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Astronomers Spot Signs of Planets Outside the Milky Way

Researchers estimate that more than a trillion planets may lurk beyond our galaxy’s bounds

Researchers estimate more than a trillion planets could exist in the elliptical galaxy at the center of this image. (University of Oklahoma)
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Scientists have confirmed the existence of over 3,600 exoplanets—planets outside our solar system—and that number's steadily increasing. But all of those planets reside within our own Milky Way galaxy. Tiny and dim in comparison to stars, planets beyond our galaxy's boundaries are extremely difficult to spot. But as Elaina Zachos at National Geographic reports, astronomers have used a technique known as microlensing to detect signals from a galaxy 3.8 billion light years away that seem to suggest it is chock full of planets, ranging in size from our moon to Jupiter.

According to a press release, astronomer Xinyu Dai and postdoctoral researcher Eduardo Guerras used data from NASA’s Chandra X-ray Observatory to conduct their study. Gravitational microlensing is not a new concept; it was predicted by Einstein and confirmed in 1979. The idea is that massive objects can warp space-time so much that the light from an object behind it is also warped, appearing to curve around the closer "lens." This effect distorts or magnifies the light of the distant object, making it visible when it otherwise would be lost in the night sky. For example, lensing recently helped researchers spot the distant galaxy SPT0615-JD, which lies 13.3 billion light years away.

As Michelle Starr at Science Alert reports, Dai and Guerras used the same technique for their analysis in an attempt to view a quasar galaxy called RX J1131-1231, which lies around 6 billion light years away. Using another galaxy about 3.8 billion light years away as the lens, the Chandra Observatory collected four images of J1131’s light bending around the gravitational field of the lens galaxy. The researchers crunched the numbers at the Oklahoma University Supercomputing Center to create a model from the data. What they found were unusual energy shifts in the light from the quasar. The model that best explains those discrepancies is that unbound planets—or planets that don’t orbit a star—are floating between the stars in the lens galaxy. The research appears in The Astrophysical Journal Letters.

“This is an example of how powerful the techniques of analysis of extragalactic microlensing can be. This galaxy is located 3.8 billion light years away, and there is not the slightest chance of observing these planets directly, not even with the best telescope one can imagine in a science fiction scenario,” Guerras says in the press release. “However, we are able to study them, unveil their presence and even have an idea of their masses. This is very cool science.”

Study leader Dai tells Zachos that based on their calculations, there are about 2,000 of these planets for every star beyond our galaxy, which means the lens galaxy could be home to more than a trillion planets. But the analysis still needs confirmation. “We hope other teams publish independent analyses to confirm our findings,” Dai says. “I think this is a case where scientific discoveries can be triggered by the spark of ideas.”

This isn’t the first time astronomers have claimed they’ve found extragalactic planets using microlensing. As MIT Technology Review reported in 2009, Italian researchers investigating stars in the Andromeda galaxy found a star that had some variability in its light—a sign that a planet about six times the size of Jupiter might be orbiting it. That discovery, however, was never further verified.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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