Astronomers have measured the mass of a Saturn-sized rogue planet floating freely in space, marking the first such confirmation for a world in this size range. Located nearly 10,000 light years away, the planet was detected through gravitational microlensing using both ground-based telescopes and the Gaia space observatory. This discovery fills a gap in our understanding of free-floating exoplanets.
A team led by Andrzej Udalski at the University of Warsaw in Poland identified the rogue planet, known by the dual names KMT-2024-BLG-0792 and OGLE-2024-BLG-0516, after spotting it independently with two ground-based telescopes. The planet's mass, calculated at about one-fifth that of Jupiter, places it in a rare category often called the "Einstein desert," where few rogue worlds between Neptune's mass and Jupiter's have been found. Lighter planets are thought to be easily ejected from their original orbits around stars, while heavier ones may form independently, like stars.
The breakthrough came from a fortunate alignment of observations. Gravitational microlensing, the technique used, bends light from distant stars around the planet, creating a detectable halo. At the time of the ground detection, the Gaia space telescope was ideally positioned to observe the same event, enabling precise mass measurement.
"What’s really great about this one is that it’s the first one that we’ve got that has a mass measurement, and that was only possible because they got Gaia observations as well as Earth-based observations," said Gavin Coleman at Queen Mary University of London.
Udalski emphasized the significance: "Mass is the main parameter deciding on the classification as a planet." He added, "This is the moment from which we may be sure that the candidate is a real planet, and free-floating planets indeed exist."
These rogue planets, ejected during the chaotic early stages of planetary system formation, could number in the billions across the galaxy. Some models suggest even our solar system lost a planet in its youth. The upcoming NASA Nancy Grace Roman space telescope, set for launch in 2027, promises to uncover many more, shedding light on planetary formation processes.
The findings appear in Science (DOI: 10.1126/science.aed5209).
