Astronomers used gravity to 'see' a dark, lonely planet drifting through the heart of the Milky Way.
Planets are usually defined by the star they orbit. Take away the star, and a planet becomes almost impossible to find. This is because almost every method astronomers use to detect planets depends on the light or motion of a star.
Without a star, a lone planet gives off almost no visible light and leaves no obvious signal for telescopes to track.Astronomers have long suspected that the Milky Way is also filled with many such homeless planets that were either expelled from their birth systems or never had a star to begin with. Until now, no one had been able to prove that these objects were truly planets. A new study has finally done so by measuring both the distance and mass of a free-floating object. This rogue planet, which is about the size of Saturn, is drifting nearly 10,000 light-years away toward the centre of our galaxy. The finding moves the idea of rogue planets from theory into direct observation. “Our discovery offers further evidence that the galaxy may be teeming with rogue planets,” Subo Dong, one of the study authors and a professor of astronomy at Peking University in China, notes.Why rogue planets have been so hard to confirmMost known exoplanets betray their presence through their stars. Some block starlight as they pass in front of it, while others tug gently on their stars, causing detectable motion. Rogue planets offer none of these clues. They produce almost no light of their own and have no star to interact with, making them effectively invisible. The only way astronomers can detect such objects is through gravity. When a massive body passes between Earth and a distant background star, its gravity bends the star’s light, briefly making the star appear brighter. This effect, known as gravitational microlensing, signals that something unseen has crossed the line of sight.Microlensing alone, however, has a serious limitation. The brightening pattern does not uniquely reveal whether the lensing object is small and nearby or larger and farther away. This uncertainty, called mass–distance degeneracy, meant that earlier detections could not rule out heavier objects such as brown dwarfs. As a result, astronomers could not say with certainty whether rogue planets actually existed.A lucky alignment solved the problemThe newly confirmed rogue planet was detected during a microlensing event. “We report a microlensing event—KMT-2024-BLG-0792/OGLE-2024-BLG-0516, which was observed from both ground- and space-based telescopes—that breaks the mass-distance degeneracy,” the study authors note.What made this event special was that the European Space Agency’s Gaia spacecraft also observed it, purely by chance. Gaia views the galaxy from a position far from Earth. Due to this separation, the timing of the microlensing signal looked slightly different from space than from the ground. “Serendipitously, this microlensing event was located nearly perpendicular to the direction of Gaia’s precession axis. This rare geometry caused the event to be observed by Gaia six times over a 16-hour period, beginning close to peak magnification,” the researchers said.That small difference allowed researchers to calculate microlensing parallax, which directly reveals how far away the lensing object must be. With the distance known, the team could finally determine the object’s mass. Their analysis showed that the planet lies about 3,000 parsecs, or just under 10,000 light-years, from Earth. Its mass is around 22 percent that of Jupiter, equivalent to roughly 70 Earth masses, placing it just below Saturn. The background star involved in the event was identified as a red giant, helping to refine the measurements. This mass is particularly significant because it falls within a range where free-floating objects had rarely been seen before—a gap between lighter planets and heavier brown dwarfs often referred to as the Einstein desert. The discovery demonstrates that this gap is not empty after all.Lonely planets won’t be impossible to findBy confirming a rogue planet with a precisely measured mass, the study provides strong support for theories suggesting that free-floating planets are common. Many likely form around stars and are later thrown out by strong gravitational forces, while others may form on their own without ever circling a star.However, the technique used here still relies on rare alignments and cannot find rogue planets at will. For now, each detection depends on chance. Hopefully, further research will overcome this limitation and provide better ways to identify the homeless planets.Moreover, upcoming missions such as NASA’s Nancy Grace Roman Space Telescope and China’s Earth 2.0 mission, which are designed to monitor large regions of the sky continuously, are also likely to make microlensing detections far more frequent.The study is published in the journal Science.
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Astronomers detect rare 'free floating' exoplanet 10,000 light-years from EarthCharles Q. Choi is a contributing writer for Space.com and Live Science. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida.
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