Physics Colloquium with Fei Dai on Planet Formation
Thousands of exoplanets have been discovered in the past 25 years. Kepler has shown us close-in, sub-Neptune exoplanets are extremely common in our Galaxy: with every Sun-like star having an order of unity chance of hosting them. The TESS mission is currently conducting an all-sky survey to discover the closest and brightest planet hosts amenable to follow-up studies using the upcoming James Webb Space Telescope. With LUVOIR/HabEx and GMT/TMT recommended by the latest NASA Decadal Survey, the detection of biosignatures on exoplanets may be just a few decades ahead of us. However, there are still major gaps of knowledge in our understanding of planet formation, evolution, and habitability. To understand a process as complex as planet formation, the most extreme cases are often most revealing. The extreme exoplanets are ideal for isolating and magnifying critical aspects of planet formation that are still missing in our current theories, that also gave rise to their observed peculiar properties.
At the hottest extreme of planet formation are the ultra-short-period planets (USP, orbital period ~1 day, <2 R_earth) which seemed impossible to have formed in-situ. Yet, they are the most observationally favorable rocky planets for mass measurement, phase curve, transmission, and mass loss studies. Our earlier investigation revealed a likely formation pathway and a prevalence of Earth-like rocky composition among these planets. Our upcoming JWST program will probe their surface mineralogy through phase curve variations. The “super-puffs” (planets with anomalously low density <0.1 g cm-3) are young planets that are extremely susceptible to rapid atmospheric erosion with a timescale that is much shorter than the system's age. Dai will introduce a scenario that involves ongoing atmospheric erosion and dust entrainment to explain the “super-puffs.” Further study of super-puffs and planetary mass loss in general will help us understand the fate of the primordial H/He atmosphere and secondary atmospheres. Finally, we will describe a few novel methods that reveal planets on “oblique orbits” with respect to their host stars and a distant binary companion. The Nice and Grand Tack models have far-reaching implications for the architecture of our solar system. Exoplanets on oblique orbits are important signposts for studying similar large-scale dynamical upheavals.