Princeton University Extrasolar Planet Discussion Group

Some small (< 1.6 R⊕) exoplanets have much higher densities than expected from stellar abundances of planet-forming elements. There are two theories - metal-rich formation hypothesis and naked core hypothesis - that explain how formation and evolution can alter the compositions and structures of rocky planets to diverge from their primordial building blocks. I will revisit in detail the naked core hypothesis, which states that high-density planets are remnant cores of giant planets that remain in a fossil-compressed state, even after envelope loss. Using a planetary interior model and assuming energy-limited atmospheric escape, I showed that a large fraction, if not all, of the iron-silicate core of a giant planet is molten during the planet's early evolution. Upon envelope loss, molten part of the planets can rapidly rebound due to low viscosity, resulting in negligible radius decrease compared to self-compressed counterparts with the same core mass fraction. Such small levels of compression are insufficient to explain the origin of high-density exoplanets. I will briefly discuss the mantle-stripping giant impact hypothesis and show that it is unlikely to explain the origin of high-density exoplanets either. Finally, I will discuss the alternative pathways to the formation of small high-density exoplanets.