Planets are often found orbiting a star or a group of multiple stars. However, planets have also been observed to drift independently without a known accompanying star system. These celestial objects, called free-floating or rogue planets, are difficult to detect because they don’t emit enough light to be spotted through current generation telescopes. On page 96 of this issue, Dong et al. (1) report the discovery of a rogue planet by a short-lived microlensing event, an astronomical phenomenon in which the gravitational effect of a foreground object magnifies the light from a background star. By combining simultaneous measurements on Earth and in space, the mass of the planet was estimated to be 22% that of Jupiter. This finding demonstrates how coordinated observations can overcome difficulties in determining both the position and mass of a rogue planet and improve the understanding of how these planets form.

Rogue planets were identified in 2000 (2) when an astronomical object with a mass similar to that of Jupiter was directly observed in a nearby star-forming region, the Orion Nebula. The count of these celestial bodies has grown over the past decades, with some of them estimated to be more massive than Jupiter. However, imaging a small rogue planet is challenging. The amount of light that rogue planets emit is substantially reduced because of their diminutive size. Without a host star, common detection techniques, such as the transit method—finding an exoplanet (a planet outside of the Solar System) by observing slight dimming of a star’s light as a planet passes in front of it—cannot be used. Currently, the only technique available to discover rogue planets is gravitational microlensing. When a celestial body moves in front of a star, the light beams from the background star are bent by the gravitational field of the foreground object and focused on an observer on Earth or a satellite in space. This microlensing phenomenon amplifies the apparent brightness of the star and enables detection of the lens (such as a planet) in the foreground.

Microlensing can provide hints to planet formation processes by probing regions of star systems that are inaccessible by other techniques, such as the transit method. Recent work (3) analyzed the distribution of the mass ratio of exoplanets to their host stars by combining new microlensing events with existing samples that have been identified within the Korea Microlensing Telescope Network survey (4). The finding created an intriguing problem for planet formation models. The observation better matched simulations without the planet migration effect (a planet moves away from its original orbit) (5) than those including the migration processes (6). This contradicted observations from the transit method, which indicated substantial planet migration (7). Microlensing events from rogue planets may help close the gap by providing clues to the planets’ formation within the original planetary systems before they became independent.

Although the mass of a planet that is gravitationally orbiting a star can be estimated through an analysis of the host star, it is challenging to determine the mass of a rogue planet empirically because of ambiguities in the relationship between the actual mass of the planet and its distance from an observer. Dong et al. overcame this limitation by detecting a microlensing event simultaneously on Earth and in space. While the Optical Gravitational Lensing Experiment and Korea Microlensing Technology Network monitored the phenomenon on Earth, the Gaia satellite, which is located 1.5 million km away from Earth (the Sun-Earth L2 point) (8), repeated observations of the same event six times (see the figure). The subtle difference in timing of the light beams from the background star reaching Earth and Gaia provided a measurement of the microlens parallax (the angle between the two lines of sight) (9). Combining this quantity with other observed properties yielded measures of the mass and distance of the planet. The celestial body was estimated to have a fraction of Jupiter’s total mass (~0.22 Jupiter mass) and to be located 3 kpc away from Earth towards the Galactic bulge (center of the Milky Way). Observations could not, however, detect a host star, confirming that the microlensing event was caused by a rogue planet.

Because the rogue planet’s mass is similar to that of Saturn (~0.3 Jupiter mass), the celestial body may have formed through processes analogous to those by which other known planets have developed. This contrasts with the formation of more massive rogue planets (more than a few Jupiter masses) that is more similar to the genesis of low mass stars and brown dwarfs (celestial objects that are more massive than planets but not as massive as stars). Thus, smaller rogue planets would form within a planetary system before they are ejected from their orbit through dynamical interactions with sibling planets (10), a star within a binary system (11), or a wide-orbiting binary stellar companion (12).

There are many possible pathways that would result in the formation of rogue planets with different properties (mass, velocity, and position). For example, planets are more likely to be ejected from a circumbinary system (a planet orbiting two stars) than from a single star system because of the increased frequency of perturbations from the binary stars (10, 12). In addition, the velocity at which rogue planets are ejected from a binary system is larger than those that are ejected through planet-planet scattering (a process in which gravitational interactions between planets change their orbits substantially) (11, 13). The rogue planet discovered by Dong et al. had large relative velocities compared with those of stars that are observed at similar distances from Earth. This implies that the rogue planet may have originated in a circumbinary system where it was likely ejected through interactions with one of the binary stars.

Microlensing events have revealed hundreds of exoplanets that are bound to a star system. Nevertheless, only a handful of rogue planets have been uncovered to date. These numbers are expected to increase in the coming years with the Nancy Grace Roman Space Telescope campaign that is scheduled for launch in 2027. This observatory will survey huge swaths of the sky 1000 times faster than the Hubble Telescope and in infrared light. This will allow the detection of thousands of new planets and the rigorous testing of planet formation models (14). The discovery of Dong et al. highlights the effectiveness of the microlensing technique and how detection of rogue planets can inform the process of planet formation. 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.