Dix Planetary Science Seminar
Abstract: In order to understand a process as complex as planet formation, the most extreme cases are often the most revealing. At the hottest extreme of planet formation are the ultra-short-period planets (USP, orbital period ~1 day, <2 R_earth) which orbit their host stars just a few stellar radii away. These highly irradiated worlds are ideal testbeds for studying processes such as surface reflectivity/thermal emission, atmospheric erosion, tidal interaction. Our uniform analysis of USP planets revealed a prevalence of Earth-like rocky composition. We also found that the orbital inclinations of USP planets display a larger scatter than longer-period planets, thus betraying a dynamically hot origin. Examination of the radius distribution of USP across host stellar types favors photoevaporation, rather than core-powered mass loss, as the main driver of the bimodal radius distribution of sub-Neptune planets. Another extreme class of planets are the "super-puffs" i.e. planets with anomalously low mean density (2-7 M_earth, 7-10 R_earth, ~0.05 g cm^-3). The extended atmosphere is susceptible to extreme hydrodynamic mass loss on a timescale that is much shorter than the system's age. Even more puzzling, super-puffs are estimated to have large scale height of (~3000 km), yet recent observations revealed completely flat transmission spectra. I will introduce a scenario that may explain these observations with a dusty, outflowing atmosphere.