Astronomers using a space telescope that detects X-ray emission have found signs of a planet outside our galaxy. According to NASA, this is a first that could launch a search for planets millions of light-years farther away than the known “exoplanets” planets.

So far, the planets we know of beyond our solar system — they are called exoplanets — have all been spotted in the Milky Way, often within 3,000 light-years of Earth, the U.S. space agency said. This potential planet — around the size of Saturn — would be in the Whirlpool Galaxy, some 28 million light-years away.

Telescopes have spotted thousands of planets by detecting dips in optical light that occur when a planet transits in front of a star, blocking some of its light. In this case, researchers using NASA’s Chandra Observatory looked for dips in the brightness of X-rays, which allow detections at greater distances.

“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,” said Rosanne Di Stefano, a senior astrophysicist at the Smithsonian Astrophysical Observatory and a Harvard lecturer who led the study published this week.

The research estimates the new potential planet would orbit a neutron star or black hole at about twice the distance of Saturn from the sun.

For now, the findings offer only a glimpse of what other galaxies could hold. It will be hard to verify the planet candidate, because it may not cross in front of the X-ray source again anytime soon.

“Unfortunately to confirm that we’re seeing a planet we would likely have to wait decades to see another transit,” said one of the authors, astrophysicist Nia Imara. “We wouldn’t know exactly when to look,” she added.

Their hunt spanned three galaxies beyond the Milky Way, using the NASA X-ray observatory and the European Space Agency’s XMM-Newton, to find that one possibility. The team said the data makes a “strong argument” that the dimming of X-rays came from the passage of a planet rather than a cloud of gas and dust.

If the planet does exist there, “it likely had a tumultuous history and violent past,” NASA said. It would have survived a supernova explosion that created the neutron star or black hole that it spins around, and it risks getting blasted with “extremely high levels of radiation” if the companion star explodes in the future.

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