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

Abstract

Some low-mass planets are expected to be ejected from their parent planetary systems during early stages of planetary system formation. According to planet formation theories, such as the core accretion theory, typical masses of ejected planets should be between 0.3 and 1.0 M ⊕ . Although in practice such objects do not emit any light, they may be detected using gravitational microlensing via their light-bending gravity. Microlensing events due to terrestrial-mass rogue planets are expected to have extremely small angular Einstein radii (1 μas) and extremely short timescales (0.1day). Here, we present the discovery of the shortest-timescale microlensing event, OGLE-2016-BLG-1928 to date ( » = t 0.0288 day 41.5 minutes E). Thanks to the detection of finite-source effects in the light curve of the event, we were able to measure the angular Einstein radius of the lens q =  0.842 0.064 E μas, making the event the most extreme short-timescale microlens discovered to date. Depending on its unknown distance, the lens may be a Mars-to Earth-mass object, with the former possibility favored by the Gaia proper motion measurement of the source. The planet may be orbiting a star but we rule out the presence of stellar companions up to the projected distance of ∼8.0 au from the planet. Our discovery demonstrates that terrestrial-mass free-floating planets can be detected and characterized using microlensing.

Evaluating the Microbial Habitability of Rogue Planets and Proposing Speculative Scenarios on How They Might Act as Vectors for Panspermia

“…Only very few rogue planets are known [ 8 , 9 , 13 ] because they are so difficult to detect. Most exoplanets are discovered through their interactions with their parent star, which clearly is not an option for rogue planets.…”

“…Life could have originated on such an Earth-type planet before it was ejected from its solar system, especially if it had a reducing atmosphere. In 2020 the first terrestrial-mass rogue planet candidate was claimed to be detected using microlensing [ 13 ], showing that Mars to Earth-sized mass rogue planets can be detected and characterized in principle using current technology. However, detections are extremely challenging, because Mars to Earth-sized planets are already difficult to identify when in orbit around a star.…”

“…However, detections are extremely challenging, because Mars to Earth-sized planets are already difficult to identify when in orbit around a star. However, on theoretical grounds, many of these terrestrial mass rogue planets should exist [ 13 ]. In the following, we focus on the possibilities for life in these terrestrial types of rogue planets.…”

Using Source Proper Motion to Validate Terrestrial Parallax: OGLE-2019-BLG-1058

“…As another example, Mróz et al (2020b) found μ rel = 10.6 mas yr −1 for OGLE-2016-BLG-1928, but a source proper motion very close to the mean of the proper motion distribution for bulge giants (i.e., μ rel = 5 ∼ 10 mas yr −1 ). Because of the source proper motion, they argue that the lens is more consistent with being a member of the disk.…”

“…Recently, the OGLE and Korea Microlensing Telescope Network (KMTNet; Kim et al 2016) surveys have detected several strong FFP candidates with detections of finite-source effects (e.g., see Mróz et al 2018Mróz et al , 2019Mróz et al , 2020aMróz et al , 2020b. The KMTNet microlensing survey is particularly useful for FFP studies because of its continuous high-cadence observations, which enable both detection of short-timescale events and characterization of finite-source effects.…”