Climate Change in Hell: Long-Term Variation in Transits of the Evaporating Planet K2-22b

TL;DR

This study investigates the long-term behavior of the dust cloud around the evaporating planet K2-22b, revealing a decline in transit depth, large dust grains, and potential planetary surface changes, with no evidence of complete disintegration.

Contribution

It provides the first long-term multi-wavelength analysis of K2-22b's dust cloud, revealing trends and constraints on dust properties and planetary stability over a decade.

Findings

Transit depth has decreased since 2014.

Dust grains are likely large or the cloud is optically thick.

The planet's mass is at least comparable to Ceres.

Abstract

Context: Rocky planets on ultra-short period orbits can have surface magma oceans and rock-vapour atmospheres in which dust can condense. Observations of that dust can inform about the composition surface conditions on these objects. Aims: We constrain the properties and long-term (decade) behaviour of the transiting dust cloud from the "evaporating" planet K2-22b. Methods: We observed K2-22b around 40 predicted transits with MuSCAT ground-based multi-optical channel imagers, and complemented these data with long-term monitoring by the ground-based ATLAS (2018-2024) and space-based TESS (2021-2023) surveys. Results: We detected signals during 7 transits, none of which showed significant wavelength dependence. The expected number of MuSCAT-detected transits is >=22, indicating a decline in mean transit depth since the K2 discovery observations in 2014. Conclusions: Lack of significant wavelength dependence indicates that dust grains are large or the cloud is optically thick. Long-term trends of depth could be due to a magnetic cycle on the host star or overturn of the planet's dayside surface magma ocean. The possibility that K2-22b is disappearing altogether is ruled out by the stability of the transit ephemeris against non-gravitational forces, which constrains the mass to be at least comparable to Ceres.