Cat’s Eye Nebula: 3000 light years away, as seen by Hubble and Gaia NASA/ESA/HEIC/The Hubble Heritage Team/STScI/AURA (background image); ESA/Gaia/DPAC/UB/IEEC (blue points)

Gaia is on a mission. The space-based observatory just released its first map of the positions and distances of stars in our galaxy, and will chart a whopping billion stars before it is through.

Such an accurate map will help pin down the Milky Way’s layout and evolution. But beyond this main science goal, the tiny spacecraft is working on plenty of side gigs that are just as exciting.

Closer to home, Gaia will map thousands of objects in the solar system. These will mostly be asteroids in the main belt encircling the sun between the orbits of Mars and Jupiter. But the haul will also include several thousand near-Earth objects – comets and asteroids that have entered our neighbourhood and could potentially collide with Earth.


When Gaia spots an object we’ve never seen before, it will alert ground-based observatories. They will take a closer look, and assess whether the object is a threat.

Gaia will also shed light on planetary systems across the universe. It is expected to uncover thousands or even tens of thousands of exoplanets, planets that orbit stars other than our sun. But it will do so via astrometric method, a technique that has never been used before.

Planets more interesting than Earth

When a planet orbits a star, its gravity tugs the star to and fro, making it wobble in the sky. But that change is puny. To an alien observer about 16 light years from Earth, Jupiter would cause the sun to move roughly 1 milli-arcsecond on the sky. That’s more than a million times smaller than the full moon. With a sensitivity as low as 5 micro-arcseconds, Gaia is the only detector sensitive enough to spot this change in the star’s position.

Because the wobble increases the further the planet is from the star, this method is biased toward planets that orbit their stars at large distances, unlike the most popular planet detection methods used so far.

“That’s very exciting, of course, because in our own solar system it’s Jupiter that dominates the physics of planetary formation,” says David Hogg at New York University. “From a planetary science point of view, it’s most interesting to find the Jupiters out there – much more interesting than finding the Earth-like planets if you want to know how planetary systems form.”

Seeing the unseen

But Gaia has more tricks up its sleeve. Astronomers will also use the positions of visible stars to map invisible dark matter. The mysterious stuff makes up nearly 85 per cent of the matter in the universe, and exerts a gravitational pull on all stars, bending their paths as they traverse their home galaxy. Any deviations that can’t be explained by the visible matter will be the handiwork of dark matter.

“Our current paradigm says that dark matter should be lumpy on small scales,” says David Spergel at Princeton University. This paradigm is based on the theory that dark matter is composed of massive slow particles that mostly interact through the force of gravity. Such lethargic particles will hang around in clusters for a while. But if we find that dark matter is smooth rather than lumpy, astronomers might have to come up with alternative explanations.

Additionally, if you add up all those bends, you can build an overall picture of the dark matter throughout the galaxy. We think that the Milky Way – and all galaxies – are embedded within massive halos of dark matter, which extend far beyond the normal, visible, matter. But we’re not sure what shapes these halos take.

Different observations of gas and stars in the Milky Way have hinted at different shapes. “We think that dark matter halos are more football-shaped, and that’s why it’s interesting to measure them in three-dimensions because we think they come in different degrees of squishiness,” says Kathryn Johnston of Columbia University.

Finally, Gaia will help elucidate the universe on its largest, cosmological, scales. Previous observations tell us that the universe is expanding, but two separate teams recently measured this expansion rate and found two strikingly different numbers. The ensuing debate could point towards exciting new physics. One of those measurements hinges on having accurate distances to stars within our own galaxy – Gaia’s specialty. So Gaia will help improve that measurement’s certainty and perhaps resolve the disagreement.

Gaia will transform our understanding of the universe from the solar system out to the edges of the cosmos. “It’s an experiment that has a tremendous potential to break through a lot of different areas,” Spergel says.