FOR THE FIRST TIME, ASTRONOMERS CONFIRM A ROGUE PLANET BY DIRECTLY MEASURING ITS MASS

In the silent, lightless reaches of the Milky Way, solitary travellers drift through the void. Known as ‘rogue planets’, these celestial objects have long eluded astronomers. They are lone planets free from any host star. Theoretically, they should be present in enormous numbers, billions or even trillions, but they are practically invisible to our instruments.

When we do get lucky and spot a candidate, measuring its distance can be almost impossible. Without a precise distance, we cannot calculate its mass. So, confirming that it is truly as small as a planet (not a failed star seen at a greater distance) is impossible.

That changed on 3 May 2024. An international team achieved the first-ever direct mass measurement of a rogue planet candidate, confirming its identity. The kicker is that they did not actually see the planet itself.

The study, published in Science, centred on a two-day-long gravitational microlensing event designated KMT-2024-BLG-0792/OGLE-2024-BLG-0516. This event was caused when the rogue planet, which was too faint to be seen, passed in front of a distant star. The planet’s gravity acted as a natural magnifying glass, temporarily intensifying the star’s light.

Direct measurements of other rogue planets’ masses have been impossible because the microlensing’s effect is influenced by both the object’s mass and its distance. Without extra information, astronomers cannot disentangle the effects of the object’s mass from its distance. Thus, they have never been able to be certain that they have seen a rogue planet.

“You need to have this fundamental measurement of mass to really know it's a planet,” says lead researcher Dong Subo, Peking University, China.

This is where the team got lucky. An independent way to measure its distance was made possible by the most opportune telescopic combination. The microlensing event was captured simultaneously by two ground-based surveys: the Korea Microlensing Telescope Network (KMTNet) and the Optical Gravitational Lensing Experiment (OGLE). Then, nearly two hours later, ESA’s Gaia space telescope saw it from orbit.

This unique alignment allowed researchers to employ parallax to calculate its distance. Much like human depth perception relies on the spacing between our eyes, the 1.5 million kilometre gap between Earth and Gaia provided the necessary perspective to calculate the planet’s distance. With this information, they could then calculate its mass, based on the brightening it caused.

The team found that the world must be about one-fifth of Jupiter’s mass, or roughly similar to Saturn’s mass.

This discovery offers compelling evidence that our Galaxy is teeming with ejected worlds. These rogue planets are often the victims of early cosmic chaos, gravitationally chucked into the void by sibling planets or passing stars. Researchers expect to find hundreds more with upcoming missions including the Nancy Grace Roman Space Telescope and the Chinese Space Station Survey Telescope.

FOR THE FIRST TIME, ASTRONOMERS CONFIRM A ROGUE PLANET BY DIRECTLY MEASURING ITS MASS

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