Astronomers have spotted tantalizing signs of a planet in a star system outside the Milky Way, which, if it's confirmed, would be the first ever found in another galaxy.

The possible planet was discovered in a spiral galaxy called Messier 51, also known as the Whirlpool Galaxy, which is more than 23 million light-years from Earth.

If it is confirmed, the planet in the Whirlpool Galaxy would be thousands of times farther away than any other alien planet that has been identified.

The possible alien world was found in an X-ray binary system, a type of star system that produces and emits X-rays and is usually made up of a normal star and a collapsed star, such as a neutron star or a black hole.

Di Stefano and her colleagues applied the same basic idea, but instead of optical light, they monitored for changes in the brightness of X-rays from the binary system in the Whirlpool Galaxy.

Di Stefano said the region that produces bright X-rays is relatively small, making it possible to detect transits that block most or all of the X-ray emissions.

"The awesome thing is that they found a planet orbiting around a neutron star that is part of a system that has been through a supernova explosion and had an interesting and complicated evolutionary history," he said.