Constraining the Bulk Compositions of M Dwarf Radius Valley Planets

Planets transiting nearby M dwarf stars are the best laboratory we have to study exoplanets with terrestrial sizes, compositions, and insolation levels. However, it is unclear how the properties of small planets orbiting M dwarfs might differ from those of the terrestrial worlds in our own solar system. We can investigate possible formation and evolution scenarios by studying the bulk compositions of planets in and around the predicted radius valley. Systems exhibiting transit timing variations (TTVs) allow us to efficiently and simultaneously measure the bulk compositions of multiple planets in the same system. TESS has revealed several dynamically interacting multi-planet systems orbiting bright nearby M stars that are accessible to ground-based telescopes. We used diffuser-assisted infrared photometry from Palomar to observe one representative M dwarf system containing two planets with sizes of 1.5 and 2.1 Earth radii that have orbital periods near the 2:1 mean-motion resonance. We combine our new Palomar transit observations, which have a timing precision 30x better than that of the TESS data, with the archival TESS data to characterize the bulk compositions of these planets. We explore whether these two planets, which straddle the proposed radius valley for M dwarfs, have bulk compositions that are consistent with the picture of planet formation and evolution in which rocky planetary cores accrete small volatile-rich atmospheres that are stripped away from the more highly irradiated planets.