There is a 73% chance that one or more vast clusters of stars lie undiscovered at the outer edges of the Milky Way, scientists recently reported, and identifying precisely how many are scattered throughout the galaxy can offer clues about how the Milky Way has grown and changed over billions of years.

These groupings of hundreds of thousands to millions of stars are known as globular clusters. For a study published Feb. 9 in Monthly Notices of the Royal Astronomical Society, researchers examined the orbits of all the known globular clusters and found that the configuration that researchers observe today should only rarely occur. 

“It’s more likely that there are some extra globular clusters out there,” said Jeremy Webb, an assistant professor of astronomy at the University of Toronto and first author of the paper, which found a reasonably good likelihood of there being two or even three undiscovered clusters at the edges of the galaxy.

Stars always form in a cluster of some kind, Webb noted. These can be small clusters of a few tens of neighboring stars, which are pulled apart very quickly by the galaxy's gravity, or larger clusters that persist for several billion years. Globular clusters are largest of all, and their own gravity has kept them bound together for nearly the entire existence of the Milky Way. 

“They give us a window into what the galaxy was like when it was first forming ten-plus billion years ago,” Webb said.

The Milky Way has 160 confirmed globular clusters found within about 145 kiloparsecs, or roughly 472,927 light-years, of the center of the galaxy. Because these clusters don’t have perfectly circular orbits, at certain points in any cluster’s orbit it will move closer to and farther from the center of the galaxy. When researchers peer at the edge of the Milky Way, they would expect to see the same number of globular clusters moving inward toward the center as there are traveling outward.

However, the European Space Agency’s Gaia satellite — which previously catalogued the locations and motions of all the known globular clusters in the Milky Way — has measured five of the six outermost clusters moving inward and only one moving outward.

“The question we asked ourselves is if this is a rare occurrence. Did it just happen to be that the globular clusters in our galaxy are in this slightly rare state," Webb asked, "or are we missing some?”

He and his colleagues used mathematical models to simulate how the known Milky Way globular clusters would move along their orbits at various points over the next 12 billion years. They estimated that the arrangement that researchers can observe today should only occur around 6% of the time if there are no additional globular clusters hidden at the edge of the galaxy.

The researchers also identified another promising sign of undiscovered globular clusters, which are packed more densely together in the inner regions of the Milky Way and become sparser farther out. 

“You expect it to trail off smoothly to zero,” Webb said. But beyond 50 kiloparsecs, “It kind of sharply drops all of a sudden, which was another indicator that maybe some were missing.”

Based on the orbits and density of the known globular clusters, Webb and his team estimated that there’s a 73% likelihood that at least one undiscovered cluster exists in the distant outskirts of the Milky Way. This estimate is based on the assumption that scientists have identified all the globular clusters within 50 kiloparsecs. But the disk-shaped center of the galaxy is hard to see through, Webb says, which means that there may be additional undiscovered clusters closer to home.

If scientists have accounted for all of the globular clusters within 150 kiloparsecs, Webb and his team estimate, then there is a 91% likelihood of there being at least one undiscovered cluster at the edges of the galaxy. The researchers estimate that the likelihood of there being two unidentified clusters is between 60% and 70%, and the figure is around 50% for three undiscovered clusters.

Figuring out how many globular clusters populate the Milky Way can give scientists clues about how many smaller clusters have formed over the galaxy’s history and how many smaller galaxies it has engulfed. This could ultimately reveal how much of the galaxy formed directly out of gases within the center of a dark matter halo and how much came from other, smaller galaxies that merged with the Milky Way, each bringing its own clump of dark matter to the mix.

Dark matter — which emits no light and can only be observed through its gravitational effects on visible matter — makes up most of the universe's material. Dark matter halos hold galaxies together and extend far beyond their centers.

Knowing how many smaller galaxies have merged with the Milky Way could allow scientists to determine the extent to which the dark matter in our galaxy is distributed in clumps versus smoothly, Webb says. 

“If we really know how our galaxy formed and evolved, it helps us put constraints on dark matter,” he said.

For future studies, Webb and his team plan to predict where the undiscovered globular clusters are most likely hiding. Upcoming surveys from the Vera C. Rubin Observatory in Chile and the European Space Agency’s Euclid space telescope may then be able find these mysterious clusters, the researchers concluded.

The article, “The likelihood of undiscovered globular clusters in the outskirts of the Milky Way,” published Feb. 9 in Monthly Notices of the Royal Astronomical Society, was authored by Jeremy J Webb and Raymond G Carlberg, University of Toronto.