Slow-moving stars at the Milky Way’s outskirts suggest our galaxy may be far lighter than previously believed, with profound implications for dark matter
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.
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.
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.
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.
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 —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.
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