Unveiling a rare super-Neptune and Neptune pair with TESS and CHEOPS

Super-Neptunes — planets slightly larger than Neptune — are an intrinsically rare population of planets unlike any found in the Solar System. How did these planets form, are they similar to the ice giants in our own Solar system, or are they hydrogen-helium rich worlds that formed close-in to their parent stars? We have a program on ESA’s CHEOPS space telescope and NASA’s TESS mission to discover and characterise these rare planets. From this program, we report the discovery of TOI-5126, a system hosting a super-Neptune, with an exterior Neptune. Of particular note, the inner planet lies in a regime that is heavily bombarded by intense stellar irradiation, and may be undergoing active atmospheric evaporation. We also report tentative detections of Transit Timing Variations — gravitational interactions between the two planets — that may help us measure the mass of these small planets. Such measurements are critical to understanding their atmospheric evolution, and are impossible to achieve via traditional radial velocity observations.