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Astronomers detect first hints of extragalactic planet

A blip recorded by the NASA’s Chandra X-ray Observatory has pointed astronomers to what might be the first planet detected passing across a star in a galaxy beyond our own — but we may not know for sure anytime soon.

The observation of an X-ray transit in the spiral galaxy M51, about 28 million light-years away in the northern constellation Canes Venatici, is reported in the journal Nature Astronomy.

Even if the detection of a planet in M51 goes unconfirmed, the Chandra observations demonstrate that X-ray transits could become a new method for tracking planets far beyond our solar system.

“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 of the Harvard-Smithsonian Center for Astrophysics, lead author of the newly published study, said in a news release.

The method is similar to the one that astronomers have used for more than a decade to detect planets in our own celestial neighborhood. They keep watch on a distant star and look for slight dips in the intensity of its light. If there’s a pattern to those dips, that could mean a planet is crossing the disk and blocking some of the starlight.

Usually, the observations are made in optical wavelengths. But Di Stefano and her colleagues focused on X-ray data associated with a binary star system known as M51-ULS-1. The binary pair is thought to consist of a star about 20 times as massive as our own sun, plus a black hole or a neutron star.

The team identified a three-hour period during which X-ray emissions from the star system decreased to zero. The characteristics of the apparent transit event would match up with a planet candidate that’s roughly the size of Saturn, orbiting the black hole or neutron star at a distance that’s about twice Saturn’s distance from the sun.

In the course of analyzing the data, the researchers checked whether the dip in starlight could have been caused by a hiccup in the star’s intrinsic X-ray emissions, by a passing cloud of dust or gas, or by some other object coming between the star system and Chandra’s vantage point. They concluded that a Saturn-sized planet provided the best explanation for the data they were seeing.

A planet spinning near such a strong X-ray source would be unlikely to harbor life as we know it. Such a planet would have had to weather the stellar explosion that gave rise to the black hole or neutron star, and it’s in a position to be blasted with another blazing jolt of radiation if the companion star blows up in a supernova.

Di Stefano and her colleagues don’t go so far as to claim that they’ve definitively discovered a planet. That would require further observations of transits — and if the researchers’ interpretation of the data is correct, the next transit won’t take place for another 70 years.

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

But that doesn’t mean the researchers are at a dead end: They plan to search the archives of X-ray observations made by Chandra and another space telescope, XMM-Newton, and look for signs of other transits with shorter cycles. They also say their method could be used to detect X-ray transits in our own Milky Way galaxy, which could point to the existence of extreme exoplanets closer to home.

In addition to Di Stefano and Imara, the authors of the Nature Astronomy study, titled “A Possible Planet Candidate in an External Galaxy Detected Through X-Ray Transit,” include Julie Berndtsson, Ryan Urquhart, Roberto Soria, Vinay Kashap and Theron Carmichael.

By Alan Boyle

Mastermind of Cosmic Log, contributing editor at GeekWire, author of "The Case for Pluto: How a Little Planet Made a Big Difference," president of the Council for the Advancement of Science Writing. Check out "About Alan Boyle" for more fun facts.

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