Astronomers have uncovered evidence of what could be the first planet identified outside of our galaxy. The candidate exoplanet is located in the spiral galaxy Messier 51 (M51), often known as the Whirlpool Galaxy.

The researchers used NASA's Chandra X-ray Observatory to detect X-ray dimming from a "X-ray binary," a system in which a Sun-like star orbits a neutron star or black hole.

The dimming is interpreted by the authors as a planet passing in front of the neutron star or black hole.

"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," lead Author of the study Rosane Di Stefano, of the Center for Astrophysics | Harvard & Smithsonian (CfA), said.

Exoplanets are worlds that exist outside of our solar system, and astronomers had only discovered exoplanets and potential exoplanets within the Milky Way up until now, almost all of which are fewer than 3,000 light-years from Earth.

The one in M51 is about 28 million light-years away from Earth, thousands of times farther than any alien worlds discovered in our galaxy.

In X-rays, material near a black hole or neutron star reaches unacceptably high temperatures and begins to glow. However, because the X-ray-generating region of space is so small, a planet passing in front of it might block much, if not all, of the X-rays, simplifying the transit detection procedure.

Because the candidate exoplanet can only physically block a small fraction of its host star's light, this would enable the detection of alien worlds far more distant than those typically detected via conventional optical light transit observations, which require the analysis of tiny variations in light.

The X-ray approach was used by Di Stefano and his colleagues to detect an exoplanet candidate in a binary system known as M51-ULS-1 in the M51 galaxy. A neutron star or a black hole orbits a companion star that is around 20 times the mass of the sun in the system.

The Chandra data-monitored X-ray transit lasted around three hours, during which time X-ray emissions decreased quickly to zero. As a result, the researchers suspect an exoplanet, around the size of Saturn, orbiting the black hole or neutron star at roughly twice the distance Saturn orbits our sun.

Although more evidence is needed to corroborate the findings, this is an extraordinary and historic find for modern astronomy, and it heralds the start of intergalactic exoplanet research.