Astronomers spot first possible exoplanet outside our galaxy

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.”

Di Stefano and colleagues detected it using Nasa’s Chandra X-ray Observatory, by searching for a characteristic dip in the brightness of X-rays caused by the passage of a planet in front of the X-ray binary.

These luminous systems typically contain a neutron star or black hole pulling in gas from a closely orbiting companion star – in this case, a star with a mass about 20 times that of the Sun. The material near the neutron star or black hole becomes superheated and glows in X-rays.

The signal they detected lasted for about three hours, during which the X-ray emission decreased to zero. Based on this and other information, Di Stefano and colleagues estimate that exoplanet candidate would be roughly the size of Saturn, and orbit the neutron star or black hole at about twice the distance of Saturn from the Sun – roughly equivalent to Uranus’ orbit. The research was published in Nature Astronomy.

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Exciting as this discovery is, more data is needed to confirm that the object is indeed an extragalactic exoplanet. One challenge is that the object’s large orbit means it will not cross in front of the X-ray binary again for about 70 years.

“Unfortunately to confirm that we’re seeing a planet we would probably have to wait decades to see another transit,” said 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.”

If a planet really does exist in this system, it is likely to have experienced a violent past. Any exoplanet in the system would have had to survive the cataclysmic supernova explosion that created the neutron star or black hole from a previously existing star. The future may also be dangerous, as at some point the companion star could also explode and blast the planet with extremely high levels of radiation.

Searching for X-ray transits in Milky Way, and other extragalactic X-ray sources, could lead to the discovery of other exoplanet candidates in unusual environments.