Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry (2108.04745v1)

Abstract: The recent discovery and initial characterization of sub-Neptune-sized exoplanets that receive stellar irradiance of approximately Earth's raised the prospect of finding habitable planets in the coming decade, because some of these temperate planets may support liquid water oceans if they do not have massive H2/He envelopes and are thus not too hot at the bottom of the envelopes. For planets larger than Earth, and especially planets in the 1.7-3.5 R_Earth population, the mass of the H2/He envelope is typically not sufficiently constrained to assess the potential habitability. Here we show that the solubility equilibria vs. thermochemistry of carbon and nitrogen gases results in observable discriminators between small H2 atmospheres vs. massive ones, because the condition to form a liquid-water ocean and that to achieve the thermochemical equilibrium are mutually exclusive. The dominant carbon and nitrogen gases are typically CH4 and NH3 due to thermochemical recycling in a massive atmosphere of a temperate planet, and those in a small atmosphere overlying a liquid-water ocean are most likely CO2 and N2, followed by CO and CH4 produced photochemically. NH3 is depleted in the small atmosphere by dissolution into the liquid-water ocean. These gases lead to distinctive features in the planet's transmission spectrum, and a moderate number of repeated transit observations with the James Webb Space Telescope should tell apart a small atmosphere vs. a massive one on planets like K2-18 b. This method thus provides a way to use near-term facilities to constrain the atmospheric mass and habitability of temperate sub-Neptune exoplanets.

• The paper demonstrates that transit spectroscopy can differentiate small, habitable H₂ atmospheres from massive, recycling ones on temperate sub-Neptunes.
• It reveals that solubility equilibria and thermochemical processes yield distinct gas signatures, with CO₂ and N₂ in thin atmospheres versus CH₄ and NH₃ in massive ones.
• The study outlines JWST observation strategies, suggesting modest exposure times can effectively characterize atmospheric composition and potential habitability of exoplanets like K2-18 b.

Analyzing the Hidden Oceans: Atmospheric Characterization of Temperate Sub-Neptunes

The paper by Hu et al. explores the potential for unveiling liquid water oceans on temperate sub-Neptune exoplanets through their transit signals. It specifically focuses on distinguishing between small hydrogen (H₂) atmospheres versus massive ones using the James Webb Space Telescope (JWST) among other technologies.

Key Findings

• Atmospheric Mass and Composition: The paper addresses the pivotal role of the atmospheric mass in determining the habitability of sub-Neptune exoplanets. The main highlight centers around discriminating between small H₂ dominated atmospheres, conducive to habitable environments, and massive atmospheres resulting in significant thermochemical recycling of gases like CH₄, NH₃, and H₂S.
• Solubility Equilibria vs. Thermochemistry: It provides an in-depth analysis of how solubility equilibria in oceans and thermochemical recycling lead to distinct gas abundance signatures. Notably, in smaller atmospheres conducive to liquid-water oceans, gases like CO₂ and N₂ prevail. In contrast, massive atmospheres are characterized by CH₄ and NH₃ due to the thermochemical recycling at high pressures and temperatures.
• Observable Atmospheric Signatures: The work emphasizes identifying H₂ atmospheres through their pressure scale height with transmission spectroscopy. It notes that differences in atmospheric compositions can be distinguished based on the presence of gases such as NH₃ and HCN vs. CO₂ and CO in different atmospheric scenarios. For instance, in small H₂ atmospheres, CO₂ and CO features are prominent, while NH₃ and HCN are nearly absent.
• Practical Implications for JWST: The paper argues that using JWST, with modest observation time, can enable atmospheric characterization of temperate sub-Neptune exoplanets like K2-18 b. The authors use model simulations to suggest that specific gas signatures (e.g., CO₂, CO, CH₄, NH₃) are identifiable.

Implications and Future Directions

The research offers meaningful implications for the field of exoplanetary science, particularly in using transit spectroscopy to infer atmosphere size and potential habitability. The robust differentiation between thin and thick atmospheres on temperate sub-Neptunes sets the stage for identifying potentially habitable worlds. With JWST and missions like ARIEL looking towards atmospheric characterization, this paper presents a near-term framework for evaluating sub-Neptune atmospheres.

Furthermore, the understanding of atmospheric processes on sub-Neptunes could inform planetary formation models. It addresses significant gaps in knowledge concerning the internal structure and composition of exoplanets larger than Earth, expanding models to accommodate a range of atmospheric possibilities including those enabling liquid water oceans.

Conclusion

Hu et al.'s work presents a comprehensive methodology for evaluating the habitability of sub-Neptunes through transit spectroscopy, setting a precedent for future exploration of potential habitable exoplanets. This research not only enhances our understanding of sub-Neptune classes but also reinforces the possibility of discovering habitable exoplanets within our observational reach in the near future.

This paper stands as a promising foundation for further advancements in characterizing the atmospheres of temperate exoplanets and understanding the diverse environments that may support life beyond Earth.