Radial velocities of M-dwarf LTT 3780 with HARPS

The two planet candidates are identified in a single Transiting Exoplanet Survey Satellite sector and validated with reconnaissance spectroscopy, ground-based photometric follow-up, and high-resolution imaging. With measured orbital periods of P_b_=0.77, P_c_=12.25days and sizes r_p,b_=1.33{+/-}0.07, r_p,c_=2.30{+/-}0.16R{earth}, the two planets span the radius valley in period-radius space around low-mass stars, thus making the system a laboratory to test competing theories of the emergence of the radius valley in that stellar mass regime.

By combining 63 precise radial velocity measurements from the High Accuracy Radial velocity Planet Searcher (HARPS) and HARPS-N, we measure planet masses of m_p,b_=2.62_-0.46_^+0.48^ and m_p,c_=8.6_-1.3_^+1.6^M{earth}, which indicates that LTT3780b has a bulk composition consistent with being Earth-like, while LTT3780c likely hosts an extended H/He envelope. We show that the recovered planetary masses are consistent with predictions from both photoevaporation and core-powered mass-loss models.