The hunt for exoplanets is venturing beyond the Milky Way. Astronomers using NASA’s Chandra X-ray Observatory have detected what may be the first signs of a planet in another galaxy. The team noticed drops in X-ray brightness that indicate a planet moving in front of a star in the Messier 51 (also known as M51) galaxy 28 million light-years away. For context, all the candidates for exoplanets in the Milky Way are no more than 3,000 light-years from Earth — this planet could easily set a distance record if confirmed.
The nature of the stars made the feat possible. Because the researchers had to focus on X-ray bright binary systems where the area of bright rays is relatively small, the transit was significantly easier to spot. Conventional detection of nearby stars requires much more sensitive light detection, as a planet can only block a small amount of light from a given star.
The planet itself is believed to be the size of 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, because those aren’t consistent with the event they recorded in M51. However, a planet would match the data.
The challenge, as you might guess, is verifying that data. The planet’s orbit could rule out another transit for about 70 years, and it wouldn’t be clear exactly when astronomers should take a closer look. This planetary candidate’s three-hour transit did not provide a large window. That’s also assuming the ‘living’ star doesn’t explode and bathe the planet in radiation.
However, if confirmation ever comes, the discovery would be very important. While there is not much doubt about the existence of planets in other galaxies, it would be helpful to have evidence of their existence. This could also significantly expand the scope of future planetary searches to include the galactic environment, not just nearby stars.
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