University of Cambridge > > Exoplanet Seminars > Exploring the atmospheric structure of water-rich sub-Neptunes

Exploring the atmospheric structure of water-rich sub-Neptunes

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If you have a question about this talk, please contact Dr Dolev Bashi.

Population studies and planetary formation models predict a class of water-rich sub-Neptunes consisting of a rocky core overlain by a water-rich envelope. Characterising such planets is difficult since differing interior structures often lead to degenerate mass and radii. Recent JWST observations aim to break some of these degeneracies by retrieving atmospheric composition, however accurate atmospheric models are required to interpret data. For example, separate analyses of the JWST transmission spectrum of water world candidate TOI -270 d predicted different interior structures – a “Hycean world” scenario with a liquid water surface and a “miscible sub-Neptune” where the water in the envelope is in a supercritical state. To distinguish these scenarios, I have developed a radiative-convective model specifically designed to model water-rich sub-Neptunes. In particular, the model accounts for the inhibition of convection due to mean molecular weight gradients induced by the condensation of water vapour in a less dense background gas. I show that this can warm a liquid water surface significantly, moving the traditional habitable zone of the planet outwards and disfavouring the presence of water oceans for sub-Neptunes with Earth-like instellations. Following on from this, I will explore the possible equilibrium states of a sub-Neptune with a supercritical water envelope. Lastly, I will discuss attempts to decipher whether sub-Neptunes have surfaces using atmospheric chemistry and the implications this has on interpreting present and future observations.

This talk is part of the Exoplanet Seminars series.

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