About 28 million years ago in the distant Whirlpool Galaxy, a blue supergiant star was having a downright miserable time.
The young, enormous star was stuck in a gravitational dance with a greedy partner—perhaps a black hole or a dense neutron star—whose gravity was so intense, it was feeding like a vampire on the star’s exterior. As the star’s plasmas were ripped away, they glowed in x-rays a million times brighter than our sun.
Then something passed between this far-flung source of x-rays and our solar system, blocking it from our perspective for several hours. Because of how long light takes to travel such incredible distances, x-ray telescopes orbiting Earth didn’t pick up the dip in the signal until 2012. Now a team of scientists is making the case that the mysterious x-ray blocker could have been a planet—among the most distant and extreme ever found.
In a study published today in the journal Nature Astronomy, researchers led by astrophysicist Rosanne Di Stefano argue that M51-ULS-1, the x-ray binary system within the Whirlpool Galaxy, may host a Saturn-size planet that orbits as far from the binary as Uranus does from our sun.
If this planet really exists, M51-ULS-1 would mark the first pinpointed star system in another galaxy that has an “extroplanet,” or a planet found outside of our home galaxy, the Milky Way.
“The fact that this candidate—and we should refer to it as a candidate—is in another galaxy blows me away,” says Di Stefano, a researcher at the Harvard-Smithsonian Center for Astrophysics. “I just feel thrilled about that element of it, in the sense of being almost humbled by it.”
The suspected planet within M51-ULS-1 is still a candidate because it hasn’t been confirmed, which would require astronomers to see multiple periodic dips in the x-rays—a clear sign of a planet orbiting the light source. However, the object’s orbit is expected to take decades to complete, which means it may take centuries to see multiple additional dips.
“It’s kind of like watching the first pitch of a baseball game. … You’ve learned something, but you don’t know the outcome yet,” says Chris Burke, an exoplanet researcher at the Massachusetts Institute of Technology who wasn’t involved with the study.
Still, the technique that uncovered this signal provides a new way to hunt for planets in distant galaxies. The study also suggests that planets could survive in more extreme star systems than previously thought. “It potentially is opening up a new parameter space for understanding planet formation,” Burke says.
Worlds beyond the Milky Way
The main methods that astronomers use to find exoplanets within the Milky Way involve observing the stars that the planets orbit, but those techniques are much less effective when they’re applied to other galaxies. “If something is a thousand times more distant, you get a million times fewer photons,” Di Stefano says. “It’s a big challenge.”
Until now, astronomers hunting for planets in galaxies other than the Milky Way have relied on gravitational lensing, a phenomenon where large objects such as stars warp the space-time around them enough to bend incoming light. If a star happens to pass in between Earth and a more distant light source, the star can temporarily magnify that distant source from our point of view, resulting in a flash of light called a microlensing event.
If a star has planets orbiting it, those worlds affect the shape of that star’s gravitational lens, just as adding a tiny blob of glass to a camera lens would subtly warp the pictures. Astronomers can detect these variations during a microlensing event and use them to infer the presence of planets around a microlensing star.
So far, 118 planets within the Milky Way have been found this way—as well as a candidate detection from outside our galaxy. In 2004 researchers looking at the Andromeda Galaxy announced that they had picked up an unusual microlensing signal, which a follow-up study in 2009 suggested could have come from a star with a planet around it.
However, this method provides very little detail about the stars or the planets orbiting them, especially at large distances. The Andromeda signal played out within an individual pixel in a telescope's camera sensor.
In 2018 Di Stefano and Harvard postdoctoral fellow Nia Imara, now at the University of California, Santa Cruz, proposed another approach for planet-hunting outside the Milky Way: looking within star systems called x-ray binaries.
An x-ray binary forms when a close-knit pair of stars dances around each other, and then one of the stars dies and collapses into a black hole or an extremely dense stellar corpse known as a neutron star. The immense gravity of the collapsed object rips material off of its companion star with such ferocity that the system glows with x-rays.
If a planet could survive in this chaotic environment, it’s possible that its orbit would happen to pass between Earth and the x-ray source, revealing the world’s presence.
Worlds in extreme environments
In the summer of 2018 Di Stefano, Imara, and their colleagues decided to trawl archival data collected by NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton telescope to hunt for fluctuations in the signals of x-ray binaries. They soon found the candidate signal from M51-ULS-1.
The researchers then checked to see whether something other than a planet could explain M51-ULS-1’s dimming, since x-ray binaries can fluctuate in brightness. So far, those alternate explanations haven’t held up.
In the 2012 signal, x-rays of all energies dimmed to practically zero—strongly implying that a solid, opaque object was blocking them from view. If the obscuring object had been a dust cloud, researchers expect it would have let at least some x-rays through.
If the blocking object were a star, it would act as a gravitational lens, which would appear to make the binary brighter during the transit, not dimmer as observed. And in all likelihood, M51-ULS-1 is too young to host a “brown dwarf”—an object bigger than a gas giant planet but smaller than a star—of the right size to explain the observations.
If a planet does exist in M51-ULS-1, it has managed to survive in a very violent, very young system. “That’s a crazy system to try to form a planet in, because there’s so much activity,” Burke says.
More planet detections around x-ray binaries could help reveal just how easily star systems can birth planets at all. Di Stefano, for one, is excited to see researchers apply her team’s method to more archival x-ray data, including for x-ray binaries within the Milky Way.
“It opens up a very wide field,” she says. “My hope would be that people take off with this.”