouds, and three-dimensional heterogeneity, while Earth-sized targets remain observationally demanding.

  • Method: Eclipse spectroscopy derives dayside emission by subtracting in-eclipse spectra from out-of-eclipse spectra.The method uses secondary eclipses, when the planet is occulted by its star.
  • Sensitivity: Eclipse spectroscopy is as demanding as transmission spectroscopy and is not feasible for planets around solar-type stars or in visible/near-infrared wavelengths when contrast is below 1 ppm.The practical prospects therefore favor mid-infrared space observations.
  • Spectral information: Eclipse spectroscopy works best around 8–30 µm, where planet-to-star contrast is larger while the planet remains sufficiently bright.Relevant molecular bands include O3, CO2, CH4, SO2, and N2O.
  • Interpretation: Thermal-emission features depend on both molecular abundances and the atmospheric temperature profile.Temperature decreases can produce absorption features, while thermal inversions can produce emission features; weak vertical gradients weaken molecular features.
  • Limitations: Retrieving atmospheric properties from thermal emission is complicated by clouds and three-dimensional heterogeneity and requires sophisticated models plus high-precision observations.The inverse problem is not straightforward even for cloud-free Earth-like atmospheres.
  • Spatial characterization: Mapping Earth-sized planetary daysides through eclipse ingress and egress would be exceedingly difficult because the planetary signal is weak.Two-dimensional eclipse maps have instead been demonstrated for hot Jupiters.
  • Facilities: JWST is the most promising near-future observatory for eclipse spectrophotometry, but detecting thermal-emission features requires a smaller-than-predicted noise floor.Mid-infrared observations are favored for potentially habitable planets.

4. Characterizing Planets with General Orbital Inclination

Non-transiting planets lack transit-specific techniques but are generally closer to Earth, benefiting other follow-up observations. Their characterization therefore relies on methods applicable across orbital inclinations.

  • Scope: Non-transiting planets are missed by techniques unique to transiting systems but are generally closer to Earth, benefiting other follow-up observations.The paper considers astrophysical, chemical, climatological, and high-contrast imaging approaches for these systems.

4.1. Astrophysical Characterization

Astrophysical characterization of non-transiting planets must address difficult radius, mass, and orbital constraints. Scattered-light measurements create radius–albedo degeneracies, radial velocities create mass–inclination degeneracies, and imaging or astrometry may provide additional constraints.

  • Radius: In visible and near-infrared scattered light, disk-integrated intensity is proportional to squared radius times planetary albedo, making radius generally degenerate with albedo.Mid-infrared thermal emission could better constrain radius, but no current projects are capable of those observations.
  • Mass: Radial velocities measure planetary mass multiplied by the sine of orbital inclination, so true mass requires an independent inclination constraint.Statistically, the expected true mass is 4/π times the measured m_p sin i value.
  • Mass: Astrometry detects the star’s periodic reflex motion across the sky, but Gaia is unlikely to detect temperate Earth-sized planets.Future astrometric capabilities for such planets are being considered in the context of LUVOIR-related studies.
  • Orbital elements: Radial velocities can constrain semi-major axis and eccentricity, while multi-epoch direct imaging can constrain orbits when radial velocities are unavailable.Orbital ephemerides also help schedule direct imaging near maximum angular separation.

4.2 Chemical/Climatological Characterization: Phase Curves

Phase curves can reveal atmospheric, surface, cloud, and gas properties by tracking orbital changes in planetary brightness and spectra. For temperate Earth-sized planets, thermal phase curves around nearby late-type stars are the most promising, but their small amplitudes demand strong instrumental and stellar stability.

