The Milky Way may be missing a trillion suns' worth of mass

Several independent groups have reported oddly sluggish orbits of stars along the Milky Way’s outer rim. One possible explanation for this is that our galaxy is extraordinarily deficient in dark matter, or another is that our core conceptions about dark matter are somehow deeply flawed.

By Adam Mann for Scientific American

There’s something strange going on with the Milky Way. Recent measurements suggest that stars at the outskirts of our galaxy are misbehaving. They’re traveling far slower than similarly situated stars in other galaxies. One possible explanation for the Milky Way’s stellar slowpokes is that our galaxy is extraordinarily deficient in dark matter, the invisible substance thought to serve as gravitational scaffolding for cosmic structures. Another is that our core conceptions about dark matter—such as how much of it exists in the universe—are somehow deeply flawed.

This head-scratcher stems from the European Space Agency’s Gaia satellite, which provides unparalleled information on the speeds and positions of nearly two billion stars in the Milky Way. Last year the Gaia team released the space-based telescope’s most precise measurements yet, spurring astronomers to refresh their galaxy-spanning assessments of stellar behavior. Several independent groups have now reported the oddly sluggish orbits of stars along the Milky Way’s outer rim, the peripheral edge of our galaxy’s luminous whorl.

Stellar speeds offer a way to weigh a galaxy; the gravitational force each particular star feels depends on the galaxy’s total mass. A Gaia-derived study released on September 27 in the journal Astronomy & Astrophysics pegged the combined mass of our galaxy’s gas, dust, stars and dark matter at some 200 billion times that of our sun—hefty for you and me but on the order of five times less than that found in several other earlier assessments. Because the Milky Way’s visible material hasn’t disappeared, one easy—and especially thought-provoking—way to explain this result is that far less dark matter is floating around than previously believed.

Then again, weighing a galaxy is a notoriously tricky business, so it’s possible that errors lurk in Gaia’s data or the new analyses that create the illusion of the Milky Way as anomalously trim. But the fact that multiple teams have seen the same result gives more substance to the findings. If true, they could force a rethink of fundamental physics and prompt a reexamination of all other galaxies in the universe.

“Let me put it this way,” says Stacy McGaugh, an astronomer at Case Western Reserve University, who wasn’t involved in any of the recent studies. “If it worked out that way, it would be revolutionary.”

In the 1970s astronomer Vera Rubin and her colleagues began measuring stellar motions in other galaxies. Stars around a galaxy’s periphery were expected to orbit at a more leisurely pace than those closer in, much like how Neptune meanders around our sun every 165 years while Mercury zips about in 88 days. Yet, strangely, Rubin and her associates found that outlying stars were traveling at roughly the same rate as their more central siblings, suggesting that an enormous reservoir of hidden material in and around each galaxy was gravitationally tugging on the far-out stars to boost their speeds. This invisible stuff, already then called dark matter, was surmised to form immense halos surrounding galaxies, outweighing the visible material by a factor of 10 for large galaxies and as much as 100-fold for dwarf galaxies.

Measuring how everything in our galaxy moves while stuck inside of it is not the easiest task. So astronomers have tended to assume that stars in the Milky Way behave much like those seen in other galaxies. The sun, located roughly 26,000 light-years from the galactic center, orbits around it at about 500,000 miles per hour (800,000 kilometers per hour), and most observations of other stars within and beyond the Milky Way have supported the idea that stellar speeds farther out should be broadly consistent with that of our home star.

The Gaia satellite, which was launched in 2013, offers the best-yet test of this simple notion via the spacecraft’s extraordinarily precise measurements of the three-dimensional positions and motions of stars in the Milky Way. But this testing has been a gradual process because the precision of Gaia’s reckoning improves in lockstep with how long it observes its stellar sample. Using Gaia, theoretical physicist Francesco Sylos Labini of the Enrico Fermi Study and Research Center in Italy and his associates saw subtle hints of a decline in the Milky Way’s stellar speeds a few years ago. Those hints became much more obvious in Gaia’s most recent data release, from 2022, which pegs stellar motions with twice the precision of a previous offering from 2018. Such improvements allow astronomers to plot the paths of stars with greater accuracy and out to much farther distances than before.

