A Snowball in Hell: The Potential Steam Atmosphere of TOI-1266c

TL;DR

This study explores the potential atmospheric states of TOI-1266c, a super-Venus exoplanet, using models to assess detectability of water-rich atmospheres and implications for planetary evolution.

Contribution

It introduces new modeling approaches to evaluate atmospheric compositions and detectability of TOI-1266c, providing insights into its nature and evolution.

Findings

Water-rich atmospheres may be detectable with JWST in under 20 hours.

Water vapor spectral features are minimally affected by high-altitude clouds.

TOI-1266c serves as a test case for theories on sub-Neptune and Venus-like planet evolution.

Abstract

TOI-1266c is a recently discovered super-Venus in the radius valley orbiting an early M dwarf. However, its notional bulk density ($\sim$2.2 g cm$^{-3}$) is consistent with a large volatile fraction, suggesting that it might have volatile reservoirs that have survived billions of years at more than twice the Earth's insolation. On the other hand, the upper mass limit paints a picture of a cool super Mercury dominated by >50\% iron core ($\sim$9.2 g cm$^{-3}$) that has tiptoed up to the collisional stripping limit and into the radius gap. Here, we examine several hypothetical states for TOI-1266c using a combination of new and updated open-source atmospheric escape, radiative-convective, and photochemical models. We find that water-rich atmospheres with trace amounts of H$_{2}$ and CO$_{2}$ are potentially detectable (SNR $>\sim 5$) in less than 20 hours of JWST observing time. We also find that water vapor spectral features are not substantially impacted by the presence of high-altitude water or ice clouds due the presence of a significant amount of water above the cloud-deck, although further work with self-consistent cloud models is needed. Regardless of its mass, however, TOI-1266c represents a unique proving ground for several hypotheses related to the evolution of sub-Neptunes and Venus-like worlds, particularly those near the radius valley.