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.

中文翻译：

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Kepler-78 和超短周期行星

与地球相比，系外行星 Kepler-78b 具有相似的大小（1.2 $R_\oplus$）和一千倍的轨道周期（8.5 小时）。它是目前最小的行星，其质量、半径和日侧亮度都被测量过。Kepler-78b 是超短周期 (USP) 行星的典范，该类别由简单的标准 $P_{\rm orb} < 1$ 天定义。我们描述了我们对开普勒数据进行的基于傅立叶的搜索，这些数据导致了 Kepler-78b 的发现，并回顾了此后对 USP 行星数量的了解。它们和热木星一样常见，而且它们几乎总是小于 2 $R_\oplus$。它们通常是致密多行星系统的成员，尽管它们往往具有相对较大的周期比和相互倾角。它们可能是“气态矮星”裸露的岩石核心，