Rogue planets, drifting through the galaxy without a host star, have previously been detected only as brief microlensing events yielding rough statistical mass estimates. An international team led from the Department of Astronomy in the School of Physics at Peking University measured the mass of one directly. The object weighs about one fifth of Jupiter's mass, placing it in the Saturn class and settling its status as a planet rather than a failed star.
The measurement exploited a geometric trick. The event, observed on 3 May 2024 and lasting about two days, was recorded from Earth by the Korea Microlensing Telescope Network, using three 1.6 metre telescopes in Chile, South Africa and Australia, and by the Optical Gravitational Lensing Experiment with a 1.3 metre telescope in Chile. The Gaia spacecraft, about 1.5 million kilometres from Earth, also captured it, and saw it roughly two hours later.
That offset is the microlens parallax effect, analogous to human depth perception, and it allowed the team to calculate the object's mass and distance separately rather than as a degenerate combination. This was the only occasion during Gaia's mission, which observed nearly two billion stars from 2014 to early 2025, that all three instruments recorded a rogue planet candidate together. The results were published in Science.
The technique matters more than the single object. Microlensing surveys and models suggest the Milky Way contains billions or even trillions of starless planets, formed either by ejection from planetary systems or by direct collapse from gas and dust. NASA's Nancy Grace Roman Space Telescope is expected to find hundreds, and China's survey telescope and Earth 2.0 mission include microlensing goals, so a validated mass-measurement method arrives at a useful moment.