Chinese scientists confirm planet identity for free-floating object

Chinese scientists have successfully achieved the first direct mass measurement of a free-floating object, confirming that it is a planet with a mass comparable to that of Saturn.

This key research finding, led by the Department of Astronomy research team at the School of Physics, Peking University, was published in Science on Friday.

Free-floating objects are mysterious celestial bodies that do not orbit any star and drift alone through interstellar space. When a free-floating object passes in front of a background star, its gravity bends the starlight like a lens, causing the star to brighten temporarily – a phenomenon known as a microlensing event, according to Dong Subo, head of the research team.

Over the past decade, scientists have detected approximately ten free-floating objects through observations with ground-based telescopes. However, their mass has yet to be accurately measured.

In 2024, the research team reported a microlensing event observed from both ground- and space-based telescopes. By combining observations from Earth and a distant spacecraft with a time difference, the mass-distance degeneracy can be broken, enabling the research team to extract the object's distance information and then determine its mass and distance separately, the research shows.

"This is analogous to observing the same scene with 'eyes' that are extremely far apart," Dong said.

"This achievement highlights the advantages and significance of space-ground coordinated observations and provides important reference for upcoming projects such as NASA's Roman Space Telescope," a reviewer for the journal Science commented.

The China Space Station Telescope, independently developed by China and planned for launch, will also support such observations.