TLDR
Astronomers found a brief brightening caused by an unseen object about the size of Mars or Earth bending starlight. The object may be wandering alone through space, showing that such small, dark worlds can be found without seeing them directly.
Summary
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1 Study Aim
The authors aimed to test whether gravitational microlensing (a temporary brightening caused by gravity bending light) could detect a terrestrial-mass rogue planet. They focused on OGLE-2016-BLG-1928, the shortest-timescale microlensing event known then. Planet-formation theories predict that small planets can be ejected from their systems. The study therefore examined whether this event matched an ejected planet rather than ordinary stellar variability. The research tested whether a tiny, unseen planet could be identified from its brief effect on a background star.
2 Study Design
The study analyzed 23 years of Optical Gravitational Lensing Experiment (OGLE) observations and data from the Korea Microlensing Telescope Network (KMTNet). The event occurred on June 18, 2016, and lasted about 41.5 minutes. The authors fitted a finite-source model, which accounts for the lens crossing the visible disk of the background star. This model measured the angular Einstein radius, the apparent size of the lensing region. They used source color and brightness to estimate the star’s angular radius. Markov Chain Monte Carlo sampling estimated model uncertainties. Binary-lens models tested for a host star, while Gaia space-based motion measurements helped assess the lens location. The researchers closely modeled one unusually short brightening and checked whether another star could explain it.
3 Findings
The authors measured an angular Einstein radius of 0.842 ± 0.064 microarcseconds and a relative lens-source motion of 10.6 ± 1.0 milliarcseconds per year. The lens mass cannot be measured precisely because its distance is unknown. If it lies in the Galactic disk, the lens would be about three Mars masses. A bulge location would allow roughly two Earth masses, but Gaia proper motion strongly favors the disk interpretation. The event’s shape does not match stellar flares. Binary-lens tests found no significant host-star evidence within about 8 astronomical units, or Earth-Sun distances. The study recommends improved coverage and more color observations for future short events. The evidence points to a tiny wandering planet, though its exact mass and freedom from a distant host remain uncertain.