SEEING THE UNSEEN

Stars aren’t the only things Gaia can detect. It can also help find dark matter, a mysterious substance that appears to make up the bulk of the Universe. “We have no idea what dark matter is,” says Gerry Gilmore, of the University of Cambridge, UK. But even if it’s invisible, Gaia can help determine where it is by measuring the effect of its gravity on things we can see.

One of these is the Sun. Astronomers have long known that the Sun (and the Earth) orbit the galactic centre once every 230 million years, moving at a velocity of about

230 kilometres per second. But we’re not going in a straight line. Rather, the galaxy’s gravity is bending our movement, keeping us from wandering off into the intergalactic void. And now, Gaia has allowed us measure that rate of deflection to astonishing precision.

The number? About

7.3 km/second/million years, Gilmore and others reported recently in Astronomy and Astrophysics.

That sounds like a lot, but a million years is a long time. Reduced to human timescales it means the Sun’s motion around the galaxy is being bent by gravity by a whopping 7.3 mm/year.

That’s about the rate at which tectonic plates move here on

Earth. But it’s more than can be accounted for based on visible matter alone. “The immediate conclusion,” Gilmore says, “is that roughly half the weight interior to the

Sun is dark matter.”

Other researchers are using Gaia data to try to find out whether dark matter is broadly dispersed or exists in clumps.

In a 2019 paper in

The Astrophysical Journal, a team led by Ana Bonaca, of the Harvard-smithsonian Center for Astrophysics, took a detailed look at a star stream called GD-1, far out in the Milky Way’s halo. What they found was odd. “We see stars pulled out of this stream,” Bonaca says. Best guess? Half a billion years ago, a ball of dark matter with a mass of about five million Suns ploughed through it, scattering stars like confetti in its gravitational wake.