A Free-floating or Wide-orbit Planet in the Microlensing Event OGLE-2019-BLG-0551

Abstract

High-cadence observations of the Galactic bulge by the microlensing surveys led to the discovery of a handful of extremely short-timescale microlensing events that can be attributed to free-floating or wide-orbit planets. Here, we report the discovery of another strong free-floating planet candidate, which was found from the analysis of the gravitational microlensing event OGLE-2019-BLG-0551. The light curve of the event is characterized by a very short duration (3 days) and a very small amplitude (0.1 mag). From modeling of the light curve, we find that the Einstein timescale, =  t 0.381 0.017 E day, is much shorter, and the angular Einstein radius, q =  4.35 0.34 E μas, is much smaller than those of typical lensing events produced by stellar-mass lenses (t 20 E days, q~0.3 E mas), indicating that the lens is very likely to be a planetary-mass object. We conduct an extensive search for possible signatures of a companion star in the light curve of the event, finding no significant evidence for the putative host star. For the first time, we also demonstrate that the angular Einstein radius of the lens does not depend on blending in the low-magnification events with strong finite source effects.

Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. II. Free-floating Planet Detection Rates*

“…Thus, in the presence of blended light, one may underestimate the true peak magnification (S. A. Johnson et al 2020, in preparation;Mróz et al 2020), making it difficult to constrain ρ precisely. This is important because ρ depends on the angular source size, which will be poorly constrained when no color measurement is made while the source is magnified.…”

“…Indeed, quite recently, multiple well-characterized, extremely short microlensing events have been discovered. Mróz et al (2018Mróz et al ( , 2019a, and Mróz et al (2020) together report a total of four FFP candidates, two of which had timescales consistent with Earth-or Neptune-mass lenses. Han et al (2020a) report the discovery of three events consistent with brown dwarf mass lenses (masses of ∼0.04 M e ), of which two are isolated and one is in a near-equal-mass binary.…”

A Terrestrial-mass Rogue Planet Candidate Detected in the Shortest-timescale Microlensing Event

“…The approximate values of these parameters can be estimated from the light curve without the need of sophisticated modeling (cf. Mróz et al 2020): the maximum magnification A and duration Δt of the event are related to…”

“…If the radius of the source star is larger than the Einstein radius, the duration of microlensing events is extended thanks to finitesource effects (Gould 1994;Nemiroff & Wickramasinghe 1994;Witt & Mao 1994). For sub-Earth-mass lenses, finite-source effects become important if the angular radius of the source, θ * , is of the order of the Einstein radius, ( ) Mróz et al 2018Mróz et al , 2019Mróz et al , 2020Kim et al 2020). These events may be caused by unbound or wideorbit (10 au) planets since microlensing observations alone are not able to rule out the presence of a distant stellar companion (as discussed in more detail by Mróz et al 2020).…”

“…For sub-Earth-mass lenses, finite-source effects become important if the angular radius of the source, θ * , is of the order of the Einstein radius, ( ) Mróz et al 2018Mróz et al , 2019Mróz et al , 2020Kim et al 2020). These events may be caused by unbound or wideorbit (10 au) planets since microlensing observations alone are not able to rule out the presence of a distant stellar companion (as discussed in more detail by Mróz et al 2020). These detections, together with short-timescale events found by Mróz et al (2017), provide strong evidence for a large population of free-floating or wide-orbit planets in the Milky Way.…”