The runaway greenhouse radius inflation effect: An observational diagnostic to probe water on Earth-sized planets and test the habitable zone concept

Planets similar to Earth but slightly more irradiated are expected to enter into a runaway greenhouse state, where all surface water rapidly evaporates, forming an optically thick H₂O-dominated atmosphere. For Earth, this extreme climate transition is thought to occur for an increase of only ∼6% in solar luminosity, though the exact limit at which the transition would occur is still a highly debated topic. In general, the runaway greenhouse is believed to be a fundamental process in the evolution of Earth-sized, temperate planets. Using 1D radiative-convective climate calculations accounting for thick, hot water vapor-dominated atmospheres, we evaluate the transit atmospheric thickness of a post-runaway greenhouse atmosphere, and find that it could possibly reach over a thousand kilometers (i.e., a few tens of percent of the Earth's radius). This abrupt radius inflation resulting from the runaway-greenhouse-induced transition could be detected statistically by ongoing and upcoming space missions. These include satellites such as TESS, CHEOPS, and PLATO combined with precise radial velocity mass measurements using ground-based spectrographs such as ESPRESSO, CARMENES, or SPIRou. This radius inflation could also be detected in multiplanetary systems such as TRAPPIST-1 once masses and radii are known with good enough precision. This result provides the community with an observational test of two points. The first point is the concept of runaway greenhouse, which defines the inner edge of the traditional habitable zone, and the exact limit of the runaway greenhouse transition. In particular, this could provide an empirical measurement of the irradiation at which Earth analogs transition from a temperate to a runaway greenhouse climate state. This astronomical measurement would make it possible to statistically estimate how close Earth is from the runaway greenhouse. Second, it could be used as a test for the presence (and statistical abundance) of water in temperate, Earth-sized exoplanets.