Free-Floating Planets, the Einstein Desert, and 'Oumuamua
  We complete the survey for finite-source/point-lens (FSPL) giant-source
events in 2016-2019 KMTNet microlensing data. The 30 FSPL events show a clear
gap in Einstein radius, $9\,\mu{\rm as}<\theta_{\rm E} <26\,\mu{\rm as}$, which
is consistent with the gap in Einstein timescales near $t_{\rm E}\sim
0.5\,$days found by Mroz et al. (2017) in an independent sample of
point-source/point-lens (PSPL) events. We demonstrate that the two surveys are
consistent. We estimate that the 4 events below this gap are due to a power-law
distribution of free-floating planet candidates (FFPs) $dN_{\rm FFP}/d\log M =
(0.4\pm 0.2)\,(M/38 M_\oplus)^{-p}$/star, with $0.9\lesssim p \lesssim 1.2$.
There are substantially more FFPs than known bound planets, implying that the
bound planet power-law index $\gamma=0.6$ is likely shaped by the ejection
process at least as much as by formation. The mass density per decade of FFPs
in the Solar neighborhood is of the same order as that of 'Oumuamua-like
objects. In particular, if we assume that 'Oumuamua is part of the same process
that ejected the FFPs to very wide or unbound orbits, the power-law index is
$p=0.92\pm 0.06$. If the Solar System's endowment of Neptune-mass objects in
Neptune-like orbits is typical, which is consistent with the results of Poleski
et al. (2021), then these could account for a substantial fraction of the FFPs
in the Neptune-mass range.
