A Clue as to the Bulk Composition of Short-Period Mini-Neptunes

The bulk composition of planets is a unique clue as to where and how they formed. However, interiors of mini-Neptunes with radii near the radius gap remain poorly constrained. We conducted follow-up observations of TESS candidates on the ground and validated four transiting short-period mini-Neptune orbiting M dwarfs. The mass-radius relationship of these four mini-Neptunes suggests that they are either rocky cores surrounded by a less massive atmosphere or water worlds. In addition, the fact that at least the three mini-Neptunes are in an eccentric orbit supports that unlike short-period rocky super-Earths such as CoRoT-7b, tidal dissipation in mini-Neptunes should be inefficient. We also considered what if a short-period mini-Neptune has a thin atmosphere onto a rocky core. A mini-Neptune likely undergoes a giant impact event in the advanced stage of planet formation. We simulated the growth of a mini-Neptune through gas accretion from a protoplanetary disk, followed by giant impacts onto itself. Although a large amount of primordial H₂/He atmosphere that a mini-Neptune has should blow off via an impact-driven shock, the remaining atmosphere can be water-rich due to chemical reactions between the H₂/He gas and vaporizing rocky material. Thus, atmospheric compositions of short-period mini-Neptunes may become fingerprints to reveal their formation processes.