Astronomers may have spotted a planet in another galaxy for the first time

The hunt for exoplanets is venturing beyond the Milky Way. Astronomers using NASA's Chandra X-ray Observatory have detected what might be the first signs of a planet in another galaxy. The team noticed dips in X-ray brightness that hint at a planet transiting in front of a star in the Messier 51 (aka M51) galaxy 28 million light-years away. For context, all the exoplanet candidates in the Milky Way are no more than 3,000 light-years from Earth — this planet would easily set a distance record if confirmed.

The very nature of stars made the feat possible. As the researchers had to focus on X-ray bright binary systems where the region of bright rays is relatively tiny, the transit was considerably easier to spot. Conventional detection of nearby stars requires much more sensitive light detection, as a planet might only block a small amount of light from a given star.

The planet itself is believed to be as large as Saturn, but would orbit its hosts (a star 20 times the mass of the Sun as well as a black hole or neutron star) at twice the distance.

Scientists didn't believe the dimming was due to gas clouds or dust, as those aren't consistent with the event they recorded in M51. A planet, however, would line up with the data.

The challenge, as you might guess, is verifying that data. The planet's large orbit could rule out another transit for roughly 70 years, and it wouldn't be clear exactly when astronomers would have to take a look. The three-hour transit of this planet candidate didn't provide a large window. That's also assuming the 'living' star doesn't explode and bathe the planet in radiation.

If there's ever a confirmation, though, the discovery would be very significant. While there aren't many doubts that planets exist in other galaxies, it would be useful to have evidence of their existence. This could also significantly widen the scope of future planetary searches to include the galactic neighborhood, not just close-by stars.

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