TOI-421 light and velocity curves

We also searched for TTVs and modelled the interior structure of the planets. Finally, we simulated the evolution of the primordial H-He atmospheres of the planets using two different modelling frameworks. We determine the planetary radii and masses of TOI-421 b and c to be R_b_=2.64+/-0.08R_{Earth}_, M_b_=6.7+/-0.6M_{Earth}_, R_c_=5.09+/-0.07R_{Earth}_, and M_c_=14.1+/-1.4M_{Earth}_.

Using these results we retrieved average planetary densities of rho_b_=0.37+/-0.05rho_{Earth}_ and rho_c_=0.107+/-0.012rho_{Earth}_. We do not detect any statistically significant TTV signals. Assuming the presence of a hydrogen-dominated atmosphere, the interior structure modelling results in both planets having extensive envelopes.

While the modelling of the atmospheric evolution predicts for TOI-421 b to have lost any primordial atmosphere that it could have accreted at its current orbital position, TOI-421 c could have started out with an initial atmospheric mass fraction somewhere between 10 and 35%. We conclude that the low observed mean density of TOI-421b can only be explained by either a bias in the measured planetary parameters (e.g. driven by high-altitude clouds) and/or in the context of orbital migration.

We also find that the results of atmospheric evolution models are strongly dependent on the employed planetary structure model.