A Geologically Robust Procedure For Observing Rocky Exoplanets to Ensure that Detection of Atmospheric Oxygen is an Earth-Like Biosignature

Abstract

In the next decades, the astrobiological community will debate whether the first observations of oxygen in an exoplanet${}^{\prime }$s atmosphere signifies 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 Earth${}^{\prime }$s, precluding use of oxygen as a biosignature. The strategy starts with the most readily obtained data: semi-major axis and stellar luminosity to ensure residence in the habitable zone; stellar XUV 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${}^{\prime }$s water content; those incompatible with less than 0.1 wt % H${}_2$O 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 CO${}_2$ are present only at low (10${}^2$-10${}^4$ ppm levels). Assuming oxygen is found, attribution to life requires the difficult 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. We show how this is the case 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.

27 Pages, 1 Figure, 0 Tables (accepted 09 June 2020, in press)

References in corpus (27)

• The Transiting Exoplanet Survey Satellite
• Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
• Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
• The GALAH Survey: Scientific Motivation
• Most 1.6 Earth-Radius Planets are not Rocky
• Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass
• Extreme Water Loss and Abiotic O${}_2$ Buildup On Planets Throughout the Habitable Zones of M Dwarfs
• Can we constrain interior structure of rocky exoplanets from mass and radius measurements?
• Stellar Abundances in the Solar Neighborhood: The Hypatia Catalog
• X-Ray Spectroscopy of Stars
• Combining high-dispersion spectroscopy (HDS) with high contrast imaging (HCI): Probing rocky planets around our nearest neighbors
• Observing the Atmospheres of Known Temperate Earth-sized Planets with JWST
• Alien Maps of an Ocean-Bearing World
• The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST
• Geodynamics and Rate of Volcanism on Massive Earth-like Planets
• Detecting the Glint of Starlight on the Oceans of Distant Planets
• On the Age of the TRAPPIST-1 System
• Direct detection of exoplanets in the 3 -- 10 micron range with E-ELT/METIS
• Phase transitions in MgSiO3 post-perovskite in super-Earth mantles
• Herschel/HIFI observations of Mars: first detection of O_2 at submillimetre wavelengths and upper limits on HCl and H_2O_2
• Detecting Ocean Glint on Exoplanets Using Multiphase Mapping
• Hydrodynamic simulations of captured protoatmospheres around Earth-like planets
• A Catalog of Stellar Unified Properties (CATSUP) for 951 FGK-Stars Within 30 pc
• The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets
• Detectability of Life Using Oxygen on Pelagic Planets and Water Worlds
• X-ray emission processes in stars
• Chandra Characterization of X-ray Emission in the Young F-Star Binary System HD 113766

Cited by in corpus (1)

• On the Utility of Transmission Color Analysis I: Differentiating Super-Earths and Sub-Neptunes