Discovering other worlds the size of our own has been a long-held dream of astronomers. The transiting planets Kepler-20e and Kepler-20f, which belong to a multi-planet system, hold a very special place among the many groundbreaking discoveries of the Kepler mission because they finally realized that dream. The radius of Kepler-20f is essentially identical to that of the Earth, while Kepler-20e is even smaller (0.87 R[Earth]), and was the first exoplanet to earn that distinction. Their masses, however, are too light to measure with current instrumentation, and this has prevented their confirmation by the usual Doppler technique that has been used so successfully to confirm many other larger planets. To persuade themselves of the planetary nature of these tiny objects, astronomers employed instead a statistical technique to "validate" them, showing that the likelihood they are planets is orders of magnitude larger than a false positive. Kepler-20e and 20f orbit their Sun-like star every 6.1 and 19.6 days, respectively, and are most likely of rocky composition. Here we review the history of how they were found, and present an overview of the methodology that was used to validate them.

中文翻译：

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在 Kepler-20 系统中发现第一颗地球大小的行星围绕太阳以外的恒星运行

发现与我们自己一样大的其他世界一直是天文学家长期以来的梦想。凌日行星 Kepler-20e 和 Kepler-20f 属于多行星系统，在开普勒任务的众多开创性发现中占有非常特殊的地位，因为它们终于实现了这个梦想。Kepler-20f 的半径与地球的半径基本相同，而 Kepler-20e 甚至更小（0.87 R[Earth]），并且是第一个获得这一区别的系外行星。然而，它们的质量太轻，无法用当前的仪器测量，这阻碍了通常的多普勒技术对它们的确认，该技术已成功用于确认许多其他较大的行星。为了说服自己这些微小物体的行星性质，天文学家采用了一种统计技术来“验证” 它们，表明它们是行星的可能性比误报大几个数量级。Kepler-20e 和 20f 分别每 6.1 天和 19.6 天绕着它们的类太阳恒星运行，它们很可能是岩石成分。在这里，我们回顾了它们被发现的历史，并概述了用于验证它们的方法。