Revealing the Population of Volatile-Rich Planets Around Cool Stars

Models predict that planets with water-rich compositions may be common around low-mass stars, but definitive evidence for the existence of water worlds has remained elusive. Recent JWST observations of sub-Neptune sized planets orbiting nearby M dwarfs found atmospheres enriched in CH_4, CO_2 and H_2O, but the mechanism for this enrichment is debated. To answer this question, it’s important to know if these two planets represent the broader population of sub-Neptunes, and to quantify the range of possible sub-Neptune atmosphere types. A first step to investigating this newly accessible subject is constraining the volatile atmospheric mass fractions of more small planets. This requires precise mass and radius measurements, but some of these systems are challenging targets for RVs. Dynamical interactions in near-resonant multi-planet systems produce transit timing variations (TTVs), and measurements of these TTVs constrain the planetary densities. Over the past three years, we have collected dozens of high SNR TTV observations using ground-based telescopes to characterize the planetary masses in multi-planet M dwarf systems, along with space-based XUV observations to measure the planetary radiation environments. Preliminary results from this survey have found several new small volatile-rich worlds, including some that are top candidates for atmospheric characterization with JWST and some of the coldest known volatile-rich rocky planets.