AAS Nova

Selections from 2019: Rethinking Carbon Monoxide in the Search for Life

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habitable exoplanet
Artist's impression of a potentially habitable exoplanet. Are there signatures can we look for when searching for where life may not be? [NASA/Ames/JPL-Caltech]

Editor’s note: In these last two weeks of 2019, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Rethinking CO Antibiosignatures in the Search for Life Beyond the Solar System

Published March 2019

Main takeaway:

A team of scientists led by Edward Schwieterman (UC Riverside) used computer simulations of planetary ecospheres/atmospheres to show that a number of planet types of interest in the search for life — like planets similar to early Earth, or planets around M-dwarf stars — can maintain an accumulation of carbon monoxide in their atmospheres.

Why it’s interesting:

It was previously thought that the presence of carbon monoxide in a planet’s atmosphere could be considered an antibiosignature — a signature that indicates that there probably isn’t life present. This is because carbon monoxide represents an unexploited source of free energy; its accumulation was thought to indicate that there’s no life available to take advantage of it. Antibiosignatures are valuable because, in the search for life on planets beyond the solar system, they can quickly tell us where we shouldn’t waste our time looking. But Schwieterman and collaborators’ work now suggests we may need to rethink the assumption that carbon monoxide can be used as such an indicator.

What this means for the search for life:

If carbon monoxide can be present in a planet’s atmosphere even when the world is inhabited, we clearly can’t use the accumulation of this gas to unambiguously rule out targets in the search for extraterrestrial life. Instead, our best bet is to continue to develop a framework that relies on the presence or absence of various combinations of gases. We may also be able to use this information together with novel approaches, like searching for seasonal variation that could be caused by the presence of life, or calculating whether the atmosphere and surface of a planet are out of equilibrium.

Citation

Edward W. Schwieterman et al 2019 ApJ 874 9. doi:10.3847/1538-4357/ab05e1