The runaway greenhouse radius inflation effect and observational impacts

Presenter: Martin TURBET
Planets similar to Earth – but slightly more irradiated – are expected to experience a runaway greenhouse transition, a state in which a net positive feedback between surface temperature, evaporation, and atmospheric opacity causes a runaway warming. This runaway greenhouse positive feedback ceases only when oceans have completely boiled away, forming an optically thick H2O-dominated atmosphere. This runaway greenhouse limit is traditionally used to define the inner edge of the Habitable Zone.

Using a hierarchy of numerical climate models, we found that as an Earth-like planet experiences a runaway greenhouse transition, the apparent thickness of its atmosphere evolves from a few tens of kilometers to possibly as high as over a thousand kilometers, depending on the initial water content of the planet. We call this phenomenon the “runaway greenhouse radius inflation effect”.

We will show how we can take advantage of this effect to test the Habitable Zone concept as well as to diagnose the statistical abundance of water in temperate, Earth-size exoplanets. We will also show how this effect can dramatically affect estimates of mass-radius relationships for water-rich terrestrial-size planets and discuss it in the context of the TRAPPIST-1 system.