Habitability and Biosignatures of Hycean Worlds

Brief

Using four-layer interior structure models and atmospheric radiative transfer, the Cambridge team maps the mass-radius space of Hycean planets (water mass fractions 10–90%, H₂-dominated atmospheres, ocean surfaces at 300–400 K and 1–1000 bar) for masses of 1–10 M⊕. The Hycean habitable zone's inner edge reaches equilibrium temperatures of ~500 K around late M dwarfs, far hotter than the terrestrial HZ, while the outer edge is physically unrestricted. A subclass, 'Dark Hycean' worlds, are tidally locked planets where only the permanent nightside is habitable; they can reach radii of 2.6 R⊕ at 10 M⊕. Trace biosignatures (DMS, DMDS, OCS, CH₃Cl, N₂O) are expected at ~1 ppmv and, per prior transmission-spectroscopy models, would be detectable with JWST for a sample of 11 nearby M-dwarf candidates including K2-18b.

Metadata

Category

Search

Venue

The Astrophysical Journal

Type

Peer-reviewed

Year

2021

Authors

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

DOI

10.3847/1538-4357/abfd9c

arXiv

2108.10888

Access

Open access

Length

4.2 M

Instruments

JWST NIRSpec, HST WFC3, ELT

Data sources

NASA Exoplanet Archive

Tags

astrobiology, biosignature, exoplanet atmospheres, habitability, transmission spectroscopy, mini-Neptune

Key points

• Hycean planets can reach radii of 2.6 R⊕ (Dark Hycean) or 2.3 R⊕ (canonical) at 10 M⊕, substantially larger than rocky super-Earths at the same mass, with the Dark Hycean upper boundary exceeding the pure-H₂O curve by up to ~0.25 R⊕.p.1
• The Hycean habitable zone inner boundary corresponds to equilibrium temperatures as high as ~500 K for late M dwarfs; the outer boundary is unrestricted to arbitrarily large orbital separations, unlike the CO₂-condensation-limited terrestrial HZ.p.1
• Dark Hycean worlds, tidally locked planets with Teq up to ~510 K, can sustain nightside ocean surfaces below 400 K and atmospheric terminator temperatures significantly higher, meaning transmission spectra probe conditions hotter than the habitable region itself.p.7
• Eleven candidate Hycean planets are identified orbiting M dwarfs with J < 10 mag; K2-18b leads the list at 8.63 ± 1.35 M⊕, 2.51 R⊕, Teq = 250 K, J = 9.8.p.7
• Biosignatures DMS, DMDS, OCS, CH₃Cl, and N₂O are expected at ~1 ppmv in Hycean atmospheres and are previously shown detectable via JWST transmission spectroscopy for H₂-rich super-Earths; the paper extends this to the larger Hycean targets.p.3
• A minimum H₂O mass fraction of 10% is set to ensure ocean survival over Gyr against photodissociation and atmospheric escape, equivalent to more than 100 times Earth's water mass fraction, even M-dwarf coronal mass ejections cannot fully strip it.p.6
• Microorganisms on Earth survive in up to ~88% H₂ concentrations in natural environments, and 100% in laboratory conditions, providing empirical support for the habitability premise of H₂-dominated Hycean oceans.p.3
• The H₂-rich atmosphere contributes less than 0.1% of total planetary mass yet can add ~0.1–0.2 R⊕ to the observable radius depending on temperature, pressure at the ocean surface, and mean molecular weight.p.5

Verbatim

• “We investigate a new class of habitable planets composed of water-rich interiors with massive oceans underlying H 2 -rich atmospheres, referred to here as Hycean worlds.”

  p.1

• “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 ⊕ ).”

  p.1

• “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.”

  p.1

• “Microorganisms on Earth are known to survive in H 2 -rich environments (Stevens & McKinley 1995; Freund et al. 2002; Gregory et al. 2019) including conditions with up to ∼ 88% H 2 concentrations in natural environments (Gregory et al. 2019), and even 100% in laboratory conditions (Seager et al. 2020).”

  p.3

• “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).”

  p.1

Most interesting

• Cold Hycean worlds, receiving negligible stellar irradiation at arbitrarily large orbital distances, are theoretically habitable, meaning the Hycean HZ has no defined outer edge.
• CH₄ is explicitly rejected as a reliable Hycean biosignature: it is a natural carbon carrier in H₂-rich atmospheres and its abiotic abundance would swamp any biological signal.
• Dark Hycean planets with Teq ~510 K can still host habitable nightside oceans; the dayside reaches 500–600 K but nightside surface temperatures remain below 400 K with inefficient day-night heat redistribution.
• The paper's internal structure model uses a fourth-order Runge-Kutta scheme solving mass continuity and hydrostatic equilibrium across four layers (Fe core, silicate mantle, H₂O ocean, H₂/He envelope), with the H₂O equation of state compiled from six separate sources spanning from 1993 to 2021.
• The enhanced H₂O mixing ratio assumed for the atmosphere (10%, or 100× solar) is observationally anchored to the upper end of H₂O abundances retrieved for K2-18b from Hubble/HST spectra (Benneke et al. 2019b).
• The paper directly set up the subsequent K2-18b JWST observations and the contested 2023 DMS detection claim, the biosignature targets and detection feasibility arguments here were the scientific motivation for those follow-up observations.