Presentation #610.01 in the session Neptunes and Mini-Neptunes.
When plotting a mass-radius diagram of confirmed exoplanets, there is a region with a degeneracy between water-worlds and planets with a hydrogen-rich atmosphere. We report on the properties of TOI-1778 b, a planet residing in this region. We combined 99 HARPS-N RVs, two TESS sectors, and two CHEOPS transits with a GP for stellar activity, and find that the planet has a moderately eccentric orbit, a period of 6.5 days, a mass of ~8.5 Earth masses, and a radius of ~2.8 Earth radii. These indicate a sub-Neptune with a H/He atmosphere that is about ~1% of the total mass and a ¼ of its radius, largely ruling it out as a water-world. The characterisation of TOI-1778 b, adding to the small sample of well-characterised sub-Neptunes, is an important step forwards on our journey to understanding planetary formation and evolution pathways, and constraining the compositional confusion that currently surrounds small exoplanets.