The hunt for exoplanets goes beyond the Milky Way. Astronomers using NASA’s Chandra X-ray Observatory have discovered what may be the first signs of a planet in another galaxy. The team noticed a decrease in X-ray brightness suggesting a planet passing in front of a star in the Messier 51 (aka M51) galaxy 28 million light-years away. For context, all 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 nature of the stars made the business possible. Since the researchers had to focus on X-ray binary systems where the range of light 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 may only block a small amount of light from a given star.
The planet itself is thought to be as large as Saturn, but would orbit its hosts (a star 20 times the mass of the Sun and a black hole or neutron star) at double distance.
Researchers did not believe the attenuation was due to gas clouds or dust, as they do not match the event they recorded in the M51. However, a planet would line up with the data.
The challenge, as you might guess, is to verify this data. The planet’s large orbit could preclude another transit in about 70 years, and it would not be clear exactly when astronomers should look. The three-hour transit of this planetary candidate did not yield a large window. It also presupposes that the ‘living’ star does not explode and bathe the planet in radiation.
However, if there is ever a confirmation, the discovery would be very significant. Although there is not much doubt that planets exist in other galaxies, it would be useful to have evidence for their existence. This could also significantly expand the scope of future planetary searches to include the galactic quarter, not just close to stars.
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