Institute for Advanced Study / Princeton University Joint Astrophysics Colloquium

The discovery of thousands of new planetary candidates over the last decade has revolutionized the field of planet formation. One of Kepler's key findings is that the most abundant planets in our galaxy, observed to date, are larger than Earth but smaller than Neptune. I will review the origin and formation of this small, close-in exoplanet population, demonstrating that the two planet populations located above and below the radius valley likely started out as one and that the smaller and closer planets lost their primordial hydrogen dominated atmospheres (super-Earths), while the larger and further away planets retained a significant fraction of their primordial envelope (sub-Neptunes). Comparing theory and observations, I will show that we can already infer important properties about the underlying planet population and I will discuss observational predictions and tests that can distinguish between photo-evaporation and core-powered mass loss models. I will further highlight the importance of jointly modeling the physics and chemistry of a planet's interior and atmosphere for correctly inferring atmospheric mass-fractions from mass-radius measurements and for determining the total hydrogen mass budgets of sub-Neptunes and interior composition of super-Earths