Abstract

There is an incredible variation in the mass and radii of exoplanets. Generally, the properties of exoplanets are inferred from interior structure models that treat the bodies as cold, differentiated and non-rotating. However, exoplanets are not always in such states. Models of accretion predict that terrestrial bodies are formed with substantial angular momentum. Rocky bodies can be hot because of proximity to their host stars or from giant impacts during accretion. We present a new code (HERCULES) that solves for the equilibrium structure of rotating bodies as a series of concentric, constant-density layers. The HERCULES code is an efficient tool for calculating the structure of rotating exoplanets with realistic equations of state. Using HERCULES and a smoothed particle hydrodynamics (SPH) code, we show that hot, rotating bodies display diverse morphologies. In particular, for rotating bodies there is a thermal limit at which the rotational velocity at the equator intersects the Keplerian orbital velocity. Beyond this corotation limit, the body forms an extended, continuous structure with a corotating region and a disk-like region, which we have named a synestia. By analyzing SPH calculations of giant impacts and N-body models of planet formation, we show that typical rocky planets reach substantially vaporized states multiple times during accretion. For the expected mean angular momentum of growing planets, most of these impact-generated states will exceed the corotation limit and be synestias. Hot, rotating structures can have a bulk density several times lower than an equivalent cold, non-rotating body. The density inferred from observations can also be inaccurate by a factor of a few, depending on the orientation of an oblate body. In addition, the range of structures for hot, rotating bodies has significant implications for the differentiation, cooling and internal dynamics of rocky bodies. Finally, synestias lead to a new mode of satellite formation that can explain the unique chemical relationship between the Earth and Moon.