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A Geologically Robust Procedure For Observing Rocky Exoplanets to Ensure that Detection of Atmospheric Oxygen is an Earth-Like Biosignature | | Carey M. Lisse
; Steven J. Desch
; Cayman T. Unterborn
; Stephen R. Kane
; Patrick R. Young
; Hilairy E. Hartnett
; Natalie R. Hinkel
; Sang Heon Shim
; Eric E. Mamajek
; Noam R. Izenberg
; | | Date: |
12 Jun 2020 | | Abstract: | In the next decades, the astrobiological community will debate whether the
first observations of oxygen in an exoplanet$’$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$’$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$’$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. | | Source: | arXiv, 2006.7403 | | Services: | Forum | Review | PDF | Favorites | |
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