Evidence of a possible planet orbiting a star beyond the Milky Way galaxy has been spotted, NASA revealed.

NASA's Chandra X-ray Observatory made the discovery, allowing further studies into exoplanets that are located farther than ever been detected, a statement on the U.S. space agency's website said. This possible candidate exoplanet is situated in the Messier 51 (M51) spiral galaxy, which is also known as the Whirlpool Galaxy. It is around 28 million light-years away from Earth.

These exoplanets are called as such since they are positioned outside of our Solar System. Most of the discovered and candidate exoplanets have been found just within the Milky Way galaxy that are less than around 3,000 light-years from Earth. This exoplanet in M51 could be thousands of times farther than similar planets in the Milky Way.

NASA Chandra X-Ray Images: Detection of Exoplanet Done at X-ray Wavelengths

Harvard and Smithsonian Center for Astrophysics scientist Rosanne Di Stefano, who led the study published in Nature Astronomy, disclosed that the search for exoplanets are being done at X-ray wavelengths, which makes it possible to find planets farther from our galaxy.

The NASA statement said the findings were based on transits, which is observed when planet in front of a star obstructs some of the star's light and generates that drop in optical light. Astronomers using Earth-based or space-borne telescopes, such as those on NASA's Kepler and TESS missions, have also sought for such dips in optical light or electromagnetic radiation that humans can observe, which makes the discovery of thousands of exoplanets possible.

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Di Stefano told BBC News that her team's approach is the "only presently implementable method to discover planetary systems in other galaxies." She added it is a "unique method" that can find "planets around X-ray binaries at any distance from which we can measure a light curve."

Di Stefano's team scoured for optical light dips using the brightness of X-rays obtained from X-ray bright binaries, NASA further said. Such luminous systems characteristically encompass a neutron star or a black hole that draws in gas from a neighboring star. Material close to the neutron star or black hole would turn superheated, thereby emitting glows in X-rays.

Method Revealed in Spotting M51 Exoplanet

Since the area generating bright X-rays is small, a planet crossing in front of this region may obstruct most if not all of the X-rays, making the transit easier to detect as the X-rays can fully vanish from sight. This allows the detection of exoplanets at greater distances than present optical light transit analysis, which must spot small dips in light since the planet only obstructs a tiny fraction of the star.

Di Stefano's team utilized this approach in spotting the M51 exoplanet candidate in the binary system named M51-ULS-1. This binary system comprises a neutron star or black hole that is orbiting a nearby star having a mass of around 20 times our Sun. The X-ray transit the team discovered using Chandra went on for three hours, as the X-ray emission turned to zero.

Based on the study, the scientists approximate the exoplanet candidate to be around the size of Saturn, which is orbiting the neutron star or black hole around twice the distance of Saturn from the Sun.

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