Compared to the Earth, the exoplanet Kepler-78b has a similar size (1.2 R⊕) and an orbital period a thousand times shorter (8.5 hours). It is currently 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 Porb<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, and they are almost always smaller than 2 R⊕. They are often members of compact multi-planet systems, although they tend to have relatively large period ratios and mutual inclinations. They might be the exposed rocky cores of "gas dwarfs," the planets between 2-4 R⊕ in size that are commonly found in somewhat wider orbits.
@article{arxiv.1803.03303,
title = {Kepler-78 and the Ultra-Short-Period Planets},
author = {Joshua N. Winn and Roberto Sanchis-Ojeda and Saul Rappaport},
journal= {arXiv preprint arXiv:1803.03303},
year = {2020}
}