Astronomers have spotted tantalizing signs of a planet in a star system outside the Milky Way, which, if it's confirmed, would be the first ever found in another galaxy.
The discovery, reported in a study published Monday in the journal Nature Astronomy, demonstrates a new technique for finding far-off worlds, and it could significantly expand the search for so-called extragalactic exoplanets.
"It's always fun when you find something that is the first of its kind," said the study’s lead researcher, Rosanne Di Stefano, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. "Once we began to find planets locally, it made sense that there were planets in other galaxies, but this is humbling and really exciting."
The possible planet was discovered in a spiral galaxy called Messier 51, also known as the Whirlpool Galaxy, which is more than 23 million light-years from Earth.
The first exoplanets, or planets outside the solar system, were discovered in the 1990s, and they required combining a number of complicated detection techniques. Since then, however, NASA missions like the Kepler Space Telescope and the Transiting Exoplanet Survey Satellite have uncovered a bonanza of worlds throughout the galaxy.
More than 4,000 exoplanets have been discovered and confirmed, but until now, they have all been in the Milky Way. Most have also been less than 3,000 light-years from Earth. If it is confirmed, the planet in the Whirlpool Galaxy would be thousands of times farther away than any other alien planet that has been identified.
The possible alien world was found in an X-ray binary system, a type of star system that produces and emits X-rays and is usually made up of a normal star and a collapsed star, such as a neutron star or a black hole.
Typically, astronomers use what's known as the "transit method" to look for planets. Transits occur when a planet orbits in front of its parent star, temporarily blocking part of it and causing an observable dip in the star's light. Di Stefano and her colleagues applied the same basic idea, but instead of optical light, they monitored for changes in the brightness of X-rays from the binary system in the Whirlpool Galaxy.
Di Stefano said the region that produces bright X-rays is relatively small, making it possible to detect transits that block most or all of the X-ray emissions.
"It's a very obvious signal," she said.
Using data from NASA's Chandra X-ray Observatory, Di Stefano and her colleagues observed that the transit lasted about three hours, and they were able to roughly gauge the object's size because it completely blocked the X-ray source. They estimate that the possible planet is the size of Saturn and that it is much farther from its star than Earth is from the sun.
Bruce Macintosh, a professor of physics at Stanford University who wasn’t involved with the research, said the discovery is exciting because, if it is verified, it shows not only that planets are common throughout the cosmos, but that they can also exist in unlikely places.
"The awesome thing is that they found a planet orbiting around a neutron star that is part of a system that has been through a supernova explosion and had an interesting and complicated evolutionary history," he said. "It's exciting that a planet can survive having its star blow up."
Confirming that there really is a planet in the X-ray binary system is likely to take time. The planet's far-out orbit means it is likely to be around 70 years before astronomers could witness another transit.
"And because of the uncertainties about how long it takes to orbit, we wouldn't know exactly when to look," a co-author of the study, Nia Imara, an assistant professor at the University of California, Santa Cruz, said in a statement.
Macintosh said that the method of studying X-ray transits is "clever" but that it's unlikely that it could be used to find hundreds of thousands of planetary candidates because it also relies on luck.
"You can only see transits when objects line up just right between you and the thing you're looking at," he said. "And you only see it when it passes in front of the target object for a few minutes or hours."
Still, Di Stefano said, it's gratifying that the new method of searching for extragalactic exoplanets, which she and her colleagues first theorized in 2018, has produced such an enticing result.
"We did not know whether we would find anything, and we were extremely lucky to have found something," she said. "Now we hope other groups around the world study more data and make even more discoveries."