2249273

Target RA (J2000): 51.640358 and deg Target Dec (J2000): -43.61356 and deg Gaia GMag: 9.018 Programme ID: CH_PR230027 PI of observing programme: Eisner Title of programme: Readying a warm sub-Neptune orbiting around the bright Sun-like star TOI-4320 for future atmospheric characterisation Abstract: Sub-Neptune sized planets are amongst the most common types of exoplanets in our Galaxy however their formation and composition are still widely debated. Through transit spectroscopy we can probe their atmospheric composition and thus help constrain theories of how these systems form and evolve. We propose to refine the ephemerides and radius of a sub-Neptune sized planet orbiting around a bright Sun-like star that was identified in the TESS data by citizen scientists taking part in the Planet Hunters TESS project. The refined orbital period that we can determine with just one additional transit observation with CHEOPS will ensure that the mid-transit time of this target will be sufficiently constrained for future atmospheric characterisation with JWST or Ariel. Furthermore the refined planet radius combined with our ongoing radial velocity campaign to determine the mass of this planet will allow us to constrain the bulk composition and thus infer the internal structure of the planet. Even though sub-Neptune sized planets are common this one stands out due to its long orbital period (~25 days) compared to the majority of TESS planets and due to the brightness of the host star (Vmag = 9.17). To date less than 3% of all known sub-Neptune size planets transit stars with a Vmag brighter than10 highlighting the unique opportunity presented by this system. [truncated! Please see actual data for full text]