Biosignatures & Paleobiology

Carbon-bearing Molecules In A Possible Hycean Atmosphere

By Keith Cowing
Status Report
September 17, 2023
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Carbon-bearing Molecules In A Possible Hycean Atmosphere
dimethyl sulfide (DMS) — Wikipedia

The search for habitable environments and biomarkers in exoplanetary atmospheres is the holy grail of exoplanet science. The detection of atmospheric signatures of habitable Earth-like exoplanets is challenging due to their small planet-star size contrast and thin atmospheres with high mean molecular weight. Recently, a new class of habitable exoplanets, called Hycean worlds, has been proposed, defined as temperate ocean-covered worlds with H2-rich atmospheres.

Their large sizes and extended atmospheres, compared to rocky planets of the same mass, make Hycean worlds significantly more accessible to atmospheric spectroscopy with the JWST. Here we report a transmission spectrum of the candidate Hycean world, K2-18 b, observed with the JWST NIRISS and NIRSpec instruments in the 0.9-5.2 μm range. The spectrum reveals strong detections of methane (CH4) and carbon dioxide (CO2) at 5σ and 3σ confidence, respectively, with high volume mixing ratios of ~1% each in a H2-rich atmosphere.

The abundant CH4 and CO2 along with the non-detection of ammonia (NH3) are consistent with chemical predictions for an ocean under a temperate H2-rich atmosphere on K2-18 b. The spectrum also suggests potential signs of dimethyl sulfide (DMS), which has been predicted to be an observable biomarker in Hycean worlds, motivating considerations of possible biological activity on the planet.

The detection of CH4 resolves the long-standing missing methane problem for temperate exoplanets and the degeneracy in the atmospheric composition of K2-18 b from previous observations. We discuss possible implications of the findings, open questions, and future observations to explore this new regime in the search for life elsewhere.

Spectral contributions of key molecular species in the 1-5 µm range. The different curves show individual contributions from different molecules to a nominal model transmission spectrum of K2-18 b shown in blue and denoted as Combined. The model assumes a mixing ratio of 10−2 for CH4 and CO2, 10−4 for H2O, 10−5 for all the other species, consistent with our retrieval estimates discussed in section 3, and an isothermal temperature profile of 250 K. Each curve corresponds to a transmission spectrum with opacity contributions from a single molecule at a time, in addition to H2-H2 and H2-He collisioninduced absorption. The spectral ranges of our JWST NIRISS and NIRSPec observations are also indicated; the NIRSpec range spans two detectors (NRS1 and NRS2) with a gap between them at 3.72-3.82 µm.

Nikku Madhusudhan, Subhajit Sarkar, Savvas Constantinou, Måns Holmberg, Anjali Piette, Julianne I. Moses

Comments: Accepted for publication in ApJ Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2309.05566 [astro-ph.EP] (or arXiv:2309.05566v1 [astro-ph.EP] for this version)
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Submission history
From: Madhusudhan Nikku
[v1] Mon, 11 Sep 2023 15:53:01 UTC (5,765 KB)

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