Image source, ESO / L. CalçadaImage caption, The finding comes from observation of an X-ray binary – a neutron star or black hole pulling in gas from a companion star

Astronomers have found hints of what could be the first planet ever to be discovered outside our galaxy.

Nearly 5,000 "exoplanets" – worlds orbiting stars beyond our Sun – have been found so far, but all of these have been located within the Milky Way galaxy.

The possible planet signal discovered by Nasa's Chandra X-Ray Telescope is in the Messier 51 galaxy.

This is located some 28 light-years away from the Milky Way.

This new result is based on transits, where the passage of a planet in front of a star blocks some of the star's light and produces a characteristic dip in brightness that can be detected by telescopes.

This general technique has already been used to find thousands of exoplanets.

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But Dr Roseanne Di Stefano and colleagues searched for dips in the brightness of X-rays received from a type of object known as an X-ray bright binary.

These objects typically contain a neutron star or black hole pulling in gas from a closely orbiting companion star. The material near the neutron star or black hole becomes superheated and glows at X-ray wavelengths.

Because the region producing bright X-rays is small, a planet passing in front of it could block most or all of the X-rays, making the transit easier to spot.

The team members used this method to detect the exoplanet candidate in a binary system called M51-ULS-1.

Future planet-hunting

This binary contains a black hole or neutron star orbiting a companion star with a mass about 20 times that of the Sun.

The X-ray transit lasted about three hours, during which the X-ray emission decreased to zero. Based on this and other information, the astronomers estimate that the candidate planet would be around the size of Saturn, and orbit the neutron star or black hole at about twice the distance Saturn lies from the Sun.

"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, from the Harvard-Smithsonian Center for Astrophysics in Cambridge, US.

The researchers freely admit that more data is needed to verify their interpretation.

One challenge is that the planet candidate's large orbit means it would not cross in front of its binary partner again for about 70 years, quashing any attempts to make a follow-up observation in the near-term.

One other possible explanation that the astronomers considered is that the dimming has been caused by a cloud of gas and dust passing in front of the X-ray source.

However, they think this is unlikely, because the characteristics of the event do not match up with the properties of a gas cloud.

"We know we are making an exciting and bold claim so we expect that other astronomers will look at it very carefully," said co-author Julia Berndtsson of Princeton University, New Jersey.

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

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