Prospects For Biological Evolution On Hycean Worlds
Recent detections of carbon-bearing molecules in the atmosphere of a candidate Hycean world, K2-18 b, with JWST are opening the prospects for characterising potential biospheres on temperate exoplanets.
Hycean worlds are a recently theorised class of habitable exoplanets with ocean covered surfaces and hydrogen-rich atmospheres. Hycean planets are thought to be conducive for hosting microbial life under conditions similar to those in the Earth’s oceans. In the present work we investigate the potential for biological evolution on Hycean worlds and their dependence on the thermodynamic conditions.
We find that a large range of evolutionary rates and origination times are possible for unicellular life in oceanic environments for a relatively marginal range in environmental conditions.
Bulk properties of temperate sub-Neptune exoplanets. The circles with uncertainties show a selected sample of temperate sub-Neptune exoplanets with confirmed measurements of radii and masses, with uncertainties below 2 earth masses, and zero-albedo equilibrium temperatures (Teq) below 600 K. The circles are color-coded by Teq as denoted by the color bar. The dashed lines show theoretical mass-radius curves of model planets of uniform interior compositions denoted in the legend and the cyan and crimson regions denote ranges of masses and radii allowed for Hycean and Dark Hycean planets, following Madhusudhan et al. (2021). A subset of these planets with Teq below ∼400 K and lying in the cyan region, marked with grey outer circles, could be candidate Hycean planets (Madhusudhan et al. 2021). Adapted from Madhusudhan et al. (2021) with data updated based on the NASA Exoplanet Archive. — astro-ph.EP
For example, a relatively small (10 K) increase in the average ocean temperature can lead to over twice the evolutionary rates, with key unicellular groups originating as early as ∼1.3 billion years from origin of life. On the contrary, similar decreases in temperatures can also significantly delay the origination times by several billion years.
This delay in turn could affect their observable biomarkers such as dimethylsulfide, which is known to be produced predominantly by Eukaryotic marine phytoplankton in Earth’s oceans.
Therefore, Hycean worlds that are significantly cooler than Earth may be expected to host simpler microbial life than Earth’s oceans and may show weaker biosignatures, unless they orbit significantly older stars than the Sun.
Conversely, Hycean worlds with warmer surface temperatures than Earth are more likely to show stronger atmospheric biosignatures due to microbial life if present.
Time-calibrated phylogenetic trees with calculated evolutionary rates at Earth’s median temperature (top) and at +10 K increase relative to Earth (bottom) with the colour indicating the evolutionary rates at the nodes. — astro-ph.EP
Emily G. Mitchell, Nikku Madhusudhan
Comments: Accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2502.07872 [astro-ph.EP] (or arXiv:2502.07872v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2502.07872
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Submission history
From: Madhusudhan Nikku
[v1] Tue, 11 Feb 2025 19:00:00 UTC (1,867 KB)
https://arxiv.org/abs/2502.07872
Astrobiology,
