Planet found outside our galaxy for first time

'Extroplanet' detected 28 million light years away

An artist's rendering of the first planet candidate identified outside of our Milky Way galaxy is pictured next the M51 galaxy. Photo: Nasa

A planet has been detected outside of the Milky Way galaxy for the first time.

Astronomers spotted the planet, which could be the size of Saturn, about 28 million light years away.

The planet in M51, a spiral galaxy also known as the Whirlpool Galaxy, would be the first “extroplanet” to be detected.

An "extroplanet" is one found outside of Earth's home galaxy.

It is thousands of times farther away than the 4,000 “exoplanets” – those orbiting a star outside Earth’s solar system – that have been found within the Milky Way.

Researchers using Nasa’s Chandra X-ray Observatory believe a dimming of X-rays in the system they were viewing was being caused by a planet passing in front of the collapsed core of a supergiant star or black hole.

A composite image of M51 with X-rays from Chandra and optical light from Nasa's Hubble Space Telescope contains a box that marks the location of the possible planet candidate. Photo: Nasa

“We know we are making an exciting and bold claim so we expect that other astronomers will look at it very carefully,” said Julia Berndtsson of Princeton University in the US, co-author of the research published in Nature Astronomy.

“We think we have a strong argument, and this process is how science works.”

The research paper details their search for X-ray transits in three galaxies beyond the Milky Way galaxy, using both Chandra and the European Space Agency’s XMM-Newton. The search of more than 200 systems produced just one successful find.

The research could open a new window to search for "extroplanets".

“We are trying to open up a whole new arena for finding other worlds by searching for planet candidates at X-ray wavelengths, a strategy that makes it possible to discover them in other galaxies,” said Rosanne Di Stefano of the Centre for Astrophysics, Harvard and Smithsonian in Cambridge, Massachusetts, who led the study.