AU Mic b, or not AU Mic b: the question of the young planet’s escaping atmosphere

AU Mic is an M1Ve star in the β Pic moving group and therefore has an age of 23 ± 3 Myr. At 9.79 ± 0.04 pc, it is one of the closest pre-main sequence stars to Earth. Two exoplanets transiting AU Mic have been discovered using data from TESS. AU Mic b orbits closer to its host, with a period of 8.46 days. It has a radius measurement of 4.20 Earth radii, placing it on the edge of the hot Neptune desert. We expect planets at this size (indicating a large gaseous envelope) and orbital period (highly irradiated) to be experiencing significant photoevaporation. This and its youth prompt the detailed study of AU Mic b’s potentially escaping atmosphere. We obtained Lyman-alpha transits of AU Mic b with HST/STIS. We present a detailed analysis of the flares within these observations. The flare-removed Lyman-alpha light curves do not exhibit any evidence of escaping neutral hydrogen. This supports theoretical work done by Owen & Murray-Clay (2021) showing that some hot Neptunes and sub-Saturns will have their escaping envelopes photoionized too quickly to be observable in Lyman-alpha.