Cloudless atmospheres for L/T dwarfs and extra-solar giant planets

TL;DR

This paper proposes a novel explanation for brown dwarf and exoplanet spectral evolution that eliminates the need for clouds, attributing observed phenomena to thermo-chemical instabilities caused by slow chemical reactions.

Contribution

It introduces a new cloudless model based on thermo-chemical instability to explain spectral changes in brown dwarfs and exoplanets, challenging the traditional cloud-based paradigm.

Findings

Explains J-band brightening without clouds.

Accounts for FeH absorption resurgence at L/T transition.

Provides a physical basis for the L to T spectral transition.

Abstract

The admitted, conventional scenario to explain the complex spectral evolution of brown dwarfs (BD) since their first detections twenty years ago, has always been the key role played by micron-size condensates, called "dust" or "clouds", in their atmosphere. This scenario, however, faces major problems, in particular the J-band brightening and the resurgence of FeH absorption at the L to T transition, and a physical first-principle understanding of this transition is lacking. In this paper, we propose a new, completely different explanation for BD and extrasolar giant planet (EGP) spectral evolution, without the need to invoke clouds. We show that, due to the slowness of the CO/CH4 and N2/NH3 chemical reactions, brown dwarf (L and T, respectively) and EGP atmospheres are subject to a thermo-chemical instability similar in nature to the fingering or chemical convective instability present in Earth oceans and at the Earth core/mantle boundary. The induced small-scale turbulent energy transport reduces the temperature gradient in the atmosphere, explaining the observed increase in near infrared J-H and J-K colors of L dwarfs and hot EGPs, while a warming up of the deep atmosphere along the L to T transition, as the CO/CH4 instability vanishes, naturally solves the two aforementioned puzzles, and provides a physical explanation of the L to T transition. This new picture leads to a drastic revision of our understanding of BD and EGP atmospheres and their evolution.