  • Method and Sensitivity: Phase-dependent planetary spectra can be extracted as time-varying components synchronized with the orbital period, with larger amplitudes for edge-on systems.Transiting planets are therefore the most favorable targets, although phase curves can also be used for non-transiting planets.
  • Method and Sensitivity: Scattered-light phase curves from potentially habitable planets are below 1 ppm of stellar light and are unfeasible to detect.Thermal phase curves around late-type stars offer better star-to-planet contrast, but their contrast is still only about 10-100 ppm and their variation amplitude is smaller.
  • What can be studied?: Broadband thermal phase curves probe global heat redistribution and can constrain whether a planet has an atmosphere or surface flow.Atmosphere-less planets produce strong day-night thermal contrast, whereas thick atmospheres tend to redistribute heat more effectively.
  • What can be studied?: Thermal phase curves can trace large-scale cloud patterns, including substellar clouds that may indirectly indicate underlying surface liquid water.This signature is associated with synchronously rotating ocean-covered planets under strong irradiation.
  • What can be studied?: Spectrally resolved phase curves can imprint atmospheric molecular signatures because wavelength-dependent opacity changes the pressure levels being probed.Different pressure levels may exhibit different horizontal temperature patterns, producing variation spectra.
  • Opportunities through 2030: JWST may measure thermal phase curves down to temperate Earth-sized planets around nearby late-type stars.For Proxima Centauri b, the estimated thermal phase variation is about 10 ppm or less with an Earth-like atmosphere and about 100 ppm without an atmosphere, assuming 60 degree inclination.

4.3. Chemical/Climatological Characterization: High-Contrast Imaging

High-contrast imaging suppresses stellar glare to access planetary light across orbital inclinations, while coronagraphs, starshades, spectral separation, and polarimetry provide complementary detection strategies. Ground-based and space-based capabilities can reach different targets, but Earth-sized planets around solar-type stars require space-based observatories and several signatures remain observationally difficult.

  • Method and Sensitivity: Direct imaging can characterize planets at all orbital inclinations but must suppress stellar glare with coronagraphs or starshades.The inner working angle must be smaller than the planet-star angular separation, and accessible targets decrease at longer wavelengths for fixed instrument sizes.
  • Method and Sensitivity: Direct imaging is most effective in visible scattered light, where habitable planets have more accessible planetary-star contrast.The relevant contrast is determined by the faint planetary signal relative to stellar light and depends on planetary albedo.
  • Opportunities through 2030: Wavefront-controlled space coronagraphs have demonstrated 6 x 10^-10 contrast in vacuum tests and are being developed toward a 1x10^-10 requirement at 3 λ/D.WFIRST is planned to demonstrate the precision wavefront control needed for high-contrast imaging from space.
  • Method and Sensitivity: Combining high-contrast imaging with high-resolution spectroscopy may detect molecular signatures at ~10^-5-10^-4 contrast, improving the contrast requirement by a factor of 10^3.This combination is presented as a promising approach for ground-based observations of potentially habitable planets around late-type stars.
  • Method and Sensitivity: Polarization alone will likely be insufficient to detect potentially habitable planets, making combined direct-imaging and polarimetric instruments more likely to be useful.High-precision observations of hot Jupiter HD 189733 yielded no conclusive detection, with upper limits of tens of ppm.
  • What can be studied?: Ocean glint can raise planetary albedo at crescent phase, but the small star-planet separation and faint scattered light make direct imaging challenging.Observing this signature may require direct-imaging missions beyond 2030.
  • What can be studied?: Polarization measurements can distinguish atmospheric Rayleigh scattering, liquid-water reflection, and water-droplet scattering through phase-dependent peaks.Earthshine observations show Rayleigh scattering dominates disk-integrated polarized light at short wavelengths, while near-infrared continuum polarization decreases to ∼9-12%.
  • Opportunities through 2030: Ten-meter ground-based telescopes have achieved 10^-4–10^-6 contrast for Jupiter-sized planets, and high-resolution spectroscopy could extend such techniques to smaller planets.An Earth-like O2 absorption feature at 1.27 µm is predicted to be detectable for habitable planets found with high-contrast imaging.

4.4. Chemical/Climatological Characterization: Spectral Separation

Spectral separation methods can recover planetary atmospheric signals from star–planet spectra and, when successful, constrain both atmospheric composition and orbital properties. Their prospects differ by spectral resolution and observing platform, with ground-based high-resolution methods facing severe contrast challenges.

