Possible extragalactic planet detected, but astronomers remain skeptical

Twenty-eight million light-years away, the Whirlpool Galaxy spirals through space like a cosmic storm, its arms studded with young stars igniting and old ones dying. In 2012, NASA's Chandra Observatory, which observes the universe in X-rays, caught something unexpected: a brief flicker from one of those arms. An X-ray source called M51-ULS-1 dimmed for about two hours, then blazed back to life. On its own, this would have been unremarkable. X-ray sources flare and fade all the time. But when a team of astronomers led by Rosanne Di Stefano at the Harvard-Smithsonian Center for Astrophysics examined the data more closely, they began to wonder if something extraordinary had happened—something that had never been confirmed before.

The source they were studying is an X-ray binary, a system of two objects locked in a gravitational dance. One is either a black hole or a neutron star; the other is a massive, luminous blue supergiant. As the compact object pulls material from its companion, that stolen gas and dust swirls into an accretion disk, heating to millions of degrees and flooding space with X-rays. Di Stefano's team proposed that the two-hour dimming wasn't caused by the binary itself, but by something passing in front of it—a planet roughly the size of Saturn, orbiting at a distance comparable to where Uranus circles our sun. They named it M51-1 and published their hypothesis in Nature Astronomy on October 25, 2021.

If confirmed, M51-1 would be the first planet ever detected beyond the Milky Way. The discovery would shatter a boundary that has held for three decades, ever since astronomers found the first exoplanet in 1992 around a neutron star called PSR1257+12. Since then, they have cataloged over 4,500 worlds orbiting distant suns—rocky planets, gas giants, super-Earths, even a world where it rains molten iron. Yet every single one has orbited a star within our own galaxy. The farthest known exoplanet lies just 28,000 light-years away. Andromeda, our nearest galactic neighbor, sits 2.5 million light-years distant. Planets almost certainly exist in those far galaxies; we simply lack the tools to see them. Until now, perhaps.

But the astronomical community is not convinced. Di Stefano's team modeled alternative explanations and concluded that a Saturn-sized planet fit the data best. Yet other astronomers point out that the researchers have not definitively ruled out other culprits: brown dwarfs, which are failed stars; M-dwarfs, common red dwarf stars; or simply variations in how the accretion disk emits X-rays. Benjamin Pope, an exoplanet researcher at the University of Queensland, was blunt: "Personally, I wouldn't bet that this is a planet." Duncan Galloway, who studies neutron star binaries at Monash University, noted that the violent environment around X-ray binaries can produce all manner of unexpected behavior. Di Stefano herself has been careful not to overstate the case. "We cannot claim that this is definitely a planet," she said, "but we do claim that the only model that fits all of the data is the planet candidate model."

The skepticism is not unusual in astronomy. Similar claims of extragalactic planets have surfaced before—in 2018, researchers thought they had found thousands using gravitational microlensing; others have pointed to candidates in Andromeda. Each time, the broader community has remained cautious. This is how science works: a team makes an argument, others scrutinize it, and the process continues. Confirmation of M51-1 will not come for many decades, when the planet is predicted to transit the X-ray binary again. But there are other paths forward. Pope suggests searching for analogous systems within the Milky Way, where optical telescopes could provide additional detail. Galloway sees value in the work regardless of what caused the dimming: the study has given astronomers a new reason to hunt through archival X-ray data for similar events.

Di Stefano and her team are already doing exactly that, scanning other X-ray binaries for signs of dimming. She believes the real verification will come not from confirming M51-1, but from finding more planets like it. "Ultimately, the best verification will be the discovery of more planets," she said. For now, M51-1 remains a candidate, a tantalizing possibility suspended between discovery and doubt. The Whirlpool Galaxy keeps its secrets, but astronomers are learning to look in new ways.