Spectroscopic data of HD 235088 (TOI-1430)

This allowed us to measure both the radius and mass (and consequently the density) of the planet with high precision, and reconstruct the evolution of its atmosphere. TOI-1430 is an active K-dwarf star born 700+/-150Myr ago and rotates in Prot~12-days. This star hosts a mini-Neptune whose orbital period is Pb=7.434133+/-0.000004-days.

Thanks to long-term photometric and spectroscopic monitoring of this target performed with TESS, HARPS-N, HIRES, and APF, we estimated a radius Rb=1.98+/-0.07R_{Earth}_, a mass Mb=4.2+/-0.8M_{Earth}_, and thus a planetary density rhob=0.5+/-0.1rho_{Earth}_. TOI-1430 b is hence a low-density mini-Neptune with an extended atmosphere, at the edge of the radius gap. Because this planet is known to have an evaporating atmosphere of He, we reconstructed its atmospheric history.

Our analysis supports the scenario in which, shortly after its birth, TOI-1430 b may have been super-puffy, with a radius 5X-13X and a mass 1.5X-2X that of today; in ~200Myr from now, TOI-1430 b should lose its envelope, showing its Earth-size core. We also looked for signals from a second planet in the spectroscopic/photometric series, without detecting any.