Astronomers may have found first known planet in another galaxy

The exoplanet appears to orbit a massive star and a dead star in the Whirlpool galaxy

The spiral-shaped Whirlpool galaxy (left) may be home to the first planet spotted outside our own Milky Way.

S. Beckwith/STScI, the Hubble Heritage Team/STScI/AURA, NASA, ESA

Astronomers spotted what they believe may be the first known planet in another galaxy.

More than 4,800 planets have been discovered orbiting stars other than our sun. But until now, all of them have been inside our Milky Way galaxy. The potential new world orbits two stars in the Whirlpool galaxy. That galaxy is some 28 million light-years from Earth. (That’s more than 250 times as far as the Milky Way is wide.) Astronomers are calling the possible exoplanet M51-ULS-1b.

Confirming its existence would be a big deal. It would suggest that there are many other planets in other galaxies waiting to be discovered. Astronomers shared their find October 25 in Nature Astronomy.

“We probably always assumed there would be planets” in other galaxies, says Rosanne Di Stefano. She’s an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. It’s in Cambridge, Mass. But planets in other galaxies have been hard to find. Why? Distant stars in telescope images blur together too much to observe them one by one. That makes it difficult to scout for planetary systems around each one.

In 2018, Di Stefano and a colleague came up with a way to overcome this challenge. That colleague, Nia Imara, also is an astrophysicist. She works at the University of California, Santa Cruz. Their idea was to search for planets in star systems called X-ray binaries.

X-ray binaries usually consist of two objects. One is a massive star. The other is what remains after a second massive star has exploded. The stellar corpse is either a neutron star or a black hole. Both types of dead stars are extremely dense. As a result, they have a super strong gravitational pull.

In an X-ray binary, the dead star pulls material from the other star. This heats up the compact object so much that it emits bright X-rays. That radiation stands out even within a crowd of other stars. And so astronomers can spot X-ray binaries, even if they are in other galaxies.

If a planet orbits stars in an X-ray binary, it could transit — cross in front of — those stars from Earth’s perspective. For a short time, the planet would block the X-rays coming from that system. That lost signal would point to the planet’s existence.

Di Stefano’s team wondered if a telescope had ever seen such a thing.

To find out, the researchers looked at old data from NASA’s Chandra X-ray telescope. Those data included observations of three galaxies — the Whirlpool, Pinwheel and Sombrero galaxies. The researchers were looking for X-ray binaries that had briefly dimmed.

The search turned up only one clear planetlike signal. On September 20, 2012, something had blocked all X-rays from an X-ray binary for about three hours. This binary was a system in the Whirlpool galaxy known as M51-ULS-1.

Recalls Di Stefano says, “We said, ‘Wow. Could this be it?’”

A discovery or a mistake?

To be sure, the researchers ruled out other possible explanations for the dip in X-ray light. For instance, they made sure it couldn’t be due to gas clouds passing in front of the stars. And it couldn’t be changes in how much X-ray light the star system emitted. But they found no such alternative explanations.

To Di Stefano and colleagues, that sealed the deal.

A Saturn-sized planet likely orbits the X-ray binary. This planet would be tens of times farther from its stars than Earth is from the sun.

“To actually find something, it’s a beautiful thing,” Di Stefano says. “It’s a humbling experience.”

This finding “is quite intriguing and would be a great discovery,” adds Ignazio Pillitteri. He works at the Italian National Institute for Astrophysics. That’s in Palermo. But this astrophysicist isn’t convinced that the new exoplanet exists. To be sure, he would like to see the planet pass in front of its stars once more.

Matthew Bailes also has doubts. He’s an astrophysicist at the Swinburne University of Technology in Melbourne, Australia. If the planet is real, finding it relied on a lot of coincidences. For one thing, its orbit needed to be perfectly aligned for observers on Earth to see it cross in front of its stars. For another, it had to pass in front of its X-ray binary while the Chandra telescope was looking.

“Maybe we were lucky,” Di Stefano admits. But, she says, “I think it’s very likely that we were not.” Instead, she suspects that there are many planets in other galaxies to find. This one just happened to be the first that the telescope glimpsed. 

Di Stefano doesn’t expect to see this particular planet again in her lifetime. It could take decades for it to pass in front of its host stars again. “The real test,” she says, “is finding more planets.”

Lisa Grossman is the astronomy writer at Science News. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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