Astronomers have found a hint of what they believe could be the first planet discovered outside our galaxy.
So far, almost 5.000 exoplanets have been found - worlds that orbit our Sun, but they are located in the Milky Way galaxy.
A possible signal of a new planet located in the Messiah 51 galaxy was discovered by the NASA-Chandra telescope.
This planet is 28 light years away from the Milky Way galaxy.
This discovery is based on transits, where the passage of a planet in front of a star blocks some of its light, creating a characteristic diminution of brightness that telescopes can detect.
Such a technique has already been used by scientists to find thousands of exoplanets.
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But Dr Rosen di Stefano and her colleagues looked for changes and dips in brightness from an object known as an X-ray glow binary.
These objects contain a neutron star or black hole that is sucking in gas from a companion star in nearby orbit.
Material near a neutron star or black hole is superheated and radiates at the same wavelengths as X-rays.
Since the area where the X-rays are generated is small, a planet passing in front could block most or all of these rays.
And that's exactly what makes it easier to spot the transit.
Team members used this method to discover an exoplanet candidate in a binary system called M51-ULS-1.
Future searches for planets
This binary system contains a black hole or neutron star orbiting a companion with a mass 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, scientists estimate that the planet is a Saturn-sized candidate.
They also estimate that it orbits a neutron star or black hole at about twice the distance that Saturn is from the Sun.
"We are trying to create a completely new space for finding other worlds.
We are looking for planet candidates at X-ray wavelengths and this could help us strategize for searches in other galaxies," said Rozana di Stefano from the Center for Astrophysics in Cambridge, USA.
The researchers are aware that more data is needed to confirm their interpretation of the new discovery.
One explanation that astronomers have considered is that the dimming was caused by a cloud of gas and dust that passed in front of the X-ray source.
However, they believe that this is unlikely, because the basic characteristics of this event do not match the properties of the gas cloud.
"We know our claim is exciting and bold, so we expect it will be looked at carefully by other astronomers," said Julia Berndston of Princeton University in New Jersey.
"We have a strong argument, and this, by the way, is the process by which science works," she concluded.
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