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 R⊕ (2.3 R⊕) for a mass of 10 M⊕ (5 M⊕). 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 that 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.
| Original language | English |
|---|---|
| Article number | 1 |
| Number of pages | 25 |
| Journal | The Astrophysical Journal |
| Volume | 918 |
| Issue number | 1 |
| Early online date | 26 Aug 2021 |
| DOIs | |
| Publication status | Published - 1 Sept 2021 |
Bibliographical note
Acknowledgments:We thank the anonymous reviewer for their valuable comments and the editorial team at AAS journals for efficiently overseeing the review and publication of our work during the challenging past year of the COVID-19 pandemic. We thank all those in Cambridge and beyond who worked on the front lines to keep us safe during the pandemic. A.A.A.P. acknowledges support from the UK Science and Technology Facilities Council (STFC) toward her doctoral studies. We thank Siddharth Gandhi for discussion on day–night energy redistribution in the self-consistent atmospheric models, Matthew Nixon for discussion on the water equation of state, and Subhajit Sarkar for discussion on JWST simulated data. We thank James Kasting for helpful comments on our manuscript. This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. This research has made use of the NASA Astrophysics Data System and the Python packages numpy, scipy, and matplotlib. Part of this work was performed using resources provided by the Cambridge Service for Data Driven Discovery (CSD3) operated by the University of Cambridge Research Computing Service (www.csd3.cam.ac.uk), provided by Dell EMC and Intel using Tier-2 funding from the Engineering and Physical Sciences Research Council (capital grant EP/P020259/1), and DiRAC funding from the Science and Technology Facilities Council (www.dirac.ac.uk).