Deciphering Sub-Neptune Atmospheres: New Insights from Geochemical Models of TOI-270 d

Abstract: The nature of sub-Neptunes is one of the hottest topics in exoplanetary science. Temperate sub-Neptunes are of special interest because some could be habitable. Here, we consider whether these planets might instead be rocky worlds with thick, hot atmospheres. Can recent JWST observations of TOI-270 d be understood in terms of such a model? We perform thermochemical equilibrium calculations to infer conditions of quenching of C-H-O-N species. Our results indicate apparent CO$_2$-CH$_4$ equilibrium between ~900 and ~1100 K. The CO abundance should be quenched higher in the atmosphere where the equilibrium CO/CO$_2$ ratio is lower, potentially explaining a lack of CO. N$_2$ is predicted to dominate the nitrogen budget. We confirm that the atmosphere of TOI-270 d is strongly enriched in both C and O$_g$$_a$$_s$ relative to protosolar H, whereas N is likely to be less enriched or even depleted. We attempt to reproduce these enrichments by modeling the atmosphere as nebular gas that extracted heavy elements from accreted solids. This type of model can explain the C/H and O$_g$$_a$$_s$/H ratios, but despite supersolar C/N ratios provided by solids, the NH$_3$ abundance will probably be too high unless there is a nitrogen sink in addition to N$_2$. A magma ocean may be implied, and indeed the oxygen fugacity of the deep atmosphere seems sufficiently low to support the sequestration of reduced N in silicate melt. The evaluation presented here demonstrates that exoplanetary geochemistry now approaches a level of sophistication comparable to that achieved within our own solar system.

Analyzing the Geochemistry of TOI-270 d: Unveiling a Temperate Sub-Neptune Atmosphere

The paper "Deciphering Sub-Neptune Atmospheres: New Insights from Geochemical Models of TOI-270 d" presents a comprehensive geochemical analysis of the exoplanet TOI-270 d, a temperate sub-Neptune. Through a series of thermochemical equilibrium calculations, the study explores the plausibility of a rocky world with a thick, hot atmosphere, an intriguing alternative to the Hycean world hypothesis.

The authors made use of data from the JWST, revealing that TOI-270 d's atmosphere is enriched with CO$_2$, CH$_4$, and H$_2$O, marking robust constraints on the atmospheric composition. Specifically, thermochemical models were employed to infer equilibrium quenching conditions between CO$_2$ and CH$_4$, indicating an equilibrium temperature range between 900 and 1100 K and pressures from 0.9 to 13 bar. Furthermore, they assert that the CO abundance in the atmosphere is potentially quenched to a state where the CO/CO$_2$ ratio is significantly lower, offering an explanation for its observed scarcity.

In addressing the nitrogen composition of TOI-270 d's atmosphere, the authors propose that N$_2$ is likely the dominant nitrogen species over NH$_3$, potentially due to the formation of a magma ocean with low oxygen fugacity that sequesters reduced nitrogen species in a silicate melt. The N/H ratio was constrained to be relatively low, reflecting a deficiency of nitrogen compared to protosolar standards.

The implications of these models are significant. Firstly, they suggest that under conditions of metal-rich enrichment, TOI-270 d's atmosphere could be analogous in certain elemental enrichments to some of the solar system's gas giants, albeit through different formation mechanisms. This finding enhances our understanding of sub-Neptune formation and throws light on the potential diversity among exoplanetary systems. Furthermore, speciation of key elements within the atmosphere indicates significant deviations from previously hypothesized atmospheric conditions, such as Hycean worlds, while providing detailed analytical frameworks for interpreting atmospheric data.

The paper concludes that the potential pathways for nitrogen depletion, involving a combination of photochemical processes and atmospheric-surface interactions, could explain the “missing ammonia” problem without requiring the invocation of the Hycean world scenario. This contributes to a broader understanding of exoplanetary atmospheres, especially those in the sub-Neptune category.

Looking forward, the study emphasizes the need for continued observational efforts to refine the chemical models of TOI-270 d, especially regarding CO and NH$_3$ detections, which could provide further insights into atmospheric dynamics and composition. Additionally, ongoing comparisons with other sub-Neptunes and solar system analogs could illuminate diverse planetary formation processes and interior-atmosphere interactions. As new observational platforms and methodologies become available, a broader geochemical perspective encompassing direct and indirect measurements will be essential for advancing this nascent field of exoplanetary science.