Atmospheric Regimes and Trends on Exoplanets and Brown Dwarfs

TL;DR

This review synthesizes current observational and theoretical knowledge of exoplanet and brown dwarf atmospheres, identifying key regimes, trends, and principles that underpin their diverse atmospheric behaviors.

Contribution

It provides a comprehensive overview of atmospheric regimes and statistical trends in exoplanets and brown dwarfs, highlighting fundamental processes and critical problems in the field.

Findings

Identification of dominant atmospheric processes affecting stability and energy transport

Analysis of how planetary parameters influence atmospheric characteristics

Establishment of simple scaling laws for atmospheric behavior

Abstract

A planetary atmosphere is the outer gas layer of a planet. Besides its scientific significance among the first and most accessible planetary layers observed from space, it is closely connected with planetary formation and evolution, surface and interior processes, and habitability of planets. Current theories of the planetary atmosphere were primarily obtained through the studies of eight large planets, Pluto and three large moons (Io, Titan, and Triton) in the Solar System. Outside the Solar System, more than four thousand extra-solar planets (exoplanets) and two thousand brown dwarfs have been confirmed in our galaxy, and their population is rapidly growing. The rich information from these exotic bodies offers a database to test, in a statistical sense, the fundamental theories of planetary climates. Here we review the current knowledge of atmospheres of exoplanets and brown dwarfs from recent observations and theories. This review highlights important regimes and statistical trends in an ensemble of atmospheres as an initial step towards fully characterizing diverse substellar atmospheres, that illustrates the underlying principles and critical problems. Insights are obtained through analysis of the dependence of atmospheric characteristics on basic planetary parameters. Dominant processes that influence atmospheric stability, energy transport, temperature, composition, and flow pattern are discussed and elaborated with simple scaling laws. We dedicate this review to Dr. Adam P. Showman (1968-2020) in recognition of his fundamental contribution to the understanding of atmospheric dynamics on giant planets, exoplanets, and brown dwarfs.