Free-floating or wide-orbit? Keck adaptive-optics observations of free-floating planet candidates detected with gravitational microlensing

TL;DR

This study uses high-resolution Keck imaging to investigate whether microlensing-detected free-floating planet candidates are truly unbound or in wide orbits, finding no host stars at expected separations and suggesting the need for deeper observations.

Contribution

The paper provides the first high-resolution imaging analysis of free-floating planet candidates, constraining the presence of potential host stars and informing their true nature.

Findings

No host stars detected at 40-60 mas separation.

Rules out 11%-36% of potential hosts depending on assumptions.

Deeper observations needed for definitive conclusions.

Abstract

Recent detections of extremely short-timescale microlensing events imply the existence of a large population of Earth- to Neptune-mass planets that appear to have no host stars. However, it is currently unknown whether these objects are truly free-floating planets or whether they are in wide orbits around a distant host star. Here, we present an analysis of high-resolution imaging observations of five free-floating planet candidates collected with the Keck telescope. If these candidates were actually wide-orbit planets, then the light of the host would appear at a separation of 40-60 mas from the microlensing source star. No such stars are detected. We carry out injection and recovery simulations to estimate the sensitivity to putative host stars at different separations. Depending on the object, the presented observations rule out 11%-36\% of potential hosts assuming that the probability of hosting a planet does not depend on the host mass. The results are sensitive to the latter assumption, and the probability of detecting the host star in the analyzed images may be a factor of 1.9 +/- 0.1 larger, if the exoplanet hosting probability scales as the first power of the host star mass, as suggested by recent studies of planetary microlensing events. We argue that deeper observations, for example with JWST, are needed to confidently confirm or refute the free-floating planet hypothesis.