Transition between Ice and Gas giants explored with TESS and Radial Velocity follow-up

Planets in the radii-range between Neptune and Saturn (R = 4–9.1 R_⊕) are important probes of the gas accretion phase of planet formation. During this short formation stage isolated cores are growing by accreting the surrounding gas in the proto-planetary disk (Pollack et al. 1996, Helled et al. 2014). The growth-rate will determine whether a planet ends up as a Saturn-like gas giant or a Neptune-like ice giant. Recent gravitational microlensing studies (Suzuki et al 2018) point towards a larger fraction of intermediate-mass planets than predicted by population synthesis (Alibert et al. 2005), challenging our current view on core accretion. Building up a large sample of planets with masses derived from radial velocities (RVs) will be necessary to corroborate this finding.

To date only a few tens of planets with radii between 9.1 and 4 R_⊕ have well-characterized RV-masses, as illustrated in the attached figure. Mass and radius are the most fundamental properties that characterize planets, giving us insight into the planet’s internal structure. We will present our follow-up RV programme which targets TESS planet candidates with radii between 4 and 9.1 R_⊕, using the high resolution spectrographs CORALIE and HARPS. Besides an analysis of false positives, we will present our most recently confirmed planetary systems. These targets include the transition between volatile-rich and volatile-poor low-mass exoplanets, the Neptune desert, bloated Neptunes and evaporated hot Jupiters. All of them will be prime targets for atmospheric structure and composition characterization.