NASA discovers first possible planet outside our galaxy

A composite image contains a box that marks the location of the possible planet candidate. (NASA/R. DiStefano et al./Grendler/M. Weiss via CNN)

Scientists may have detected signs of a planet transiting a star outside of the Milky Way, in what could be the first planet ever to be discovered outside our galaxy.

The possible exoplanet was discovered in the Whirlpool Galaxy -- the spiral galaxy Messier 51 (M51) -- by NASA's Chandra X-ray Observatory, NASA said in a press release on Monday.

An exoplanet is a planet outside of our solar system that normally orbits a star other than our own sun in our galaxy. Until now, all other exoplanets have been found in the Milky Way, and most of them have been found less than 3,000 light-years from Earth.

This newly discovered possible exoplanet in the Whirlpool Galaxy would be about 28 million light-years away -- thousands of times farther away than those in the Milky Way.

"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.

The team searched for dips in the brightness of X-rays from X-ray bright binaries, which usually contain a neutron star -- when a massive star collapses -- or black hole pulling in gas from a closely orbiting star. The material near the neutron star or black hole becomes superheated and glows in X-rays.

The region creating bright X-rays is small, and so a planet passing in front of it would be easy to spot, as it would block most, or all, of the X-rays. This allows exoplanets to be detected at much greater distances.

However, researchers will have to wait a long time to confirm whether they have discovered an extragalactic exoplanet. Due to its large orbit, the planet candidate would not cross in front of the binary partner for another 70 years, meaning it could take decades to confirm the observation.

"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."

If the planet does exist, experts say that it would have had to survive a supernova explosion that created the neutron star or black hole. And in the future, the companion star could also explode as a supernova and blast the planet once again with extremely high levels of radiation.

Researchers will search the archives of both Chandra, which has substantial datasets for some 20 galaxies, and European Space Agency satellite XMM-Newton, for more exoplanet candidates in other galaxies. They add that another interesting line of research is to search for X-ray transits in Milky Way X-ray sources to discover new nearby planets in unusual environments.

The study was published in the journal Nature Astronomy.