Kepler-78 and the Ultra-Short-Period planets

Joshua N. Winn, Roberto Sanchis-Ojeda, Saul Rappaport

Research output: Contribution to journalReview articlepeer-review

71 Scopus citations


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 h). 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.

Original languageEnglish (US)
Pages (from-to)37-48
Number of pages12
JournalNew Astronomy Reviews
StatePublished - Nov 2018

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Planets
  • Time-series photometry


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