Scientists have directly measured the mass and distance of a free-floating planet drifting through the Milky Way, using simultaneous observations from Earth and space. The planet, with a mass similar to Saturn, likely formed around a star before being ejected into interstellar space. This discovery highlights new methods for studying these elusive objects.
Astronomers have achieved a milestone in planetary science by determining the mass and position of a rogue planet wandering the galaxy without a host star. The object was detected through a microlensing event, where its gravity temporarily bent light from a distant background star. Unlike typical microlensing detections, which often leave distance and mass ambiguous, this case benefited from coordinated observations.
A team led by Subo Dong combined data from multiple ground-based surveys with measurements from the European Space Agency's Gaia space telescope. The slight timing differences in light arrival—known as microlensing parallax—enabled precise calculations. The planet's mass is approximately 22 percent that of Jupiter, akin to Saturn, and it resides about 3,000 parsecs from the Milky Way's center.
Researchers suggest this low-mass world originated in a planetary system and was later disrupted by gravitational interactions, such as encounters with other planets or unstable companions. Free-floating planets like this are challenging to spot due to their faintness, but their numbers are expected to rise with advanced telescopes.
In a related perspective, Gavin Coleman emphasized the potential of such observations. "Simultaneous space- and ground-based observations of microlensing events could be applied in the planning of future exploratory missions and could lead to a better understanding of how planets form across the Galaxy," he noted. The upcoming NASA Nancy Grace Roman Space Telescope, set for launch in 2027, may further this research.
This finding, published in Science (2026; 391(6780):96), underscores the diversity of planetary fates and the power of multi-perspective astronomy.