Planet outside Milky Way galaxy: This unique discovery is baffling scientists

A new discovery made by NASA's Chandra X-ray Observatory is baffling astronomers.

Potentially representing what would be the first planet seen to transit a star outside of the Milky Way, scientists have found evidence for a possible planet candidate in the M51 ("Whirlpool") galaxy.

It is located about 28 million light-years away from Earth. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km).

The study, led by Rosanne Di Stefano from the Centre for Astrophysics, was published today in Nature Astronomy.

''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 Di Stefano.

Astronomers used the transit technique to detect a dimming of X-rays from an "X-ray binary" which they believe is a planet passing in front of the neutron star or black hole.

Stars burst to life within clouds of interstellar gas and dust scattered throughout galaxies.

Leftover material spinning around a new star then coalesces into planets, and circumplanetary disks surrounding some planets similarly yield moons.

More than 4,400 planets have been discovered outside our solar system, called exoplanets.

Scientists have discovered exoplanets using transits with optical light telescopes, which detect the range of light humans can see with their eyes and more.

This includes both ground-based telescopes and space-based ones like NASA's Kepler mission. 

These optical light transit detections require very high levels of sensitivity because the planet is much smaller than the star it passes in front of, and, therefore, only a tiny fraction of the light is blocked. 

NASA expects to pinpoint thousands more previously unknown worlds, perhaps hundreds of them Earth-sized or ''super-Earth'' sized which will not be any larger than or twice as big as our home planet.

Scientists are eager to learn more about how they are born as cold gas and dust consolidate in these disks surrounding new stars.

Those are believed the most likely to feature rocky surfaces or oceans and are thus considered the best candidates for life to evolve.