The interior and atmosphere of the habitable-zone exoplanet K2-18b (2002.11115v1)

Abstract: Exoplanets orbiting M dwarfs present a valuable opportunity for their detection and atmospheric characterisation. This is evident from recent inferences of H$_2$O in such atmospheres, including that of the habitable-zone exoplanet K2-18b. With a bulk density between Earth and Neptune, K2-18b may be expected to possess a H/He envelope. However, the extent of such an envelope and the thermodynamic conditions of the interior remain unexplored. In the present work, we investigate the atmospheric and interior properties of K2-18b based on its bulk properties and its atmospheric transmission spectrum. We constrain the atmosphere to be H$_2$-rich with a H$_2$O volume mixing ratio of $0.02-14.8$%, consistent with previous studies, and find a depletion of CH$_4$ and NH$_3$, indicating chemical disequilibrium. We do not conclusively detect clouds/hazes in the observable atmosphere. We use the bulk parameters and retrieved atmospheric properties to constrain the internal structure and thermodynamic conditions in the planet. The constraints on the interior allow multiple scenarios between rocky worlds with massive H/He envelopes and water worlds with thin envelopes. We constrain the mass fraction of the H/He envelope to be $\lesssim 6$%; spanning $\lesssim 10^{-5}$ for a predominantly water world to $\sim6$% for a pure iron interior. The thermodynamic conditions at the surface of the H$_2$O layer range from the super-critical to liquid phases, with a range of solutions allowing for habitable conditions on K2-18b. Our results demonstrate that the potential for habitable conditions is not necessarily restricted to Earth-like rocky exoplanets.

Interior and Atmosphere of the Habitable-Zone Exoplanet K2-18b

The paper by Madhusudhan et al. presents a comprehensive paper of the atmospheric composition and interior structure of the exoplanet K2-18b, situated in the habitable zone of its M dwarf host star. The paper employs both observational data and theoretical models to provide constraints on various possible scenarios for the internal composition of K2-18b, which exhibits a bulk density between that of Earth and Neptune.

Atmospheric Analysis

The authors conducted an atmospheric retrieval using the transmission spectrum, confirming a hydrogen-rich atmosphere with significant water vapor (H$_2$O) concentrations, ranging from 0.02% to 14.8%. The atmospheric composition also shows a conspicuous depletion of methane (CH$_4$) and ammonia (NH$_3$), indicating a chemical disequilibrium. The analysis did not conclusively detect clouds or hazes, suggesting their influence might be marginal. These results are consistent with previous observational inferences, establishing a robust profile of K2-18b's atmosphere.

Interior Composition

The paper's internal structure models explore various plausible interior compositions, particularly focusing on the mass fraction distributions across a four-layer structure: a core composed of iron and silicates, a water layer, and a hydrogen/helium envelope. The authors provide constraints on the mass fraction of the H/He envelope, determined to be less than 6%. Numerous configurations are possible, ranging from predominantly water worlds with minimal gaseous envelopes to those with substantial rocky cores and thin atmospheres.

Implications for Habitability

The findings expand our understanding of habitability beyond Earth-like conditions. Notably, the paper illustrates that planets having hydrogen atmospheres, like K2-18b, could sustain surface pressures and temperatures conducive to liquid water. The thermodynamic conditions at the H$_2$O-H/He boundary potentially support habitable environments, challenging the notion that habitability is exclusive to rocky exoplanets similar to Earth.

Conclusion and Future Prospects

The constraints offered by the authors on K2-18b significantly enrich the dialogue on exoplanetary habitability. The potential liquid water conditions highlight the variety of planetary environments that could maintain life. Looking ahead, advanced observations with the James Webb Space Telescope hold promise to provide more precise characterizations of such exoplanetary systems. Given the findings of chemical disequilibrium, the search for biosignatures on K2-18b and similar planets merits continued scientific inquiry, opening new avenues in the paper of life beyond our solar system.

By probing the compositions and conditions of exoplanets such as K2-18b, this work contributes to astrobiology and planetary science, refining models of atmospheres and interiors while expanding our search for habitable worlds.