Two super-Earths orbiting TOI-402

We then derive robust estimates from the HARPS RVs for the orbital parameters of these two planets by modelling stellar activity with a Gaussian process and using the marginalised posterior probability density functions obtained from our analysis of TESS photometry for the orbital period and time of transit. By modelling TESS photometry and the stellar host characteristics, we find that TOI-402.01 and TOI-402.02 have periods of 4.75642+/-0.00021 and 17.1784+/-0.0016 days and radii of 1.70+/-0.06 and 2.52+/-0.11 (precision 3.6 and 4.2%), respectively.

By analysing the HARPS RV measurements, we find that those planets are both super-Earths with masses of 7.20+/-0.81 and 8.79+/-1.68 (precision 11.3 and 19.1%), and small eccentricities compatible with zero at 2{sigma}. Although having rather similar masses, the radii of these two planets are very different, putting them on different sides of the radius gap. By studying the temporal evolution under X-ray and UV (XUV) driven atmospheric escape of the TOI-402 planetary system, we confirm, under the given assumptions, that photo-evaporation is a plausible explanation for this radius difference.

Those two planets, being in the same system and therefore being in the same irradiation environment are therefore extremely useful for comparative exoplanetology across the evaporation valley and thus bring constraints on the mechanisms responsible for the radius gap.