NASA scientists have detected the first possible planet outside of Milky Way Galaxy, about 28 million light years from Earth. The possible exoplanet was discovered in the spiral Messier 51 (M51) ‘Whirlpool’ galaxy after researchers detected a temporary dimming in X-rays in a binary system, the space agency said in a press release. The researchers looked for X-ray transits in three galaxies, using NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton.

Researchers see this dimming as a possible sign of a planet passing in front of an X-ray source around a neutron star or black hole orbiting a companion star. Exoplanets are defined as planets outside of our Solar System.

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Until now, all other exoplanets have been found in the Milky Way, mostly less than 3,000 light-years from Earth.

“We are trying to open up a whole new arena for finding other worlds by searching for planet candidates at X-ray wavelengths, a strategy that makes it possible to discover them in other galaxies,” Rosanne Di Stefano, lecturer instructor in astronomy at the Center for Astrophysics at the Harvard & Smithsonian in Cambridge, Massachusetts, who led the study, said in a statement.

However, researchers will have to wait for decades to confirm the observation as the exoplanet candidate will take another 70 years to cross its binary partner due to its large orbit.

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“Unfortunately to confirm that we’re seeing a planet we would likely have to wait decades to see another transit,” co-author astrophysicist Nia Imara, of the University of California at Santa Cruz, said in a statement. “And because of the uncertainties about how long it takes to orbit, we wouldn’t know exactly when to look.”

Researchers will scour archives at Chandra for substantial datasets of some 20 galaxies, and European Space Agency satellite XMM-Newton, for more exoplanet candidates in other galaxies. The study was published in the journal Nature Astronomy.