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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-characterised RV-masses. Mass and radius are the most fundamental properties that characterise planets, giving us insight into the planet’s internal structure. We will present our most recent confirmation and mass-charactrisations of TESS planet candidates (see figure for preliminary results) with radii between 4 and 9.1 R⊕, using the high resolution spectrographs CORALIE and HARPS. This region of parameter space includes the transition between volatile-rich and volatile-poor low-mass exoplanets, the Neptune desert, bloated Neptunes and evaporated hot Jupiters. These exoplanets will be prime targets for atmospheric structure and composition characterisation.

Louise Dyregaard Nielsen: Transition between Ice and Gas giants explored with TESS and RV follow-up