Mass estimates of the young TOI-451 transiting planets: multidimensional Gaussian Process on stellar spectroscopic and photometric signals

The young TOI-451 planetary system, aged 125 Myr, provides a unique opportunity to test theories of planetary internal structures and atmospheric mass-loss through examination of its three transiting planets. We present an exhaustive photometric and spectroscopic follow-up to determine the orbital and physical properties of the system. We perform multidimensional Gaussian Process regression with the code pyaneti on spectroscopic time-series and NGTS/LCO light curves to disentangle the stellar and planetary signal in ESPRESSO radial velocities. We show how contemporaneous photometry serves as an activity indicator to inform RV modelling within a multidimensional Gaussian Processes framework. We argue that this can be exploited when spectroscopic observations are adversely affected by low signal-tonoise and/or poor sampling. We estimate the Doppler semi-amplitudes of k_b = 2.6^+1.1_−1.2 m s⁻¹, k_c = 1.2^+1.0_−0.8 m s⁻¹ , and k_d = 2.7 ± 1.2 m s⁻¹. This translates in 2σ mass estimates for TOI-451 b and d of M_b = 4.7^+2.1_−2.2 M_⊕ and M_d = 10.2^+4.6_−4.5 M_⊕; as well as a mass upper limit for TOI-451 c of M_c < 11.5 M_⊕. The derived planetary properties suggest that planets c and d contain significant hydrogen-rich envelopes. The inferred parameters of TOI-451 b are consistent with either a rocky world that still retains a small hydrogen envelope or a water world. These insights make the TOI-451 system an ideal laboratory for future follow-up studies aimed at measuring atmospheric compositions, detecting atmospheric mass-loss signatures, and further exploring planetary formation and evolution processes.