The Surface and Interior Conditions of Temperate Sub-Neptune TOI-270 d

TL;DR

This study couples JWST-derived atmospheric constraints with the HyRIS interior model to map the range of plausible TOI-270 d interiors. By generating self-consistent envelope P–T profiles via GENESIS and exploring four envelope cases, the authors identify viable solutions spanning Hycean worlds with surface oceans, Mini-Neptunes with distinct H$_2$O layers, and Gas Dwarfs with thick H$_2$ envelopes and potential magma oceans. Habitable hycean conditions require extremely small envelope mass fractions ($x_{ m env} \\lesssim 3.5\times10^{-5}$) and substantial water fractions ($x_{ m H_2O} \\gtrsim 60\%$), with ocean depths of a few hundred kilometers; other scenarios depend sensitively on temperature structure and miscibility of water and hydrogen. The work highlights the critical role of atmospheric data in breaking interior degeneracies and outlines future observational and theoretical steps needed to robustly characterize TOI-270 d’s surface and interior state.

Abstract

Sub-Neptune planets, with no analogue in our solar system, provide a wealth of information about exoplanet diversity, formation & evolution, and habitability. Their robust characterisation requires the coupling of physically informed atmosphere and interior models with precise atmospheric data to break compositional degeneracies. Recent 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$$3.5\times10^{-5}$ and pressures $\lesssim$100 bar for the envelope temperature structures considered. We consider mini-Neptune interiors that are sufficiently warm for H$_2$O to be mixed with the H$_2$-rich envelope. Finally, we consider possible gas dwarf interiors, finding H$_2$-rich envelope mass fractions of $\sim$$1-5$ % are required to satisfy the bulk properties, with surface pressures $\sim10^4-10^5$ bar. Further theoretical and experimental studies in addition to future atmospheric observations will aid the characterisation of the possible interior and surface conditions on TOI-270 d.

Figures (9)

• Figure 1: Temperature structures adopted for the envelope, generated with GENESIS using the retrieved abundances of Holmberg2024. These cases correspond to each combination of $T_\mathrm{int}$ of $25$ and $50$ K, and Rayleigh enhancement factor $a$ of $100$ and $1500$. The profiles, generated to $1000$ bar, are extended using an adiabat. We also show the evening $P$-$T$ profile from Figure 10 of Benneke2024, which is only provided to 10 bar pressures.
• Figure 2: Mass-radius curves for different planet compositions, including the standard isothermal $M$-$R$ curves shown by dashed lines. The hycean mass-radius plane from Madhusudhan2021 is also shown. The mass and radius measurements for TOI-270 d are shown. VE21 is VanEylen2021, ME22 is MikalEvans2023 and K22 is Kaye2022.
• Figure 3: Model atmospheric $P$-$T$ profiles relative to the phase diagram for a 100% H$_2$O surface below the atmosphere. At a given point on a $P$-$T$ profile the phase of H$_2$O in the background corresponds to the phase of a pure H$_2$O surface assuming that point represents the boundary between the atmosphere and a pure H$_2$O interior Madhusudhan2020Piette2020Rigby2024. Thus, each $P$-$T$ profile effectively represents the locus of pressure and temperature values permissible at the boundary between the atmosphere and the interior for that model. The H$_2$O phase boundaries for pure H$_2$O are obtained from Dunaeva2010. The critical curve for H$_2$/H$_2$O mixtures is shown in the grey dashed line, above which H$_2$ and H$_2$O would be expected to exist as one phase Gupta2024. The atmospheric $P$-$T$ profiles are generated using self-consistent models as discussed in Section \ref{['Method:PTprofiles']}.
• Figure 4: Left: Ternary diagram showing the best-fit compositions for TOI-270 d, for an Earth-like core composition (blue) and a $100\%$ Fe core (red). In both cases we adopt Case 1 as the atmospheric $P$-$T$ profile. Right: Mass fractions of the H$_2$-rich envelope against core mass fraction, for Earth-like (blue) and $100\%$ Fe (red) core compositions. The red and blue shaded regions indicate combinations that reproduce the observed mass and radius to within $1\sigma$. The best fit solutions in each case are shown by the black solid line. The dashed blue lines show constant H$_2$O mass fractions at $25\%$ intervals.
• Figure 5: Locus of possible HHB conditions for TOI-270 d for the Case 1 envelope $P$-$T$ profile. An interior adiabat is also shown, corresponding to the hycean cases in Section \ref{['Res:Hycean']}, with surface conditions at 360 K and 18 bar.