  The sizes of small planets have been known to be bi-modal, with a gap
separating planets that have lost their primordial atmospheres (super-Earths),
and the ones that retain them (mini-Neptunes). Here, we report evidences for
another distinct population at smaller sizes. By focussing on planets orbiting
around GK-dwarfs inward of 16 days, and correcting for observational
completeness, we find that the number of super-Earths peak around 1.4 Earth
radii and disappear shortly below this size. Instead, a new population of
planets (sub-Earths) appear to dominate at sizes below ~ 1 Earth radius, with
an occurrence that rises 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 proto-planetary disks, have
a narrow mass distribution. The 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 towards longer
orbital periods, currently invisible, may be the true terrestrial analogues.
This strongly motivates new searches.
