The discovery surprised scientists, in part because the structure might spell trouble for theories of dark matter, the mysterious, invisible substance that's thought to make up about 23 percent of the mass in the universe.
The finding is only the latest to question dark matter's existence—last week, for instance, astronomers announced that they'd failed to detect dark matter in the sun's neighborhood, even though the substance should be there, according to accepted theory.
In the new study, led by Marcel Pawlowski of the University of Bonn in Germany, astronomers reconstructed the locations of the Milky Way's known satellites using sources ranging from 20th-century photographic plates to recent images from the Sloan Digital Sky Survey.
The team found that the Milky Way's roughly 20 companions—including dwarf galaxies and blobs of stars known as globular clusters—are distributed in a tidy plane that orbits at a right angle to our galactic disk.
"This is completely contrary to what we expect from theory," said study co-author Pavel Kroupa, also of the University of Bonn.
"You should be able to look in any direction and still find some satellite galaxies." That's because current models of galaxy formation—which are based on dark matter's existence—predict that the Milky Way's companions originally came from many different directions and so should have settled into a more or less spherical distribution.
"The logical implication of this [discovery] is that there is no dark matter," Kroupa said.
Cosmology in "Shambles"?
According to the standard theory of galaxy formation, dark matter was the gravitational scaffold upon which normal matter coalesced to form galaxies in the early universe.
As larger galaxies such as the Milky Way formed, the theory goes, leftover material amassed into hundreds of smaller satellites spread evenly around their host galaxies.
To explain the odd arrangement of satellites around the Milky Way, the Bonn team proposes that our home galaxy collided with a galactic neighbor about 11 billion years ago, which corresponds with the age of the oldest known satellite dwarf galaxy.
According to this idea, the Milky Way stripped material from the other galaxy, and gravity gathered the debris to form dwarf galaxies and globular clusters, which have remained in a plane around the Milky Way ever since, study leader Pawlowski said.
The team asserts that this model deals a significant blow to dark matter, since it shows that galaxies can form without the theoretical substance.
"It means that we have to completely and utterly rethink cosmology," Kroupa said. "Cosmology is basically in a shambles now."
Dark Matter Still Viable
But other astronomers aren't quite ready to give up on dark matter.
"Although the alignment they find is intriguing, it is very premature to conclude that dark matter is in trouble," said Ken Rines, of Western Washington University in Bellingham.
"Galaxy formation is a very tricky business. We certainly don't understand the details, and these details may ... explain the alignment that they find," Rines said.
"Although dark matter may eventually be proven wrong, the alignment of dwarf galaxies is more likely a puzzle than a fatal flaw."
Sukanya Chakrabarti, an astrophysicist at Florida Atlantic University, is also skeptical that the new study rewrites galaxy formation.
The study team's galaxy-collision scenario can explain the positions of the satellite dwarf galaxies, she concedes.
But that model doesn't explain why the satellites act as if they have more mass than can be explained by their visible matter alone—one of the main reasons scientists think dark matter exists.
Any alternative to dark matter "must not only reproduce where the stuff is but what its mass is as well," Chakrabarti said.
Study author Kroupa counters that "this is not a problem, because it has already been demonstrated many times with other data that other theories of gravity—such as MOND—describe galaxies excellently, including the satellite galaxies."
Short for Modified Newtonian dynamics, MOND is a tweaked version of Newton's theory of gravity, which proponents say can explain the observed motions of stars and galaxies without resorting to dark matter.
Still, Chakrabarti said, it's also not surprising that current dark matter simulations can't explain the Milky Way satellites' unusual orientation.
While the simulations do a good job of modeling the evolution of large-scale structure in the universe, they're less reliable when it comes to modeling the scale of individual galaxies, which involves interactions between many more variables, she said.
"If you neglect these things and you're trying to do a very detailed analysis—like where all the satellite galaxies are distributed—you're going to come up with some discrepancies," Chakrabarti said.
"The current simulations of galaxy formation are incomplete," she continued, "but that doesn't imply that dark matter isn't a viable notion."
The Milky Way's satellite "structure" will be detailed in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.