The sizes of small planets are known to be bimodal, with a gap separating planets that have lost their primordial atmospheres (super-Earths) and the ones that retain them (mini-Neptunes). Here, we report evidence for another distinct population at smaller sizes. By focusing on planets orbiting around GK dwarfs inward of 16 days and correcting for observational completeness, we find that the number of super-Earths peaks around 1.4 Earth radii and disappears shortly below this size. Instead, a new population of planets (sub-Earths) appears to dominate at sizes below ∼1 Earth radius, with an occurrence that increases with decreasing size. This pattern is also observed in ultra-short-period planets. The end of super-Earths supports earlier claims that super-Earths and mini-Neptunes, planets that likely form in gaseous protoplanetary disks, have a narrow mass distribution. Sub-Earths, in contrast, can be described by a power-law mass distribution and may be explained by the theory of terrestrial planet formation. We therefore speculate that they are formed well after the gaseous disks have dissipated. The extension of these sub-Earths toward longer orbital periods, currently invisible, may be the true terrestrial analogs. This strongly motivates new searches.

已知小行星的大小是双峰的，将失去原始大气层的行星（超级地球）和保留它们的行星（迷你海王星）隔开。在这里，我们报告了另一个较小规模的不同种群的证据。通过关注在 16 天内围绕 GK 矮星运行的行星并校正观测完整性，我们发现超级地球的数量在 1.4 个地球半径附近达到峰值，并在此大小以下不久消失。相反，一个新的行星群（亚地球）似乎在小于 1 个地球半径的大小中占主导地位，并且随着大小的减小而增加。在超短周期行星中也观察到这种模式。超级地球的终结支持了早期的说法，即超级地球和迷你海王星，行星可能形成于气态原行星盘，质量分布较窄。相比之下，亚地球可以用幂律质量分布来描述，也可以用类地行星形成理论来解释。因此我们推测它们是在气态盘消散后形成的。这些亚地球向更长的轨道周期延伸，目前是不可见的，可能是真正的地球类似物。这强烈地激发了新的搜索。