Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment

Abstract

We describe how environmental context can help determine whether oxygen (O2) detected in extrasolar planetary observations is more likely to have a biological source. Here we provide an in-depth, interdisciplinary example of O2 biosignature identification and observation, which serves as the prototype for the development of a general framework for biosignature assessment. Photosynthetically generated O2 is a potentially strong biosignature, and at high abundance, it was originally thought to be an unambiguous indicator for life. However, as a biosignature, O2 faces two major challenges: (1) it was only present at high abundance for a relatively short period of Earth's history and (2) we now know of several potential planetary mechanisms that can generate abundant O2 without life being present. Consequently, our ability to interpret both the presence and absence of O2 in an exoplanetary spectrum relies on understanding the environmental context. Here we examine the coevolution of life with the early Earth's environment to identify how the interplay of sources and sinks may have suppressed O2 release into the atmosphere for several billion years, producing a false negative for biologically generated O2. These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. We review the most recent knowledge of false positives for O2, planetary processes that may generate abundant atmospheric O2 without a biosphere. We provide examples of how future photometric, spectroscopic, and time-dependent observations of O2 and other aspects of the planetary environment can be used to rule out false positives and thereby increase our confidence that any observed O2 is indeed a biosignature. These insights will guide and inform the development of future exoplanet characterization missions. Key Words: Biosignatures—Oxygenic photosynthesis—Exoplanets—Planetary atmospheres. Astrobiology 18, 630–662.

Dead zones of classical habitability in stellar binary systems

“…Ultimately, additional observational campaigns are desired. Those should also encompass the search for biosignatures, including molecules and molecular disequilibria that are apparently irreproducible through non-biological processes; see, e.g., Seager et al (2016), Meadows et al (2018), Olson et al (2018), Schwieterman et al (2018), and Arney (2019) for background information. Fig.…”

Detectability of Life Using Oxygen on Pelagic Planets and Water Worlds

“…First, radial velocity, transit, and/or transit timing variation (TTV) methods can identify the mass, planet radius, and orbital radius that together can be used to classify an exoplanet as Earthlike and whether it is in its stars habitable zone. Second, visible and infrared transmission spectroscopy or reflectance spectroscopy can characterize atmospheric gases such as ozone, diatomic oxygen, carbon dioxide, water vapor, and possibly methane (Deming et al 2009;Domagal-Goldman et al 2016;Kaltenegger 2017;Kaltenegger & Traub 2009;Lovis et al 2017;Meadows 2017;Meadows et al 2018;Schwieterman et al 2018). The presence of oxygen (i.e., O 2 and O 3 ) in the atmosphere of a rocky planet is widely considered a robust biosignature (Meadows et al 2018).…”

“…To emphasize this point, the James Webb Space Telescope (JWST) is estimated to be able to characterize the atmospheres of only 5 exoplanets over its mission lifetime, creating the need to maximize the chances of a true positive detection (Brown & Soummer 2010). At present, the exoplanet community is beginning to develop systematic guiding principles to help select targets and prioritize exoplanets for follow-up observations (Meadows et al 2018;Desch et al 2018;National Academies 2018). The search criterion that has served the community well until nowhabitability, as defined by orbital parameters allowing the possibility of sustaining liquid water on the surfaceis no longer sufficiently restrictive.…”

“…On Earth, atmospheric oxygen exists because of life. But oxygen will not indicate life on all planets (Meadows 2017;Meadows et al 2018). A number of false positive scenarios (e.g., for Venus and Mars, or for water-rich planets around M dwarfs) have been identified that could give rise to abundant oxygen in the atmosphere of a planet by purely non-biological processes (e.g., Domagal-Goldman et al 2014;Luger & Barnes 2015;Meadows 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.…”