JWST Thermal Emission of the Terrestrial Exoplanet GJ 1132b

TL;DR

This study uses JWST's MIRI instrument to measure the thermal emission of GJ 1132b, revealing a hot, likely atmosphere-free rocky surface with no significant atmosphere detected, supporting the 'Cosmic Shoreline' hypothesis.

Contribution

First thermal emission measurement of GJ 1132b with JWST, constraining its surface temperature and ruling out thick atmospheres with certain compositions.

Findings

Measured dayside temperature of 709±31 K.

Ruled out Earth-like and thick Venus-like atmospheres with specific compositions.

Detected a featureless blackbody emission spectrum.

Abstract

We present thermal emission measurements of GJ 1132b spanning 5--12 um obtained with the Mid-Infrared Instrument Low-Resolution Spectrometer (MIRI/LRS) on the James Webb Space Telescope (JWST). GJ 1132b is an M-dwarf rocky planet with Teq=584 K and an orbital period of 1.6 days. We measure a white-light secondary eclipse depth of 140+/-17 ppm, which corresponds to a dayside brightness temperature of Tp,dayside= 709+/-31 K using improved star and planet parameters. This measured temperature is only 1 sigma below the maximum possible dayside temperature of a bare rock (i.e., assuming a zero albedo planet with no heat redistribution, Tmax = 746+14/-11 K). The emission spectrum is consistent with a featureless blackbody, which agrees with a wide range of possible surface compositions. By comparing forward models to the dayside emission spectrum, we rule out Earth-thickness (P ~ 1 bar) atmospheres with at least 1% H2O, atmospheres of any modeled thickness (10^-4 -- 10^2 bar) that contain at least 1% CO2, and thick, Venus-like atmospheres (P>~100 bar) with at least 1 ppm CO2 or H2O. We therefore conclude that GJ 1132b likely does not have a significant atmosphere. This finding supports the concept of a universal 'Cosmic Shoreline' given the high level of bolometric and XUV irradiation received by the planet.