Habitability and Biosignatures of Hycean Worlds
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Abstract
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.
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1538-4357