  • High-resolution spectroscopy: High-resolution spectra can identify Doppler-shifted planetary atmospheric lines against telluric and stellar lines while measuring the planet’s line-of-sight velocity.The method targets spectra with resolving power ℝ≳100,000.
  • Observational limitations: The method is expected to remain ground-based, while temperate Earth-sized planets present contrasts of roughly 10^-10–10^-6, far below the roughly 10^-3 contrast reached so far.This contrast gap is a central limitation for applying the technique to temperate terrestrial planets.
  • Alternative spectral regimes: Medium-resolution spectra may retain identifiable high-frequency features through theoretical-model fitting even when individual atmospheric lines are unresolved.This approach differs from resolving individual Doppler-shifted lines.
  • Alternative spectral regimes: Low-resolution star–planet spectra can potentially reveal molecules absent from the stellar atmosphere, but success depends on accurate stellar spectra and a low observational noise floor.Molecular attribution is safest when the candidate molecule is not expected in the host star.
  • High-resolution spectroscopy: Detecting Doppler-shifted lines reveals targeted molecules and can yield the planet’s true mass and orbital inclination when combined with stellar radial-velocity measurements.The line-of-sight velocity supplies information complementary to stellar RV data.
  • Opportunities through 2030: High-contrast imaging combined with ultra-stable high-resolution spectroscopy may access nearby temperate Earth-sized planets, while ELT collecting area could characterize Proxima Centauri b in about six nights.ELTs also offer smaller inner working angles and access to more targets around late-type stars.
  • Mid-infrared observations: In the mid-infrared, planetary atmospheric signatures may be searched for at planet-to-star contrasts of roughly 10–100 ppm without starlight suppression if noise is sufficiently low.This regime is especially relevant for temperate planets around late-type stars.

5. Contextual Information

Context beyond the planet’s immediate observables is essential for interpreting potential biosignatures. Host-star properties, system architecture, and better-characterized comparison planets can constrain planetary environments while retaining substantial model dependence.

  • Host-star context: Host-star characterization is central because stellar properties affect inferred planetary mass and radius and shape the planet’s climate through the stellar spectral energy distribution.Relevant properties include stellar radius, mass, age, effective temperature, and spectral energy distribution.
  • Host-star context: The host star’s ultraviolet spectrum influences atmospheric photochemistry, temperature, composition, and the detectability and reliability of biosignatures such as O3 and CH4.UV radiation may also produce abiotic potential biosignature gases such as O2.
  • Host-star context: Stellar high-energy radiation is linked to magnetic activity, which correlates negatively with stellar age and rotation period and can be estimated using activity indicators such as Ca II H and K.These measurements provide observational context for evaluating planetary environments.
  • System architecture: Planetary-system architecture may inform volatile inventories because migration of hot Jupiters could deliver material from beyond the snow line to inner-system regions.The passage presents this as an indirect implication for Earth-sized habitable-zone planets.
  • System architecture: Companion planets can drive long-term changes in a terrestrial planet’s orbit and obliquity, potentially producing dramatic global surface-temperature changes.The effect is especially relevant in the absence of strong tidal forces.
  • Gaseous comparison planets: Characterizing larger gaseous planets will be easier and can provide radius–mass relations, gas fractions, atmospheric maps, and clues about formation scenarios.Transit, radial-velocity, phase-variation, and direct-imaging observations contribute complementary information.
  • Gaseous comparison planets: Gas-giant atmospheric properties and core masses may constrain planet-forming regions, but detailed disk inferences and links to terrestrial habitability remain strongly dependent on formation and geophysical models.The paper emphasizes that the complexity of planet formation limits such deductions.

6. Prospects Beyond 2030

Beyond 2030, mission concepts span initial searches for nearby habitable planets, broad surveys of habitable zones, and mid-infrared atmospheric characterization. Their capabilities trade target yield, integration time, wavelength coverage, and instrument complexity.

  • Mission concepts: NASA-supported mission studies examine direct imaging of habitable planets across a range of science capabilities, costs, and architectures.The studies discussed include HabEx and LUVOIR, while OST targets transiting terrestrial planets spectroscopically.
  • HabEx: HabEx would use a 4–6.5 m unobscured telescope with coronagraphs and/or starshades to search the habitable zones of up to 40 nearby solar-type stars.For η⊕=10%, it would find a handful of terrestrial planets for spectroscopic follow-up, with weeks of integration per target at ℝ~140 for a 4 m aperture.
  • LUVOIR: LUVOIR would survey 200–500 habitable zones and conservatively yield 20–50 terrestrial planets using a 9–15 m segmented telescope.Its larger collecting area would enable habitable-zone spectra in roughly a day of integration and permit rotational-brightness measurements for at least a dozen targets.
  • LUVOIR: LUVOIR is also studying sub-microarcsecond astrometry to measure the stellar reflex motion of habitable-zone terrestrial planets and determine their masses.This capability would complement direct-imaging spectroscopy.
  • OST: OST would characterize nearby transiting terrestrial exoplanet atmospheres with transmission and emission spectroscopy across 6–600 µm, including a biosignature-focused instrument.Its baseline is a segmented 9 m off-axis telescope with up to five instruments.
  • OST: Mid-infrared emission spectra can distinguish wet Earth-like, dry Venus-like, and Mars-like atmospheres through features from CH4, CO2, O3, NH3, N2O, SO2, and H2O.The favorable signal-to-noise range is approximately 8–30 µm.
  • OST: Mid-infrared transmission spectra add constraints at the terminator and are less affected by high-altitude aerosols than shorter-wavelength observations.This provides a complementary atmospheric probe to dayside emission measurements.

