Simulating the escaping atmospheres of hot gas planets in the solar neighborhood

TL;DR

This study uses hydrodynamic simulations to analyze the atmospheric escape of 18 hot gas planets, revealing how planetary mass and size influence thermospheric stability, cooling, and observable Ly$ extalpha$ signals, aiding future atmospheric characterization.

Contribution

It introduces a coupled simulation approach for hot gas planet atmospheres, providing new insights into mass-loss rates, thermospheric stability, and observable signatures based on planetary properties.

Findings

Massive, compact planets have stable, hot thermospheres with weak absorption but strong Ly$ extalpha$ emission.

Smaller planets exhibit strong Ly$ extalpha$ absorption with weak emission due to cool, strong winds.

WASP-80 and GJ 3470 are promising targets for detecting atmospheric Ly$ extalpha$ signals.

Abstract

Absorption of high-energy radiation in planetary thermospheres is believed to lead to the formation of planetary winds. The resulting mass-loss rates can affect the evolution, particularly of small gas planets. We present 1D, spherically symmetric hydrodynamic simulations of the escaping atmospheres of 18 hot gas planets in the solar neighborhood. Our sample only includes strongly irradiated planets, whose expanded atmospheres may be detectable via transit spectroscopy. The simulations were performed with the PLUTO-CLOUDY interface, which couples a detailed photoionization and plasma simulation code with a general MHD code. We study the thermospheric escape and derive improved estimates for the planetary mass-loss rates. Our simulations reproduce the temperature-pressure profile measured via sodium D absorption in HD 189733 b, but show unexplained differences in the case of HD 209458 b. In contrast to general assumptions, we find that the gravitationally more tightly bound thermospheres of massive and compact planets, such as HAT-P-2 b are hydrodynamically stable. Compact planets dispose of the radiative energy input through hydrogen Ly$\alpha$ and free-free emission. Radiative cooling is also important in HD 189733 b, but it decreases toward smaller planets like GJ 436 b. The simulations show that the strong and cool winds of smaller planets mainly cause strong Ly$\alpha$ absorption but little emission. Compact and massive planets with hot, stable thermospheres cause small absorption signals but are strong Ly$\alpha$ emitters, possibly detectable with the current instrumentation. The absorption and emission signals provide a possible distinction between these two classes of thermospheres in hot gas planets. According to our results, WASP-80 and GJ 3470 are currently the most promising targets for observational follow-up aimed at detecting atmospheric Ly$\alpha$ absorption signals.