2299052

Target RA (J2000): 90.159462 and deg Target Dec (J2000): 0.13178 and deg Gaia GMag: 11.23 Programme ID: CH_PR140084 Programme Manager: ULMER-MOLL PI of observing programme: Benz Title of programme: Axis 1 - CompoSubNeptu Abstract: Sub-Neptunes are the most common type of known planets in the galactic neighborhood. In spite of that their interior composition and therefore their origin has to date not been constrained observationally. The reason is the degeneracy in possible compositions that can explain their observed properties. Two end member compositions are theoretically suggested: dry rocky cores + about 1% of H/He or ocean planets containing about 50% water in mass. The former is suggested by evolutionary evaporation models the later by formation models. These compositions correspond to fundamentally different in situ formation versus orbital migration pathways. If observationally it should turn out that close-in sub-Neptunian planets are indeed water worlds this could be seen as one of the largest successes of planet formation theory. The opposite case would mean that one or several elements of our current understanding of the origin of planets is fundamentally flawed. It is therefore of paramount importance to answer the fundamental question “What are sub-Neptunes made of?”. With this program we propose a three-faceted approach to answer this question: 1) is there a grouping in density? 2) Preparing for the JWST opportunity. 3) Is the ice mass fraction correlated with the stellar composition? We propose to use CHEOPS for the precise radius determination of a carefully chosen sample of TESS planet candidate whose radii make them susceptible to be either ocean worlds or rocky cores with H/He envelopes. [truncated! Please see actual data for full text]