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.

Prospects for Life on Temperate Planets around Brown Dwarfs

“…The next question that arises has to do with the relative ease of detecting biosignatures. We will draw upon the scaling relations presented in Fujii et al (2018) for our purpose. For starters, we note that the transit depth and transit probability for Earth-like planets around brown dwarfs are on the order of 1%, both of which are higher than (or similar to) the estimates for planets around main-sequence stars.…”

“…For starters, we note that the transit depth and transit probability for Earth-like planets around brown dwarfs are on the order of 1%, both of which are higher than (or similar to) the estimates for planets around main-sequence stars. If we opt to carry out transmission spectroscopy for an Earth-analog using the JWST, 9 the resultant signal-to-noise ratio (SNR), after invoking Fujii et al (2018), is expressible as:…”

“…Alternatively, one can analyze the thermal emission of the planet by computing the spectra of the system during secondary eclipses and taking the difference. For this procedure, the SNR for an Earth-analog using the parameters of the JWST is (Fujii et al 2018):…”

Geoscience for Understanding Habitability in the Solar System and Beyond

“…Berdyugina et al, 2016). An excellent summary of recent advances in exoplanetary biosignature science can be found in a series of review papers from the NASA Nexus for Exoplanet System Science (NExSS) and Astrobiology Program, namely: Schwieterman et al (2018) who provide an all-round review of the topic; Meadows et al (2018) who focus on oxygen; Catling et al (2018) who discuss a Bayesian approach; Walker et al (2018) who propose future methods for biosignature assessment and Fujii et al (2018) who summarize future relevant missions. A review of atmospheric biosignatures with a focus on photochemical responses in an exoplanetary context is provided by Grenfell (2017).…”

Fourth-order Coronagraph for High-contrast Imaging of Exoplanets with Off-axis Segmented Telescopes

“…The next step is the characterization of the atmosphere of the potentially habitable planets using a spectroscope. The prediction of features in planet spectra has attracted a lot of debate from an astrobiological perspective (Angel et al 1986;Kaltenegger 2017;Catling et al 2018;Kiang et al 2018;Meadows et al 2018;Schwieterman et al 2018;Fujii et al 2018). At mid-infrared wavelength, the non-thermal equilibrium of the atmospheres due to biological activity can be investigated from the thermal emission and transmission spectra.…”