Kepler-78 and the Ultra-Short-Period Planets

Kepler-78b is a planet of comparable size to the Earth (1.2 $R_\oplus$), but with an orbital period a thousand times shorter (8.5 hours). Currently it is the smallest planet for which the mass, radius, and dayside brightness have all been measured. Kepler-78b is an exemplar of the ultra-short-period (USP) planets, a category defined by the simple criterion $P_{\rm orb} < 1$ day. We describe our Fourier-based search of the Kepler data that led to the discovery of Kepler-78b, and review what has since been learned about the population of USP planets. They are about as common as hot Jupiters, almost always smaller than 2 $R_\oplus$, and often members of compact multi-planet systems. They might be the exposed rocky cores of "gas dwarfs," the planets between 2-4 $R_\oplus$ in size that are commonly found in somewhat wider orbits.