| Last year the European Space Agency’s Gaia space telescope released its most precise measurements yet of the speeds and positions of nearly two billion stars in the Milky Way. The data suggested something puzzling. Stars toward the Milky Way’s outer rim (which is about 50,000 light-years from the galaxy’s center) seem to be orbiting far slower than similarly-situated stars in other galaxies. Stellar speeds within a galaxy can be used to estimate its total mass, and in this case, a study based on the surprisingly sluggish stars found the Milky Way coming up short: Our galaxy’s gas, dust, stars and dark matter weigh in at some 200 billion times the mass of our sun, several times less than the weight reported by many earlier assessments. The Milky Way, it seems, may be “missing” about a trillion suns’ worth of mass.What’s going on: Because the Milky Way’s visible material hasn’t disappeared (which astronomers estimate to be a mass of about 60 billion suns), one easy—and especially thought-provoking—way to explain this result is that far less dark matter is in our galaxy than previously believed. A less exciting but far more likely explanation, however, is that the analysis or even Gaia’s data are somehow flawed. Or, it may merely be that the anomalously slow stars are genuine, but stars farther out (and beyond Gaia’s high-precision measurements) reverse the trend and exhibit expected higher speeds—we just haven’t seen them yet.What the experts say: If confirmed, the result “would be revolutionary,” says Stacy McGaugh, an astronomer at Case Western Reserve University who wasn’t involved in any of the recent studies. |
Francesco Sylos Labini 