Constraining small planet compositions for future missions

Small transiting exoplanets discovered by K2 and TESS are key targets for future atmospheric and interior characterisation, yet many remain limited by poorly constrained masses and rapidly degrading transit ephemerides. Without targeted follow-up, several of these systems will become effectively unobservable on the timescales relevant for next-generation missions.

We present a uniform transit and radial-velocity analysis of six such systems, combining new targeted CHEOPS observations with archival K2, TESS, and ground-based photometry, together with high-precision RVs from several telescopes. Our joint transit–RV modelling achieves typical mass and radius precisions of <15% and <5%, enabling robust bulk density and interior composition constraints spanning rocky, steam-rich, and volatile-dominated regimes.

A global fit to all available transit timings shows that the inclusion of CHEOPS, as well as collating all new TESS transits since initial validation, reduces projected mid-2040s timing uncertainties by factors of 10–100, restoring sub-hour ephemeris precision critical for HWO and Ariel scheduling. Several systems also show RV evidence for additional non-transiting companions, further enhancing their value as long-term benchmarks for atmospheric and dynamical characterisation.