Two new free-floating or wide-orbit planets from microlensing

Abstract

Planet formation theories predict the existence of free-floating planets that have been ejected from their parent systems. Although they emit little or no light, they can be detected during gravitational microlensing events. Microlensing events caused by rogue planets are characterized by very short timescales tE (typically below two days) and small angular Einstein radii θE (up to several μas). Here we present the discovery and characterization of two ultra-short microlensing events identified in data from the Optical Gravitational Lensing Experiment (OGLE) survey, which may have been caused by free-floating or wide-orbit planets. OGLE-2012-BLG-1323 is one of the shortest events discovered thus far (tE = 0.155 ± 0.005 d, θE = 2.37 ± 0.10μas) and was caused by an Earth-mass object in the Galactic disk or a Neptune-mass planet in the Galactic bulge. OGLE-2017-BLG-0560 (tE = 0.905 ± 0.005 d, θE = 38.7 ± 1.6μas) was caused by a Jupiter-mass planet in the Galactic disk or a brown dwarf in the bulge. We rule out stellar companions up to a distance of 6.0 and 3.9 au, respectively. We suggest that the lensing objects, whether located on very wide orbits or free-floating, may originate from the same physical mechanism. Although the sample of ultrashort microlensing events is small, these detections are consistent with low-mass wide-orbit or unbound planets being more common than stars in the Milky Way.

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

“…8 Typically the source radius crossing time as defined by r q m = = t t E rel * * is used as a proxy for the timescale of the event (e.g., Skowron et al 2011); however, we account for nonzero impact parameter u 0, * similar to Mróz et al (2019a). We follow their definition, except we use the variable t c instead for their Equation (A1).…”

“…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.…”

“…This has been discussed before by several authors (Di Stefano & Scalzo 1999;Han & Kang 2003;Han et al 2005), all of which propose pathways to determine whether a planetary-mass lens is bound or free-floating. Mróz et al (2018Mróz et al ( , 2019a place limits on the presence of a host photometrically, but detailed modeling of the magnification curve and photometric follow-up can also be used to determine whether the lens is isolated (Han & Kang 2003;Han et al 2005;Henderson & Shvartzvald 2016). As an example, detailed modeling has been used to determine the true, bound nature of FFP candidates by Bennett et al (2012) and Han et al (2020b).…”

Candidate Brown-dwarf Microlensing Events with Very Short Timescales and Small Angular Einstein Radii

“…From detailed investigation, it is found that the event is produced by a wide-separation planet and the analyses will be presented in a separate paper. Mróz et al (2018Mróz et al ( , 2019, respectively. They pointed out that the lens of OGLE-2016-BLG-1540 was likely to be a Neptunemass free-floating planet in the Galactic disk and the lens of OGLE-2017-BLG-0560 is either a Jupiter-mass free-floating planet in the disk or a BD in the bulge.…”

Spitzer Microlensing Parallax Reveals Two Isolated Stars in the Galactic Bulge

“…Gravitational microlensing is potentially a powerful tool for probing isolated objects with various masses such as free-floating planets, brown dwarfs, low-mass stars, and black holes. At the low-mass end, microlensing has detected several free-floating planet candidates based on their short microlens timescale, t E 2 days (Sumi et al 2011;Mróz et al 2017Mróz et al , 2018Mróz et al , 2019, including a few possible Earth-mass objects. Such discoveries are crucial for testing theories about the origin and evolution of free-floating planets (Veras & Raymond 2012;Pfyffer et al 2015;Ma et al 2016; Barclay et al 2017;Clanton & Gaudi 2017).…”