Astronomers measure the mass of a rogue planet drifting through the galaxy

Most known planets orbit one or more stars, but evidence is mounting that some worlds travel through the galaxy on their own. These lonely objects are known as free-floating or rogue planets, and they have no identified stellar companion. Because they produce very little light, astronomers usually detect them only by watching how their gravity briefly alters the light from a distant background star. This effect is called microlensing. One major challenge with microlensing is that it usually does not reveal how far away the planet is, which makes it hard to calculate its mass independently. As a result, many details about this hidden population of solitary planets have remained uncertain.

In the new research, Subo Dong and colleagues describe the discovery of a free-floating planet spotted during a short microlensing event. What makes this finding unusual is that the event was observed at the same time from Earth and from space. The team used data from several ground-based surveys along with observations from the Gaia space telescope. Small differences in the timing of the light seen from these widely separated locations made it possible to measure the microlensing parallax. When combined with finite-source point-lens modeling, this information allowed the researchers to determine both the planet's mass and where it is located in the galaxy.

The planet has a mass about 22 percent that of Jupiter and lies roughly 3,000 parsecs from the center of the Milky Way. With a mass similar to Saturn, the researchers suggest that it most likely formed within a planetary system rather than developing on its own like a small star or brown dwarf. Scientists think that low-mass rogue planets are born around stars and later forced out of their orbits by gravitational disturbances, such as close encounters with other planets or unstable stellar companions.