Photograph by Babak Tafreshi, National Geographic Creative
Scientists have long thought that exoplanets—planets beyond the solar system—were restricted to the confines of our Milky Way. After all, our galaxy is a warped disc about a hundred thousand light-years across and a thousand light-years thick, so it's incredibly difficult to see beyond that. But now, a new study is saying there could be extragalactic exoplanets.
The study, published February 2 in The Astrophysical Journal Letters, gives the first evidence that more than a trillion exoplanets could exist beyond the Milky Way.
Beyond Our Galaxy
Using information from NASA's Chandra X-ray Observatory and a planet detection technique called microlensing to study a distant quasar galaxy , scientists at the University of Oklahoma found evidence that there are approximately 2,000 extragalactic planets for every one star beyond the Milky Way. Some of these exoplanets are as (relatively) small as the Moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars.
"We can estimate that the number of planets in this [faraway] galaxy is more than a trillion," says Xinyu Dai, the astronomy and astrophysics professor who led the study.
A microlensing image highlights possible exoplanets beyond the Milky Way.
Courtesy of the University of Oklahoma
Microlensing works like magnification, says co-author Eduardo Guerras. It's a highly nuanced process that looks at frequencies emitted by moving celestial objects, meant to observe how they distort and magnify light that comes in from the objects near them. This light then illuminates things that aren't otherwise visible.
"This microlensing is amplifying something that is very small and changing colors, which makes no sense," Guerras says, "or it's amplifying a small region of a bigger object and that object has different colors."
Since these objects are so distant—the extragalactic bodies are some 3.8 billion years away—microlensing is the only way to get a sense of their shape. The researchers know they're looking at planets because of the speed at which they're moving.
"You can have this effect with stars, but it would be much, much less likely. It would be way less frequent," Guerras says. "If you have only one planet, the chances of observing it twice is astronomically small."
In Search of Exoplanets
Considering scale, detecting exoplanets can be tricky. Directly viewing exoplanets within the Milky Way is nearly impossible, so astrophysicists have to sift through data and use other detection techniques that give way to evidence of planet signatures. Normally, it takes multiple methods to confirm if there is actually an exoplanet out there, and in some cases, detections have turned out to be false positives.
"These stars are really far away. There's no way you can observe them by any [traditional] means," Guerras says.
The researchers are hoping that with the publication of their study, other scientists will pick up the data and develop another technique to verify whether or not these extragalactic planets exist.
"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."
Extra on Exoplanets
Exoplanets have been discovered in our Milky Way galaxy in the past. In fact, 5,287 planets have been confirmed and thousands more could still be out there. Previous efforts have been databased and archived.
In our galaxy, there's about one planet around every star, which means that there could be up to a trillion planets in the Milky Way. Many of these exoplanets could be Earth-size.
Dai says the study opens up the new field of studying starless planets beyond our galaxy, and could help us compare free extragalactic exoplanets with their intragalactic counterparts.
A previous version of this story misspelled Eduardo Guerras' name. The story has been updated.
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