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


Flowchart describing an observational campaign designed to efficiently select planets for expensive observational searches for oxygen in their atmospheres so that oxygen would be a reliable biosignature (i.e., attributable to life) on those planets, if detected. The observations range from those currently being undertaken, to those requiring future ground- and space-based observations. The least time- and resource-intensive observations possible for large numbers of planets are listed first, at the top, and the most expensive and difficult measurements, possible for only a handful of exoplanets, are at the bottom, in the last part of the flowchart.

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 % 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 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.

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

Comments: 27 Pages, 1 Figure, 0 Tables (accepted 09 June 2020, in press)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM); Solar and Stellar Astrophysics (astro-ph.SR)
Journal reference: Astrophysical Journal Letters 2020
Cite as: arXiv:2006.07403 [astro-ph.EP] (or arXiv:2006.07403v1 [astro-ph.EP] for this version)
Submission history
From: Carey Lisse
[v1] Fri, 12 Jun 2020 18:19:26 UTC (646 KB)
https://arxiv.org/abs/2006.07403
Astrobiology, Astrochemistry

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