First detections of escaping atmospheres from young mini Neptunes

We report the detections of photoevaporative atmospheric escape from four young mini Neptunes (< 3 R_⊕, < 1 Gyr), the smallest and youngest planets for which escaping atmospheres have been detected. Out of the five we observed, we obtained one detection in Ly alpha and three detections in the metastable helium line at 1083 nm. These detections suggest that most mini Neptunes have primordial hydrogen/helium envelopes. When combined with 3D hydrodynamic models, our helium observations show that the mass loss is vigorous enough to strip most of the envelope within ~1 Gyr, although additional modeling work is still needed to match the detailed properties of the outflow. These data constitute strong evidence that photoevaporation dramatically shapes the demographics of small exoplanets, creating the super Earth population and carving out the radius gap separating super Earths from mini Neptunes.