Can Isotopologues Be Used as Biosignature Gases in Exoplanet Atmospheres?

Isotopologue ratios are anticipated to be one of the most promising signs of life that can be observed remotely. On Earth, carbon isotopes have been used for decades as evidence of modern and early metabolic processes.

In fact, carbon isotopes may be the oldest evidence for life on Earth, though there are alternative geological processes that can lead to the same magnitude of fractionation. However, using isotopologues as biosignature gases in exoplanet atmospheres presents several challenges. Most significantly, we will only have limited knowledge of the underlying abiotic carbon reservoir of an exoplanet. Atmospheric carbon isotope ratios will thus have to be compared against the local interstellar medium or, better yet, their host star.

A further substantial complication is the limited precision of remote atmospheric measurements using spectroscopy. The various metabolic processes which cause isotope fractionation cause less fractionation than anticipated measurement precision (biological fractionation is typically 2 to 7%). While this level of precision is easily reachable in the laboratory or with special in situ instruments, it is out of reach of current telescope technology to measure isotope ratios for terrestrial exoplanet atmospheres.

Thus, gas isotopologues are poor biosignatures for exoplanets given our current and foreseeable technological limitations.

Cool (T < 389 K) terrestrial and sub-Neptune planets (Rp < 4 R⊕) shown along the x-axis in temperature and the y-axis by alphabetical order. Marker size is relative to planet size. Marker color is coded to the ln(TSM). There are very few small, cool planets with a high TSM. The best temperate terrestrial candidates by TSM are TRAPPIST-1 c, d, e, f, and g. The highest TSM small sub-Neptunes (1.5 R⊕ < Rp < 2.75 R⊕) candidates are LP 791-18 c and TOI-270 d. The highest TSM of cool planets between from 2.75 R⊕ < Rp < 4 R⊕ are Kepler-51 d and TOI-1231 b. — astro-ph.EP

Ana Glidden, Sara Seager, Janusz J. Petkowski, Shuhei Ono

Comments: 16 pages, 4 figures. Published in Life on December 11, 2023. The final authenticated version is available online at: this https URL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2401.15153 [astro-ph.EP] (or arXiv:2401.15153v1 [astro-ph.EP] for this version)
Journal reference: Life 2023, 13(12), 2325
Related DOI:
https://doi.org/10.3390/life13122325
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Submission history
From: Ana Glidden
[v1] Fri, 26 Jan 2024 19:00:13 UTC (235 KB)
https://arxiv.org/abs/2401.15153
Astrobiology, Astrochemistry,