Simultaneous ground- and space-based observations of a newly discovered free-floating planet have enabled direct measurement of its mass and distance from Earth, according to a new study. The findings offer insights into the diverse and dynamic pathways by which planets can be cast adrift into interstellar space. Although studies to date have only revealed a handful of such free-floating planets, detections are expected to increase in the coming years, particularly with the NASA Nancy Grace Roman Space Telescope campaign that is scheduled for launch in 2027, notes Gavin Coleman in a related Perspective. “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.”

Planets are most often found bound to one or more stars, yet a growing body of evidence shows that some wander the galaxy alone. These objects, called free-floating or rogue planets, lack any known stellar companion. And, since they don’t emit very much light, they reveal themselves only through their subtle gravitational effects – a phenomenon called microlensing. One of the main limitations of this discovery method is that it cannot determine the distance to these planets, making independent measurement of their mass difficult. As a result, much about this elusive population of solitary worlds remains speculative.

Here, Subo Dong and colleagues report the discovery of a new free-floating planet detected via a fleeting microlensing event. However, unlike previous detections, Dong et al. uniquely observed this microlensing event simultaneously from both Earth and space, using several ground-based surveys alongside the Gaia space telescope. Tiny differences in the timing of the light reaching these distantly separated vantage points enabled measurement of the microlensing parallax, which, when combined with finite-source point-lens modeling, allowed the authors to determine the planet’s mass and location. It is ~22% the mass of Jupiter and roughly 3,000 parsecs from the center of the Milky Way. Because this planet’s mass is comparable to that of Saturn, Dong et al. argue that it likely formed within a planetary system, rather than in isolation like a small star or brown dwarf. Such low-mass rogue planets are thought to be born around stars and later expelled from their orbital confines through gravitational upheavals, such as interactions with neighboring planets or unstable stellar companions.