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

Abstract

We report a new free-floating planet (FFP) candidate, KMT-2017-BLG-2820, with Einstein radius θ_E ≃ 6 μas, lens-source relative proper motion μ_(rel) ≃ 8 mas yr⁻¹, and Einstein timescale t_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 database 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 Mróz et al. in the OGLE-IV database. The θ_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 ≾ θE/μas ≾ 30) would argue for the latter. Whether each of the 12 (six FSPL and six PSPL) FFP candidates is truly an FFP or simply a very wide-separation planet can be determined at first adaptive optics (AO) light on 30 m telescopes, and earlier for some. If the latter, a second epoch of AO observations could measure the projected planet–host separation with a precision of O(10 au). At the present time, the balance of evidence favors the unbound-planet hypothesis.

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