The Occurrence-weighted Median Planets Discovered by Transit Surveys Orbiting Solar-type Stars and Their Implications for Planet Formation and Evolution

Planet occurrence and primordial atmospheric retention probability increase with period. The heavily irradiated short-period planets discovered by duration-limited transit surveys like that being executed by the Transiting Exoplanet Survey Satellite (TESS) may be very different from the lightly irradiated longer-period planets that are now known to be a much more common outcome of the planet formation process. We show that an occurrence-weighted mass--radius relation for the low-mass planets discovered so far by transit surveys orbiting solar-type stars requires both occurrence-weighted median Earth-mass and Neptune-mass planets to have a few percent of their masses in hydrogen/helium (H/He) atmospheres. It also implies core masses M_c in the range 2 M_Earth < M_c < 8 M_Earth that can retain their primordial atmospheres. In contrast to Uranus and Neptune that have at least 10% of their masses in H/He atmospheres, these occurrence-weighted median Neptune-mass planets are H/He poor. The implication is that they experienced collisions or formed in much shorter-lived and/or hotter parts of their parent protoplanetary disks than Uranus and Neptune's formation location in the protosolar nebula.