The surface and interior conditions of temperate sub-Neptune TOI-270 d

ABSTRACT Sub-Neptune planets, with no analogue in our Solar system, provide a wealth of information about exoplanet diversity, formation and evolution, and habitability. Their robust characterization requires the coupling of physically informed atmosphere and interior models with precise atmospheric data to break compositional degeneracies. Recent James Webb Space Telescope (JWST) observations of the temperate sub-Neptune TOI-270 d revealed detections of CH$_4$ and CO$_2$ in its H$_2$-rich atmosphere, with tentative inferences of H$_2$O and CS$_2$ and a non-detection of NH$_3$. We conduct a theoretical exploration of the range of possible interiors for TOI-270 d based on the current observational constraints. We carry out internal structure modelling using a coupled atmosphere-interior model, including self-consistent atmospheric temperature structures informed by JWST observations. The bulk properties permit solutions spanning mini-Neptune, gas dwarf and hycean scenarios, with a wide range of possible surface conditions, which are strongly dependent on the atmospheric properties, including the presence of clouds/hazes. We explore the solutions allowing for surface water oceans on TOI-270 d, including under potentially habitable conditions. The atmospheric mass fractions permitting habitable surface conditions are found to be $\lesssim$$3.5\times 10^{-5}$ and pressures $\lesssim$100 bar for the envelope temperature structures considered. We consider mini-Neptune interiors that are sufficiently warm for H$_2$O to be mixed with the H$_2$-rich envelope. Finally, we consider possible gas dwarf interiors, finding H$_2$-rich envelope mass fractions of $\sim$$1\!-\!5$ per cent are required to satisfy the bulk properties, with surface pressures $\sim 10^4\!-\!10^5$ bar. Further theoretical and experimental studies in addition to future atmospheric observations will aid the characterization of the possible interior and surface conditions on TOI-270 d.