Kepler-78 and the Ultra-Short-Period Planets

TL;DR

Kepler-78b is a small, ultra-short-period exoplanet similar in size to Earth, discovered through Fourier analysis of Kepler data, and it exemplifies the characteristics and population of USP planets.

Contribution

This paper reports the discovery of Kepler-78b using Fourier-based analysis and reviews the properties and prevalence of ultra-short-period planets.

Findings

Kepler-78b has a 8.5-hour orbital period and Earth-like size.

USP planets are as common as hot Jupiters and mostly smaller than 2 Earth radii.

They often exist in multi-planet systems with large period ratios.

Abstract

Compared to the Earth, the exoplanet Kepler-78b has a similar size (1.2 $R_\oplus$) 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 $P_{\rm orb} < 1$ 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, and they are almost always smaller than 2 $R_\oplus$. 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 $R_\oplus$ in size that are commonly found in somewhat wider orbits.