The runaway greenhouse on subNeptune waterworlds

TL;DR

This paper explores the runaway greenhouse effect on water-rich subNeptune planets, highlighting how extreme water vapor conditions cause significant radius inflation and internal state transitions, with implications for observational detection.

Contribution

It develops a model for post-runaway planetary states, including internal structure changes and radius inflation, using the AQUA equation of state database for water-rich subNeptunes.

Findings

Runaway greenhouse causes drastic internal state changes.

Radius inflation correlates with runaway conditions.

Observational signatures are limited by data degeneracies.

Abstract

The implications of the water vapor runaway greenhouse phenomenon for water-rich subNeptunes are developed. In particular, the nature of the post-runaway equilibration process for planets that have an extremely high water inventory is addressed. Crossing the threshold from sub-runaway to super-runaway conditions leads to a transition from equilibrated states with cold deep liquid oceans and deep interior ice-X phases to states with hot supercritical fluid interiors. There is a corresponding marked inflation of radius for a given mass, similar to the runaway greenhouse radius inflation effect noted earlier for terrestrial planets, but in the present case the inflation involves the entire interior of the planet. The calculation employs the AQUA equation of state database to simplify the internal structure calculation. Some speculations concerning the effect of $\mathrm{H_2}$ admixture, silicate cores and hot vs. cold start evolution trajectories are offered. Observational implications are discussed, though the search for the mass-radius signature of the phenomena considered is limited by degeneracies and by lack of data.