Exoplanet Biosignatures: Observational Prospects

Abstract

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 the 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 its context. Characterization of temperate Earth-sized planets in the coming years will focus on those around nearby late-type stars. The James Webb Space Telescope (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 Wide Field InfraRed Survey Telescope (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. Key Words: Exoplanets-Biosignatures-Characterization-Planetary atmospheres-Planetary surfaces. Astrobiology 18, 739-778.

FIG. 1.

Schematic figure showing astrophysical, chemical, climatological, and astrobiological characterizations and the possible contributions from the current and future missions (HCI = high-contrast imaging; HRS = high-resolution spectroscopy).

FIG. 2.

Examples of atmospheric and surface spectral features of temperate terrestrial planets. Upper panel: Atmospheric signatures, which can in principle be probed through both transmission spectra and emergent spectra. The continuous features of molecules at shorter wavelengths are absorption cross section at approximately 300 K, taken from MPI-Mainz UV/VIS Spectral Atlas of Gaseous Molecules of Atmospheric Interest (Keller-Rudek et al., 2013), shown in log scale from 10⁻²⁶ to 10⁻¹⁶ cm²/molecule. Original data sources are Yoshino et al. (1988) for O₂; Brion et al. (1998) for O₃; Selwyn et al. (1977) for N₂O; Cheng et al. (2006) for NH₃; Mérienne et al. (1997), Coquart et al. (1995), and Vandaele et al. (1998) for NO₂. The lines at longer wavelengths are line intensities at 296 K and 1 atm in log scale from 10⁻²⁶ to 10⁻¹⁶ cm² cm⁻¹/molecule taken from the HITRAN2012 database (Rothman et al., 2013). Lower panel: thermal radiation and reflectance spectra of surface materials, which can be probed in emergent light. The reflectance is shown in linear scale from 0 to 1. All data but biological pigments are taken from the ECOSTRESS Spectral Library (Baldridge et al., ; Meerdink et al., unpublished data). The data of biological pigments are from VPL spectral databases (Schwieterman et al., 2015).

FIG. 3.

Transmission spectra of Earth observed at the lunar eclipse, taken from Pallé et al. (2009). The spectral resolution is ∼ 960 in the optical and ∼ 920 in the near-IR.

FIG. 4.

O₂ 1.27 μm features in spectrum of an Earth atmosphere with varying spectral resolution.

FIG. 5.

Modeled thermal emission spectra of cloud-free Earth-like planets around the Sun (black), AD Leo (red), an M0 star (green), an M5 star (blue), and an M7 star (magenta), taken from Rauer et al. (2011). Reproduced with permission © ESO.

FIG. 6.

A modeled scattered light spectrum of Earth (blue solid line) and the mock observation of an Earth twin at 5 pc away assuming a LUVOIR-type telescope with 12 m diameter and 30 hr of integration time, with resolution (blue points with error bars), generated at http://jt-astro.science:5106/coron_model. The theoretical line and the noise model are based on Robinson et al. (2011) and Robinson et al. (2016), respectively.

FIG. 7.

The star-planet contrast and the star-planet separation of known planetary systems (points), and the performance of existing and future high-contrast imaging instruments (lines). This is the October 2017 version of a plot maintained by the NASA Exoplanet Exploration Program Office. The orange points correspond to the near-IR brightness of known self-luminous directly imaged planets, while the open circles show their theoretical I-band contrast. The black points show the theoretical V-band contrast of planets detected by the RV method. The Solar System planets at 10 pc at the maximum separation are presented in colored points; the dashed lines from these points indicate their orbital phase variations as seen from a direction inclined 30 degrees from the ecliptic. The self-luminous planets detected to date are at contrasts of 10⁻⁶ and brighter, while 10⁻⁹ contrasts are needed to detect Jupiter in scattered light and 10⁻¹⁰ to detect Earth as seen from outside. The data sources for the instrumental performance lines are as follows: The JWST NIRCam and HST ACS curves were provided by John Krist for Lawson et al. (2012). The GPI curve is for H band and provided by Bruce Macintosh (personal communication). The SPHERE-Sirius curve is for K band (Vigan et al., 2015, Fig. 2). The 2017 WFIRST CGI curve was provided by deputy instrument scientist Bertrand Mennesson (personal communication). The starshade curve is from Stuart Shaklan (personal communication). The performance curves shown for it are preliminary as of October 2017 and subject to revision.

FIG. 8.

Flight configuration of a 34 m starshade in exo-Earth search mode (bandpass of 425–605 nm) was studied as part of the Exo-S Extended Study (https://exoplanets.nasa.gov/internal_resources/225; Seager et al., 2015). This particular configuration attempts to maximize exo-Earth yield and assumes a 3-year mission; other options considered included both larger and smaller starshades with shorter and longer mission lifetimes.

FIG. 9.

Seven-band diurnal light curves of the disk-integrated scattered light of Earth, obtained by the EPOXI mission (Cowan et al., ; Livengood et al., 2011). Left panel: The equatorial observation started on March 18, 2008, with phase angle 57°. Right panel: The north-polar observation started on March 27, 2009, with phase angle 87°.

FIG. 10.

Solid lines: Thermal emission spectra of Earth, Venus, and Mars. The Earth spectrum is from Robinson et al. (2011). The spectra of Venus and Mars were modeled using the radiative transfer code SMART, assuming the 1D atmospheric profiles of each planet. Venus data is from Giada Arney, and Mars data is from Robinson and Crisp (2018). Dashed lines: Blackbody emission from a planet of the same radius with the approximately maximum brightness temperature of each planet in this range. See also Selsis et al. (2008) and Kaltenegger (2017).