Dynamical masses of two young transiting Neptune-size planets in the young 400 Myr star HD63433

Despite the discovery of thousands of exoplanets, the mechanisms of planet formation and evolution have not been well tested with observations, mainly because the lack of a complete photometric and spectroscopic characterization of young planets, which presents a big challenge due to the high stellar activity of young stars. By determining the masses of young exoplanets, we can establish their composition, and investigate their evolution with age. For these observations, we can deduce if they were formed without gas (rocky planets) from their origin, or if they are born as small gaseous planets that lose their atmosphere rapidly (~tens of Myr) by photoevaporation (Murray-Clay et al. 2019) or slowly (~1 Gyr) by core-powered mass loss (Ginzburg et al. 2018). HD63433 is a very bright and fast rotating (6.46 days) solar-type star of the young (~400Myr) Ursa Major moving group hosting two transiting mini-Neptune-size exoplanets (Mann et al. 2020) with orbital periods of 7.11 (HD63433 b) and 20.55 (HD63433 c) days. We have collected 150 Radial Velocity (RV) measurements of HD63433 with CARMENES spectrograph over 1.4 years to determine the mass of the planets (Mallorquín et al. 2022, in prep). We have jointly modeled the transits, the keplerian planetary signal, and stellar activity with Gaussian Processes combining the TESS photometry and the CARMENES RV dataset. Our results reveal that the outer planet is detected in the CARMENES time series with an amplitude of 4.1+-1.0 m/s at the level of 4-sigma, which correspond to an absolute mass of Mp=17.4+-4.3 mass of the Earth for HD63433 c, making it the smallest planet with a robust mass determination at young ages. We report an upper limit of Mp