6.2 Ideas for the Far Future

Far-future concepts aim to obtain richer spatial and spectral information about Earth-sized exoplanets, but most remain technologically or financially distant. Their potential advantages are accompanied by substantial engineering and environmental constraints.

  • Scope of far-future concepts: The far-future concepts discussed are not near-term projects because their technological development and funding challenges remain unresolved.The paper does not examine those challenges in detail.
  • Space interferometry: Mid-infrared space interferometers such as Darwin and TPF-I were proposed for direct imaging of Earth-sized planets but are not currently under active study.A contrast of 10^-7 is needed around solar-type stars in the mid-infrared, with more demanding requirements at longer wavelengths.
  • Surface-resolving imaging: A visible interferometric imager could spatially resolve an exoplanetary surface, with a 10 x 10 pixel map revealing albedo patterns, clouds, oceans, continents, and possible topography.Time-series mapping could also measure planetary rotation and atmospheric circulation.
  • Lunar observatories: Lunar telescopes could benefit from an atmosphere-free location and long target access, but lunar temperature swings, the roughly 330-hour night, and dust contamination offset those advantages.The rigid lunar surface may also simplify construction of large telescopes and interferometers.
  • Very large ground telescopes: Ground telescopes larger than ELTs are unlikely to be pursued for another two or three decades.Earlier concepts included 100 m-class telescopes and the proposed 74 m Colossus telescope.

7. Summary: Ideal Timeline

Near-term characterization will concentrate on a small number of potentially habitable planets around nearby late-type stars, using JWST and ELTs to investigate atmospheres and biosignatures. Solar-type-star targets will require space-based direct imaging, while later missions could broaden biosignature searches; interpretation must combine contextual evidence with improved analysis methods.

  • Nearby late-type stars: A small number of transiting potentially habitable planets around nearby late-type stars may receive intensive JWST follow-up to determine whether they possess atmospheres and signs of habitability.If these are found, JWST or ground-based transit spectroscopy may pursue biosignatures.
  • Nearby late-type stars: ELTs will offer near-term opportunities to detect atmospheric signatures around late-type-star HZ planets and test theories of atmospheric loss and replenishment.Planned techniques include high-resolution transmission spectroscopy, high-contrast imaging, and high-resolution high-contrast observations.
  • Solar-type stars: PLATO, combined with ground-based or transit-timing mass measurements, will establish mass-radius relations for HZ terrestrial planets around relatively distant solar-type stars.These measurements are intended to provide prior knowledge for future directly imaged targets.
  • Solar-type stars: WFIRST could directly image Earth-sized HZ planets around solar-type stars with a starshade at 10^-10 contrast and obtain low-resolution atmospheric spectra.Without a starshade, its coronagraph is expected to reach about 2x10^-9 contrast at 130 mas separations, enabling detection of larger, potentially terrestrial planets.
  • Beyond 2030: Beyond the initial roughly 10-year search, OST, HabEx, and LUVOIR represent three paths toward expanded biosignature investigations using broader wavelength coverage, scattered-light spectroscopy, or surveys of hundreds of stars.OST would offer higher sensitivity than JWST and expanded wavelength coverage compared with ELTs; HabEx would target nearby solar-type stars; LUVOIR would survey many stars.
  • Interpretation and limitations: Biosignature claims require environmental context, comprehensive planetary and system characterization, and theoretical models because false positives must be examined and retrievals can be degenerate or biased.The paper identifies improved data-analysis techniques and interpretive frameworks as essential, while noting that future observations will face substantial hurdles.
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