A procedure for observing rocky exoplanets to maximize the likelihood that atmospheric oxygen will be a biosignature

TL;DR

This paper proposes an observational strategy for exoplanet studies that prioritizes planets where oxygen is a reliable biosignature, emphasizing the importance of land and water presence for detectability.

Contribution

It introduces a ranking method for exoplanets based on the likelihood of oxygen being a true biosignature, considering planetary surface conditions.

Findings

Land and surface water are essential for oxygen as a biosignature.

Landless planets may host life but are unlikely to produce detectable oxygen.

Prioritizing planets with land and water improves biosignature reliability.

Abstract

Here we advocate an observational strategy to help prioritize exoplanet observations. It starts with more easily obtained observational data, and ranks exoplanets for more difficult follow-up observations based on the likelihood of avoiding planets for which oxygen is a false positives or even an inconclusive signature of life. We find that for oxygen to be a robust biosignature, both land and surface water must be present. Landless exoplanets have much slower biogeochemical cycles, so while oxygenic photosynthesizing life could exist on such planets, it could not produce oxygen at a rate competitive with abiotic rates such as photolysis. These habitable planets, whose life would not be detectable, should be avoided.