The Gaia Paradigm

ESA

No matter where on Earth you stand, if you have a view of the night sky, and if it is dark enough, you can see the Milky Way. The Milky Way is our home, and its faint clouds of light and shadow have inspired human cultures across the globe. And yet, our view of the Milky Way is limited by our perspective. In many ways, we have learned more from other galaxies than from our own. But when the Gaia spacecraft launched in 2013, all of that changed.

It is difficult to map the galaxy you live in. Nebulae and star clusters hide much of our galaxy from view. It’s rather like trying to map the size and shape of New York City while standing in the center of Times Square. It was only in 1918 that Harlow Shapley was able to determine the Sun was not at our galaxy’s center, and well into the 1920s, astronomers debated whether the Milky Way was an island universe containing all creation.

ESA/Gaia/DPAC, Stefan Payne-Wardenaar

We’ve learned a great deal since then, but the Gaia spacecraft was designed to take our understanding of the Milky Way to a new level. Its mission was to create a map of our galaxy in unprecedented detail. It precisely mapped the positions, distances, motions and spectra of more than two billion stars and other objects. From this, it was found that the Milky Way is not a simple galaxy in a humble corner of the cosmos. Its stars tell a history of turbulent change, driven by past galactic collisions and mergers. There are arched trails of stars that are the remnants of smaller galaxies the Milky Way has consumed, and stars that have been flung away at such great speed that they will eventually escape our galaxy to drift through the intergalactic abyss.

The Gaia data also revealed several surprises. For example, the Milky Way is not a flat spiral disk like many other galaxies; its outer edge has a warped shape, which wobbles as the galaxy rotates. This dynamic behavior is likely caused by interactions with other galaxies. Gaia also found that our galaxy is not dominated by two prominent spiral arms. Instead, the Milky Way is filled with a delicate flower of fainter arms. It is also a barred spiral galaxy with a central bulge that is more spheroidal than spherical. And this is just the first detailed view of our home. The complete set of observations will be available through two more upcoming data releases, which will give us an even more detailed mapping.

Gaia’s mission is now over. Yesterday, on March 27, 2025, the ESA’s European Space Operations Centre deactivated its subsystems and sent the spacecraft into a retirement orbit. All that remains is the data it gathered for more than a decade and the stories that data can tell us.