A Geologically Robust Procedure for Observing Rocky Exoplanets to Ensure that Detection of Atmospheric Oxygen Is a Modern Earth-like Biosignature

Abstract

In the next decades, the astrobiological community will debate whether the first observations of oxygen in an exoplanet’s atmosphere signify life, so it is critical to establish procedures now for collection and interpretation of such data. We present a step-by-step observational strategy for using oxygen as a robust biosignature to prioritize exoplanet targets and design future observations. It is premised on avoiding planets lacking subaerial weathering of continents, which would imply geochemical cycles drastically different from modern Earth’s, precluding use of oxygen as a biosignature. The strategy starts with the most readily obtained data: orbital semimajor axis and stellar luminosity to ensure residence in the habitable zone and stellar X-ray/ultraviolet flux to ensure an exoplanet can retain a secondary (outgassed) atmosphere. Next, high-precision mass and radius information should be combined with high-precision stellar abundance data to constrain the exoplanet’s water content; those incompatible with <0.1 wt% H2O can be deprioritized. Then, reflectance photometry or low-resolution transmission spectroscopy should confirm an optically thin atmosphere. Subsequent long-duration, high-resolution transmission spectroscopy should search for oxygen and ensure that water vapor and CO2 are present only at low (∼102–104 ppm levels). Assuming oxygen is found, attribution to life requires the most difficult step, acquisition of a detailed, multispectral light curve of the exoplanet, to ensure both surface land and water. Exoplanets failing some of these steps might be habitable, even have observable biogenic oxygen, but should be deprioritized because oxygen could not be attributed unambiguously to life, and life therefore would not be detectable on such planets. Finally, we show how to use this scheme for the solar system, the 55 Cnc system, and the TRAPPIST-1 system, in which only the Earth and TRAPPIST-1e successfully pass through our procedure.

Variability due to climate and chemistry in observations of oxygenated Earth-analogue exoplanets

“…Moreover, there is the possibility of detecting an exoplanet-exomoon system which has O 2 and CH 4 in the spectra, but not on the same celestial body (Rein et al 2014). In the coming decades, observational data, computational procedures which identify the sources of biologicallyrelevant gases (Catling et al 2018;Lisse et al 2020), and simula-tions that utilise state-of-the art climate models, will all be needed to determine the most habitable exoplanets (Shields 2019), and indeed to determine if any are inhabited.…”

A Catalog of Habitable Zone Exoplanets

“…Exoplanet discoveries over the past several decades have revealed a vast diversity of planetary architectures (Ford 2014;Winn & Fabrycky 2015), and shown that terrestrial planets are far more common than their giant planet counterparts (Borucki 2016). In these ongoing exoplanet searches, discovering those planets that may harbor life has been a primary objective for the astrobiology community (Fujii et al 2018;Schwieterman et al 2018;Glaser et al 2020;Lisse et al 2020). A potential pathway toward the identification of such worlds is to constrain the stellar and planetary parameter space that may allow for the presence of surface liquid water.…”

Blue Marble, Stagnant Lid: Could Dynamic Topography Avert a Waterworld?

“…The concurrence of land and water on a planet's surface will affect its climate state (Turbet et al 2016;Del Genio et al 2019;Rushby et al 2019;Graham & Pierrehumbert 2020;Zhao et al 2021), the planetary context of potential biosignatures (Schwieterman et al 2018;Glaser et al 2020;Lisse et al 2020;Krissansen-Totton et al 2021), and perhaps its likelihood to host the prebiotic chemistry that leads to the origin of life (Patel et al 2015;Rimmer et al 2018;Rosas & Korenaga 2021;Van Kranendonk et al 2021). Planetary land/ ocean fractions emerge in a compromise between water's total budget and distribution between surface and interior reservoirs and the size of the basins carved out by topography (e.g., Simpson 2017).…”