The Milky Way's disk grew upside down

Most of the stars in the Milky Way live in one of two disks. The thin disk is where young stars form today, close to the plane of the galaxy. The thick disk is puffier, stretching farther above and below that plane, and it is home to much older stars. How those two disks formed has been argued about for years, because the answer depends on knowing how old the stars are — and ages are the hardest number in astronomy to pin down for the oldest stars.

A new study led by David Mirabal at the Instituto de Astrofísica de Canarias (IAC) in Tenerife used precise star ages from the ChronoGal project, tied to data from ESA's Gaia satellite, to measure how the disk's thickness changed over time. Gaia has measured the positions, motions, and brightness of more than a billion stars. ChronoGal supplies ages accurate to better than 5 percent — an error of around 500 million years even for stars older than 10 billion years.

The result is not a steady flattening. The younger a star is, the closer it sits to the plane, but the rate changes sharply. Around 10.8 billion years ago there is a turning point. Before that, the disk flattened very fast: in about 2 billion years its thickness fell from more than 4,100 light-years (1.26 kiloparsecs) to around 1,600 light-years (0.5 kiloparsecs). After the turning point, the thinning slowed to a crawl, easing down to a thickness of about a tenth of a kiloparsec for the youngest stars.

That pattern is hard to explain if stars were always born in a thin disk and later got shaken out of the plane by encounters with gas clouds, spiral arms, or the galactic bar. That kind of shaking would thicken the disk slowly and continuously, not in a fast early drop followed by a long slow slide. The data instead fit the model astronomers call "upside down": the disk did not start thin and get puffed up. It started thick, because the gas forming stars in the young galaxy was highly turbulent, and it settled thinner as that gas calmed down.

In that picture, the thick disk formed during the early turbulent stage, when everything was churned up, and the thin disk formed later, in a much calmer galaxy. The old stars up high are not exiles pushed away from the plane. They were born there, when the whole disk was still thick.

Mirabal calls it the first clear observational evidence favoring one of the two competing models for the galaxy's early stage — possible because ChronoGal's ages stayed precise even at the oldest end, exactly where earlier attempts ran out of resolution. The study, "A two-stage settling of the Milky Way disk revealed by precise ChronoGal ages," is published in The Astrophysical Journal Letters (2026), DOI 10.3847/2041-8213/aeb0a3. The IAC announced the result on October 19, 2026.

For anyone who pictures the Milky Way as a flat disk that simply got messier with age, the Gaia ages run the story the other way. The galaxy's first stars were born in a thick, restless layer, and the thin, orderly disk we live in now is the late, calm chapter.

Source: Instituto de Astrofísica de Canarias, October 19, 2026 — "Research by the IAC reveals that the Milky Way's disc formed in two very distinct stages"; Mirabal et al., The Astrophysical Journal Letters, 2026, DOI 10.3847/2041-8213/aeb0a3.