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.

arXiv comment27 Pages, 1 Figure, 0 Tables (accepted 09 June 2020, in press)

References in corpus (80)

• The Transiting Exoplanet Survey Satellite
• Habitable Zones Around Main-Sequence Stars: New Estimates
• Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
• The California-Kepler Survey. III. A Gap in the Radius Distribution of Small Planets
• Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics
• The PLATO 2.0 Mission
• Clouds in the atmosphere of the super-Earth exoplanet GJ 1214b
• Evolution of the Solar Activity over Time and Effects on Planetary Atmospheres: I. High-energy Irradiances (1-1700 A)
• The GALAH Survey: Scientific Motivation
• Most 1.6 Earth-Radius Planets are not Rocky
• Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass
• Temperate Earth-sized planets transiting a nearby ultracool dwarf star
• The Mass-Radius Relation for 65 Exoplanets Smaller than 4 Earth Radii
• Extreme Water Loss and Abiotic O${}_2$ Buildup On Planets Throughout the Habitable Zones of M Dwarfs
• The Transiting Exoplanet Survey Satellite: Simulations of planet detections and astrophysical false positives
• Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life
• New Neighbors from 2MASS: Activity and Kinematics at the Bottom of the Main Sequence
• Chandra X-Ray Observatory (CXO):Overview
• Mass-Radius Relation for Rocky Planets based on PREM
• A New Family of Planets ? "Ocean Planets"
• Can we constrain interior structure of rocky exoplanets from mass and radius measurements?
• The nature of the TRAPPIST-1 exoplanets
• Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment
• A Revised Exoplanet Yield from the Transiting Exoplanet Survey Satellite (TESS)
• 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
• A Detailed Model Grid for Solid Planets from 0.1 through 100 Earth Masses
• NEXXUS: A comprehensive ROSAT survey of coronal X-ray emission among nearby solar-like stars
• Observing the Atmospheres of Known Temperate Earth-sized Planets with JWST
• Atmospheric reconnaissance of the habitable-zone Earth-sized planets orbiting TRAPPIST-1
• Alien Maps of an Ocean-Bearing World
• The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST
• The cosmic shoreline: the evidence that escape determines which planets have atmospheres, and what this may mean for Proxima Centauri b
• A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c
• The Sun in Time: Activity and Environment
• A Volume-limited Sample of 63 M7-M9.5 Dwarfs II. Activity, magnetism, and the fade of the rotation-dominated dynamo
• Geodynamics and Rate of Volcanism on Massive Earth-like Planets
• The Chemical Homogeneity of Sun-like Stars in the Solar Neighborhood
• Coronal Evolution of the Sun in Time: High-Resolution X-Ray Spectroscopy of Solar Analogs with Different Ages
• Detecting the Glint of Starlight on the Oceans of Distant Planets
• Connecting Flares and Transient Mass Loss Events in Magnetically Active Stars
• Detecting Oceans on Extrasolar Planets Using the Glint Effect
• How formation time-scales affect the period dependence of the transition between rocky super-Earths and gaseous sub-Neptunes and implications for ${\eta }_{\oplus }$
• Coronal properties of planet-bearing stars
• Detectability of biosignatures in anoxic atmospheres with the James Webb Space Telescope: A TRAPPIST-1e case study
• Scaling the Earth: A Sensitivity Analysis of Terrestrial Exoplanetary Interior Models
• Chemical Abundances in Field Red Giants from High-Resolution H-Band Spectra using the APOGEE Spectral Linelist
• 55 Cancri: Stellar Astrophysical Parameters, a Planet in the Habitable Zone, and Implications for the Radius of a Transiting Super-Earth
• On the Age of the TRAPPIST-1 System
• Characterizing Earth Analogs in Reflected Light: Atmospheric Retrieval Studies for Future Space Telescopes
• Impact of atmospheric refraction: How deeply can we probe exo-Earth's atmospheres during primary eclipse observations?
• Colors of a Second Earth: Estimating the fractional areas of ocean, land, and vegetation of Earth-like exoplanets
• Identifying Planetary Biosignature Impostors: Spectral Features of CO and O4 Resulting from Abiotic O2/O3 Production
• Carbon cycling and habitability of Earth-size stagnant lid planets
• Direct detection of exoplanets in the 3 -- 10 micron range with E-ELT/METIS
• A Volume-limited Sample of 63 M7--M9.5 Dwarfs I. Space Motion, Kinematic Age, and Lithium
• False negatives for remote life detection on ocean-bearing planets: Lessons from the early Earth
• Exoplanet Classification and Yield Estimates for Direct Imaging Missions
• The Star-Planet Connection I: Using Stellar Composition to Observationally Constrain Planetary Mineralogy for the Ten Closest Stars
• Phase transitions in MgSiO3 post-perovskite in super-Earth mantles
• The Interior Dynamics of Water Planets
• Herschel/HIFI observations of Mars: first detection of O_2 at submillimetre wavelengths and upper limits on HCl and H_2O_2
• Water Cycling Between Ocean and Mantle: Super-Earths Need Not be Waterworlds
• Detecting Ocean Glint on Exoplanets Using Multiphase Mapping
• Characterization of the L 98-59 multi-planetary system with HARPS: two confirmed terrestrial planets and a mass upper limit on the third
• The Pressure and Temperature Limits of Likely Rocky Exoplanets
• Pebble Accretion and the Diversity of Planetary Systems
• Hydrodynamic simulations of captured protoatmospheres around Earth-like planets
• Atmospheric Seasonality as an Exoplanet Biosignature
• A Catalog of Stellar Unified Properties (CATSUP) for 951 FGK-Stars Within 30 pc
• A False Positive For Ocean Glint on Exoplanets: the Latitude-Albedo Effect
• The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets
• Is the Pale Blue Dot unique? Optimized photometric bands for identifying Earth-like exoplanets
• Detectability of Life Using Oxygen on Pelagic Planets and Water Worlds
• X-ray emission processes in stars
• A Simple Analytical Model for Rocky Planet Interiors
• Expected performances of the Characterising Exoplanet Satellite (CHEOPS). I. Photometric performances from ground-based calibration
• A Catalog of Stellar Evolution Profiles and the Effects of Variable Composition on Habitable Systems
• Chandra Characterization of X-ray Emission in the Young F-Star Binary System HD 113766

Cited by in corpus (5)

• Calculation of Electric Quadrupole Linestrengths for Diatomic Molecules: Application to the H2, CO, HF and O2 Molecules
• Variability due to climate and chemistry in observations of oxygenated Earth-analogue exoplanets
• Blue marble, stagnant lid: Could dynamic topography avert a waterworld?
• An ab initio spectroscopic model of the molecular oxygen atmospheric and infrared bands
• On the Utility of Transmission Color Analysis I: Differentiating Super-Earths and Sub-Neptunes