This year alone, four different papers have revealed a precipitous decline in the speeds of stars out to 100,000 light-years from the Milky Way’s center. The recent Astronomy & Astrophysics study refers to this falloff as “Keplerian,” meaning it is like that seen in the planets in our solar system, whose motions were first accurately described by 17th-century German astronomer Johannes Kepler.

Such a finding flies in the face of all expectations. Minus a few minor deviations, plots of stellar orbits in other galaxies consistently show stars from center to rim all whirling with similar speed, as if held in dark matter’s gravitational grip. “But for the moment—and this is what is very interesting—we do not find any other galaxies showing this Keplerian decline,” says François Hammer of the Paris Observatory, a co-author of the recent Astronomy & Astrophysics study.

In a broad sense, the idea that the Milky Way is unique among all galaxies contradicts a basic tenet of cosmology, which holds that there’s nothing special about any particular place in the universe. The findings create more specific headaches because of the extrapolated lower mass estimate of 200 billion suns for our galaxy. Astronomers are quite confident in their measurements for the visible material in the Milky Way, which amount to a mass of circa 60 billion suns. If both figures are correct, this implies that the dark-to-ordinary matter ratio is just 2.3 to 1—far less than the 10:1 ratio found in galaxies of similar size.

Given that the perception of a downsized Milky Way emerges from several independent analyses, some researchers believe that while the decline may be genuine, it’s not representative of our galaxy as a whole. Stars even farther out and currently beyond the limits of Gaia’s high-precision scrutiny may well display a corresponding rise in speeds to offset the anomalous dip. “I’d be very surprised if it just keeps going because then there’ll be a lot of things that break all at once,” says astrophysicist Lina Necib of the Massachusetts Institute of Technology, a co-author of one of the other papers on the decline in stellar speeds, which was posted on the preprint server arXiv.org.

Her idea is backed by multiple lines of evidence. The Large Magellanic Cloud, which sits around 160,000 light-years from the galactic center, is a satellite galaxy that orbits our own at more than 650,000 mph (one million kilometers per hour)—a value consistent with standard dark matter models. Another line of evidence comes from stellar streams—remnants of small galaxies and star clusters that got too close to the Milky Way and were shredded by its gravity. These stellar streams arc out to great distances and provide estimates of our galaxy’s mass that line up with the weightier approximations.

There’s also the possibility that these different teams are inadvertently misinterpreting their data in some way. At the University of Pennsylvania, astronomer Robyn Sanderson makes simulated Milky Ways on a computer and then imagines what sorts of maps a virtual Gaia satellite would see if placed inside them. Any such plot requires certain assumptions that affect its results, she says, such as the overall shape of the galaxy’s distribution of dark matter. “My group has looked at how those overly simplistic assumptions—which everybody knows are overly simplistic—lead to a strange result where the model still describes the data but doesn’t necessarily correspond with the realities of the underlying system,” she says.

Sanderson, who wasn’t involved in any of the papers, is skeptical of drawing firm conclusions from them. She points out that while Gaia provides unrivaled 3-D information, the uncertainties on its stellar-speed measurements grow the farther out in the galaxy it looks.

Future data from facilities such as the Vera C. Rubin Observatory (originally called the Large Synoptic Survey Telescope and renamed in 2019) will hopefully be able to find stars in the outer parts of the Milky Way that can help settle the debate. Gaia’s next release, expected at the end of 2025, could also provide more accurate information. Hammer is eager to more closely examine other galaxies and see if their stellar speeds might also show declines similar to the Milky Way’s.

For McGaugh, the episode represents part of a normal, healthy churn expected from any mature research community. “It’s going to take a while to settle out, but I think we’ll learn things in the process,” he says. Necib agrees and says she finds the current debate more exciting than troubling. “Yeah, it’s weird,” she says, “which honestly makes for cool science. I love when things are weird.”

(Adam Mann is a journalist specializing in astronomy and physics. His work has appeared in National Geographic, the Wall Street Journal, Wired, and elsewhere.)