KMT-2017-BLG-2820 and the Nature of the Free-Floating Planet Population

We report a new free-floating planet (FFP) candidate, KMT-2017-BLG-2820, with Einstein radius ${\theta }_{\text{e}}\simeq 6\text{muas}$, lens-source relative proper motion ${\mu }_{\text{rel}}\simeq 8\text{masyr}$, and Einstein timescale $t_{\text{e}}=6.5$hr. It is the third FFP candidate found in an ongoing study of giant-source finite-source point-lens (FSPL) events in the KMTNet data base, and the sixth FSPL FFP candidate overall. We find no significant evidence for a host. Based on their timescale distributions and detection rates, we argue that five of these six FSPL FFP candidates are drawn from the same population as the six point-source point-lens (PSPL) FFP candidates found by \citet{mroz17} in the OGLE-IV data base. The ${\theta }_{\text{e}}$ distribution of the FSPL FFPs implies that they are either sub-Jovian planets in the bulge or super-Earths in the disk. However, the apparent “Einstein Desert” ($10\text{la}{\theta }_{\text{e}}\mathrm{/}\text{muas}\text{la}30$) would argue for the latter. Whether each of the 12 (6 FSPL and 6 PSPL) FFP candidates is truly an FFP, or simply a very wide-separation planet, can be determined at first adaptive optics (AO) light on 30m telescopes, and earlier for some. If the latter, a second epoch of AO observations could measure the projected planet-host separation with a precision $\mathcal{O}(10\text{au})$. At the present time, the balance of evidence favors the unbound-planet hypothesis.