Objects hanging in the vacuum of the cosmos orbiting a star have been detected throughout the galaxy, these exoplanets have been a source of enhancing our understanding of the conditions beyond our solar system. Astronomers have now gone a step further and detected the first planet not just beyond the solar system but beyond the entire stretch of the Milky Way Galaxy.

This is the first time that signs of a planet transiting a star beyond the Milky Way galaxy has been detected. The unique discovery was led by Nasa’s Chandra X-ray Observatory that has peaked into the spiral galaxy Messier 51 (M51), also called the Whirlpool Galaxy.

Astronomers have over the years detected over 4,000 such exoplanets, some Earth-like, some hot Jupiters and others decaying due to astronomical events in their region. Almost all of them are within the realm of the Milky Way, less than about 3,000 light-years from Earth. But the exoplanet in M51 would be about 28 million light-years away, meaning it would be thousands of times farther away than those in the Milky Way.

“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 Center for Astrophysics, who led the study. The astronomers searched for dips in the brightness of X-rays received from X-ray bright binary systems that typically contain a neutron star or black hole pulling in gas from a closely orbiting companion star.

To locate exoplanets, astronomers study the light emerging from the star and whenever a plant passes through it, it blocks the light from the star producing a characteristic dip. Astronomers using both ground-based and space-based telescopes have searched for dips in optical light, electromagnetic radiation humans can see, enabling the discovery of thousands of planets.

To detect the newest candidate, astronomers studied X-rays. Since 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 because the X-rays can completely disappear. The team used it to detect exoplanet candidates in a binary system called M51-ULS-1, located in M51.

This binary system contains a black hole orbiting a companion star with a mass about 20 times that of the Sun. Nasa said that The X-ray transit they found using Chandra data lasted about three hours, during which the X-ray emission decreased to zero. Based on this and other information, the researchers estimate the exoplanet candidate in M51-ULS-1 would be roughly the size of Saturn.

“Unfortunately, to confirm that we’re seeing a planet we would likely have to wait decades to see another transit. And because of the uncertainties about how long it takes to orbit, we wouldn’t know exactly when to look,” co-author Nia Imara of the University of California said in a statement.

Astronomers speculate that if a planet actually exists in this massive system it likely had a tumultuous history and violent past since it would have had to survive a supernova explosion that created the neutron star or black hole. They further speculate that at some point the companion star could also explode as a supernova and blast the planet once again with extremely high levels of radiation.