Potential detections of M-dwarf rocky planet atmospheres, and a trend in atmosphere occurrence

One of the most exciting opportunities enabled by JWST is the characterization of atmospheres on terrestrial planets orbiting M dwarfs (M-Earths). However, transmission spectroscopy of M-Earths with JWST has been ambiguous due to issues such as stellar contamination. We present thermal emission observations of four hot M-Earths (GJ 367b, Gl 486b, GJ 1132b, and LTT 1445Ab) observed with JWST/MIRI+LRS between 5-12 microns. A phase curve of GJ 367b, the hottest planet in this sample, shows a maximally hot dayside and zero heat redistribution, consistent with an airless body. However, secondary eclipses of the cooler planets Gl 486b, GJ 1132b, and LTT 1445Ab all show colder daysides than expected for a bare rock. These detections represent the strongest evidence yet of M-Earth atmospheres. We combine these detections with other JWST M-Earth results and find a trend suggesting that planets at irradiation temperatures below 800 K retain atmospheres, while hotter planets are airless.