Astronomers believe to have found the first planet outside our galaxy

Astronomers have found convincing indications of what could be the first planet ever to be discovered outside the Milky Way Galaxy.

NASA has nearly confirmed 5,000 “exoplanets” orbiting stars beyond our Sun. All of these have been located within our galaxy thus far.

On Monday, NASA said in a press release that a possible Saturn-sized exoplanet was discovered in the “Whirlpool Galaxy” — a spiral galaxy formally known as Messier 51 (M51) — by the space agency’s Chandra X-ray observatory.

The located exoplanet candidate is some 28 million light-years away outside the Milky Way.

The new finding used a transit method technique, where the orbit of a planet in front of a star blocks some of the star’s light and yields a dip in brightness that telescopes can detect.

Spiral galaxy Messier 51 (M51), known as the “Whirlpool Galaxy,” with a box containing the exoplanet candidate.
Image credits: NASA/CXC/SAO/R. DiStefano, et al.; Optical: NASA/ESA/STScI/Grendler

This method is responsible for discovering over 75% of the exoplanets in our galaxy, so it’s not surprising to see scientists attempting to implement it on an extragalactic scale.

“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,” Rosanne Di Stefano, professor in astronomy at the Center for Astrophysics at the Harvard & Smithsonian in Cambridge, Massachusetts, who led the study, said in a statement.

Since the Chandra observatory is an X-ray telescope, Dr. Rosanne Di Stefano and colleagues searched for dips in the brightness of X-rays coming from a type of object known as ‘X-ray bright binary’.

These objects typically contain a neutron star — when a massive star runs out of fuel and collapses — or a black hole pulling in gas from another star nearby. This gas forms around the neutron star or black hole creating a hot, dense, circular disk that glows in X-rays.
Transit animation of exoplanet candidate around its host object — either a black hole or neutron star.
Video credits: NASA/CXC/A.Jubett

The circular disk radiating these X-rays is small, so a planet passing through it would be easy to spot, as it would block many X-rays.

However, researchers will have to wait sometime to confirm whether they have an actual extragalactic planet on their hands. Because of its large, elongated orbit, the planet candidate will not cross in front of its host object (i.e. a neutron star or black hole) for another 70 years. Meaning it could take decades to confirm the observation.

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

For now, researchers will continue searching the archives of both Chandra, which contains datasets on over 20 galaxies, and European Space Agency satellite XMM-Newton, for addition exoplanet candidates lurking in other galaxies.

“Extraordinary claims require extraordinary evidence” — Carl Sagan

Featured image credit: Mark Garlick

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