Habitability And Biosignatures Of Hycean Worlds


The Hycean mass-radius (M-R) plane. The M-R plane of regular Hycean planets is shown in cyan, and that of Dark Hycean planets is shown in red, which includes the cyan region. Dashed lines show M-R curves for homogeneous compositions of 100% iron (gray), 100% silicate (green), Earth-like composition (brown: 32.5% Fe + 67.5% silicate), and 100% H2O at 300 K and 1 bar surface conditions (blue), as shown in the legend. The concentric black circles show the two case studies used in Section 3. The black circles with error bars show transiting exoplanets with observed masses and radii, color-coded by their equilibrium temperature (Teq), defined in eq (2), assuming full day-night energy redistribution and a Bond albedo of 0.5. Only planets orbiting host stars with J mag < 13 are shown. We note that while planets with masses and radii shown in the Hycean regions can be Hycean candidates, other internal structures may also be admissible by the data (see, e.g., Section 2.3). A list of promising Hycean candidates is shown in Table 1. Exoplanet data obtained from the NASA Exoplanet Archive.

We investigate a new class of habitable planets composed of water-rich interiors with massive oceans underlying H2-rich atmospheres, referred to here as Hycean worlds.

With densities between those of rocky super-Earths and more extended mini-Neptunes, Hycean planets can be optimal candidates in the search for exoplanetary habitability and may be abundant in the exoplanet population. We investigate the bulk properties (masses, radii, and temperatures), potential for habitability, and observable biosignatures of Hycean planets.

We show that Hycean planets can be significantly larger compared to previous considerations for habitable planets, with radii as large as 2.6 Earth radii (2.3 Earth radii) for a mass of 10 Earth masses (5 Earth masses). We construct the Hycean habitable zone (HZ), considering stellar hosts from late M to sun-like stars, and find it to be significantly wider than the terrestrial-like HZ.

While the inner boundary of the Hycean HZ corresponds to equilibrium temperatures as high as ~500 K for late M dwarfs, the outer boundary is unrestricted to arbitrarily large orbital separations. Our investigations include tidally locked `Dark Hycean' worlds that permit habitable conditions only on their permanent nightsides and `Cold Hycean' worlds that see negligible irradiation.

Finally, we investigate the observability of possible biosignatures in Hycean atmospheres. We find that a number of trace terrestrial biomarkers which may be expected to be present in Hycean atmospheres would be readily detectable using modest observing time with the James Webb Space Telescope (JWST). We identify a sizable sample of nearby potential Hycean planets that can be ideal targets for such observations in search of exoplanetary biosignatures.

Nikku Madhusudhan, Anjali A. A. Piette, Savvas Constantinou

Comments: Accepted for publication in The Astrophysical Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2108.10888 [astro-ph.EP] (or arXiv:2108.10888v1 [astro-ph.EP] for this version)
Submission history
From: Madhusudhan Nikku
[v1] Tue, 24 Aug 2021 18:00:01 UTC (6,283 KB)
https://arxiv.org/abs/2108.10888
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