Habitability and Biosignatures of Hycean Worlds (2108.10888v2)

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 andCold 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.

• The paper introduces Hycean worlds as a new class of exoplanets with water-rich interiors beneath hydrogen-rich atmospheres, expanding traditional habitability models.
• It demonstrates that these planets display unique mass-radius relationships and an extended habitable zone, tolerating higher equilibrium temperatures than Earth-like planets.
• The study outlines a method for detecting biosignatures such as DMS and nitrous oxide using transmission spectroscopy with JWST, offering actionable targets for future research.

An Overview of "Habitability and Biosignatures of Hycean Worlds"

The paper "Habitability and Biosignatures of Hycean Worlds," authored by Madhusudhan et al., introduces a new category of potentially habitable exoplanets named "Hycean" worlds. These are defined by their substantial water-rich interiors beneath hydrogen-rich atmospheres. The authors explore the mass-radius characteristics, potential habitability, and detectable atmospheric biosignatures of these planets.

Key Findings

1. Mass-Radius Plane of Hycean Worlds: Hycean planets differ significantly from rocky super-Earths and gas-rich mini-Neptunes. According to the paper, the presence of extensive water layers under high-pressure hydrogen atmospheres allows these planets to have substantially larger radii – up to 2.6 $R_\oplus$ for a 10 $M_\oplus$ planet, challenging previous notions of habitable planet sizes. The authors reconstruct the theoretical bounds for Hycean planets within this mass-radius space.
2. Hycean Habitability Zone (HZ): Unlike the conventional habitable zone determined for Earth-like planets, the Hycean HZ, as described by the authors, is considerably wider. For stars ranging from late M to early G types, Hycean planets can remain habitable even with equilibrium temperatures far exceeding those deemed tolerable for Earth-like conditions. The extended range arises from the ability of these worlds to maintain a liquid water ocean at pressures up to 1000 bar and temperatures reaching 395 K.
3. Subcategories of Hycean Worlds: The research identifies nuanced subcategories such as the "Dark Hycean" worlds, which remain habitable on their permanently night-lit faces due to inefficient day-night heat redistribution, even at equilibrium temperatures of approximately 510 K. They also propose "Cold Hycean" worlds, where habitability is sustained through internal heat flux with minimal stellar irradiation, allowing survival in interstellar space. These classifications broaden the criteria under which life might be sustained beyond traditional models.
4. Biosignature Detection: The paper introduces a promising avenue for detecting biosignatures in Hycean atmospheres using transmission spectroscopy, particularly through instruments on the James Webb Space Telescope (JWST). The authors list potential biomarkers, including dimethyl sulfide (DMS) and nitrous oxide (N$_2$O), which could indicate biological activity even in hydrogen-dominated atmospheres. Their paper suggests that these biosignatures can be detected with feasible telescope time allocations, even at the part-per-million level, especially for nearby Hycean candidates like K2-18b.

Implications and Future Directions

This paper's proposition of Hycean worlds represents a significant expansion of the characteristics and environments considered potentially habitable, extending beyond the traditional habitable zone of rocky planets with CO$_2$-dominated atmospheres. The wider habitable zone and larger radii of Hycean planets make them attractive targets for atmospheric characterization and biosignature detection using current and upcoming telescope technologies. The paper’s findings suggest that habitable conditions could exist in regimes previously deemed inhospitable, including high-temperature hydrogen-rich environments and isolated planets without stellar insolation.

The research discussed in the article opens new avenues in exoplanet exploration, suggesting that the search for extraterrestrial life may find fruitful prospects in planetary types that differ significantly from Earth analogs. Future research could further develop techniques for confirming the presence of these biosignatures and expand the scope of atmospheric modeling to include dynamic processes specific to these exotic worlds. Additionally, as observational technologies advance, the identification and confirmation of such planets could significantly enhance our understanding of life's potential spread across the cosmos.