Astronomers may have found planet beyond Milky Way for first time

Researchers say they detected evidence of a planet orbiting a pair of stars — one of which most likely underwent a supernova explosion — 23 million light-years from Earth.

A box indicates the location of a possible exoplanet detected outside the Milky Way in a composite image from the Hubble Space Telescope and the Chandra X-ray Observatory.NASA / CXC / SAO / R. Di Stefano

The possible alien world was found in an X-ray binary system, a type of star system that produces and emits X-rays and is usually made up of a normal star and a collapsed star, such as a neutron star or a black hole. 

Typically, astronomers use what’s known as the “transit method” to look for planets. Transits occur when a planet orbits in front of its parent star, temporarily blocking part of it and causing an observable dip in the star’s light. Di Stefano and her colleagues applied the same basic idea, but instead of optical light, they monitored for changes in the brightness of X-rays from the binary system in the Whirlpool Galaxy.

Di Stefano said the region that produces bright X-rays is relatively small, making it possible to detect transits that block most or all of the X-ray emissions.

“It’s a very obvious signal,” she said.

Using data from NASA’s Chandra X-ray Observatory, Di Stefano and her colleagues observed that the transit lasted about three hours, and they were able to roughly gauge the object’s size because it completely blocked the X-ray source. They estimate that the possible planet is the size of Saturn and that it is much farther from its star than Earth is from the sun.

Bruce Macintosh, a professor of physics at Stanford University who wasn’t involved with the research, said the discovery is exciting because, if it is verified, it shows not only that planets are common throughout the cosmos, but that they can also exist in unlikely places.

“The awesome thing is that they found a planet orbiting around a neutron star that is part of a system that has been through a supernova explosion and had an interesting and complicated evolutionary history,” he said. “It’s exciting that a planet can survive having its star blow up.”

Confirming that there really is a planet in the X-ray binary system is likely to take time. The planet’s far-out orbit means it is likely to be around 70 years before astronomers could witness another transit.

“And because of the uncertainties about how long it takes to orbit, we wouldn’t know exactly when to look,” a co-author of the study, Nia Imara, an assistant professor at the University of California, Santa Cruz, said in a statement.

Macintosh said that the method of studying X-ray transits is “clever” but that it’s unlikely that it could be used to find hundreds of thousands of planetary candidates because it also relies on luck.

“You can only see transits when objects line up just right between you and the thing you’re looking at,” he said. “And you only see it when it passes in front of the target object for a few minutes or hours.”

Still, Di Stefano said, it’s gratifying that the new method of searching for extragalactic exoplanets, which she and her colleagues first theorized in 2018, has produced such an enticing result.

“We did not know whether we would find anything, and we were extremely lucky to have found something,” she said. “Now we hope other groups around the world study more data and make even more discoveries.”

Source: NBC News