Gaia and ground telescopes fix mass and distance of a rogue planet

A team of astronomers has, for the first time, measured both the mass and distance of a free-floating “rogue” planet using microlensing data from ground-based observatories and the space telescope Gaia. The object, catalogued as KMT-2024-BLG-0792 and OGLE-2024-BLG-0516 by separate teams, produced a microlensing event in which the gravity of the unseen lens briefly magnified the light of a background star.

Microlensing can reveal an object’s mass, but without a distance measurement the mass remains ambiguous—a problem known as mass–distance degeneracy. In this case a fortuitous geometry allowed Gaia to observe the event multiple times over a 16-hour window, providing the parallax needed to break that degeneracy.

Observations from two different vantage points and a timing offset in the light curve let researchers calculate the microlensing parallax and thus the lens distance. The team determined the object has about 22% of Jupiter’s mass—just under Saturn’s mass—and lies roughly 3,000 parsecs (just under 10,000 light years) away.

Spectral analysis identified the lensed background star as a red giant. The result contributes to understanding of the so-called “Einstein desert,” a gap in the radial distribution that separates planets from brown dwarfs.

Key Topics

Science, Gaia, Microlensing Parallax, Einstein Desert, Red Giant