A possible Saturn-sized planet identified in the distant Whirlpool Galaxy could be the first exoplanet to be detected outside the Milky Way.
The exoplanet candidate appears to be orbiting an X-ray binary – made up of a normal star and a collapsed star or black hole – with its distance from this binary roughly equivalent to the distance of Uranus from the sun.
The discovery opens up a new window to search for exoplanets – planets orbiting stars beyond our Sun – at greater distances than ever before. Although nearly 5,000 exoplanets have been detected so far, all of them are in the Milky Way galaxy – with few further than about 3,000 light years from Earth.
An exoplanet in the spiral Messier 51 (M51) galaxy – also called the Whirlpool Galaxy because of its distinctive shape – would be about 28m light years away.
Dr Rosanne Di Stefano of the Center for Astrophysics at Harvard and Smithsonian in Cambridge, US, who led the research, said: “Since the 1750s, it has been conjectured that the dim distant nebulas, now called galaxies, are island universes: large, gravitationally-bound stellar populations similar to our home, the Milky Way. Our discovery of the planet candidate … gives us the first peek into external populations of planetary systems, extending the reach of planet searches to distances roughly 10,000 times more distant.”