The Milky Way’s disk was born thick and took 3 billion years to flatten

The Milky Way did not start as the thin, flat disk we see today. Precise ages for the stars around the Sun show that the galaxy’s first disk was a thick, churning layer of gas and young stars, and that it flattened fast before slowing to a crawl that still continues.

That answers a question astronomers have argued over for four decades: were the old stars of the Milky Way’s thick disk born high above the plane, or were they born flat and kicked upward later by spiral arms, gas clouds and the galactic bar? The new measurements point clearly to the first answer, at least for the galaxy’s youth.

The result comes from ChronoGal, a project at the Instituto de Astrofísica de Canarias (IAC) in Tenerife that turns data from the European Space Agency’s Gaia satellite into star ages. Its lead author, IAC pre-doctoral researcher David Mirabal, called it “the first clear observational evidence favouring one of the two models for the Galaxy’s early stage.”

The two stages are sharply different. In the first, the typical height of the disk’s stars above the midplane fell from more than 4,100 light-years to about 1,600 light-years in roughly 2 billion years. After a turning point about 10.85 billion years ago, the thinning slowed by a factor of about fifty, and the newest stars now form in a layer around 400 light-years thick.

How Gaia ages turned a 40-year argument into a measurement

Gaia has measured the positions, motions and brightness of more than a billion stars, but it does not stamp an age on any of them. ChronoGal gets ages statistically. The team builds a colour-magnitude diagram, a plot of how bright and how blue or red each star is, and fits it with synthetic stars of known age and chemistry from stellar evolution models. The best combination tells them how many stars formed at each age and metallicity.

The method reaches age precision better than 5%, which means an error of about 500 million years even for stars older than 10 billion years. That precision is the whole story. Earlier studies grouped stars by chemistry, as a stand-in for age, and the groups mixed stars of very different ages, which blurred exactly the early, fast changes this team was looking for.

The researchers worked in two cylinders centred on the Sun: one about 815 light-years in radius for stars on cool, flat orbits, and one about 2,450 light-years in radius for stars on hotter, more inclined orbits. They sliced both into eight layers up to about 3,260 light-years above and below the plane, corrected for every star lost to dust or too faint for Gaia, and grouped the results into populations that share an age and a metallicity.

For each population they then measured the scale height, a standard yardstick for how far above the plane its stars spread before thinning out. Plotting scale height against age gave two straight lines with a sharp bend between them. A model fitted to the data put the bend at 10.85 billion years ago, give or take about 130 million years.

Born puffy, not shaken loose

Two ideas competed. In the heating model, stars are always born in a thin, cold layer, and over billions of years encounters with gas clouds, spiral arms and the bar pump up their orbits, so old stars sit higher simply because they have been shaken longer. In the upside-down model, the young galaxy’s gas was itself turbulent and thick, so the first stars were born high, and each later generation formed in a calmer, thinner layer.

Heating is slow and steady. It cannot easily explain a disk that lost more than half its thickness in about 2 billion years, especially in a young galaxy that was still swallowing smaller galaxies, which should have kept it hot. The last big collision, with a dwarf galaxy known as Gaia-Sausage-Enceladus, happened about 10 billion years ago, roughly when the fast phase ended. The authors conclude that the whole turbulent, upside-down stage lasted about 3 billion years.

The comparison makes the scale easy to feel. The oldest populations in the study spread higher than the thick disk’s usual textbook scale height of about 2,900 light-years. The Sun, born about 4.6 billion years ago, belongs to the calm phase, when the settling rate had dropped to roughly 120 light-years per billion years. In the authors’ words, the puffed-up oldest stars are “a structural fossil of the early Galaxy’s assembly phase.”

The pattern also matches cosmological simulations. Models of Milky Way-like galaxies, including the FIRE-2 runs, predict a bursty, chaotic early era in which stars are born on disordered orbits, followed by a fast collapse into a rotating thin disk. A separate study this year dated the moment the Milky Way started to spin as a disk to about 12 billion years ago, with a wide margin, which fits the start of the fast thinning.

What the star ages do not settle

The measurement covers only the solar neighbourhood, a few thousand light-years around the Sun. The Milky Way is about 100,000 light-years across, and the inner and outer disk may have settled on different schedules. Stars that migrated from the inner galaxy, which are unusually rich in metals, were left out of the fit because they would not follow the local trend.

The fast early phase rests on few points. The uncertainty on the old-age slope is large: the settling rate in the fast phase could be about 45% lower or 75% higher than the best estimate. For the two oldest populations, the team could only set lower limits on the scale height, and some populations in the hotter sample had to be separated by hand, although the authors report the same conclusion when they divide the data uniformly.

Heating has not been ruled out either. For the calm phase, the gradual thinning fits both models equally well. Simulations suggest the two processes act together, with heating contributing 30% to 40% of the overall rise in scale height with age. And every age in the study depends on stellar evolution models, so a systematic error in those models would shift the whole timeline.

Common questions about the Milky Way’s thick and thin disks

What is the Milky Way’s thick disk?

It is a puffier, older layer of stars that overlaps the thin disk where the spiral arms and most young stars sit. Its stars are typically older than about 10 billion years, poorer in metals and on more tilted orbits. The new study suggests it formed first, from turbulent gas, rather than being heated up from the thin disk later.

How do astronomers measure the age of a star?

For single stars it is hard, because a star’s colour and brightness change slowly over billions of years. ChronoGal fits the colours and brightness of large samples of stars at once against stellar models, which gives ages to better than 5% for whole populations, about 500 million years for the oldest ones.

How thick is the Milky Way’s disk?

It depends on the stars’ age. The newest stars form in a layer whose scale height is about 400 light-years, while the oldest populations in this study reach more than 4,100 light-years. The galaxy itself is about 100,000 light-years across, so even the thick disk is thin for its width.

What does upside-down formation mean?

It means the disk formed from the top down: the first stars were born in a thick, turbulent layer of gas, and each later generation formed in a thinner, calmer one. The opposite idea is that every star was born thin and only drifted upward over time.

What comes next

The paper appeared in The Astrophysical Journal Letters on 16 October 2026, and the IAC announced the result on 19 October. ChronoGal is funded by a European Research Council Advanced Grant and has already applied the same Gaia method to the inner galaxy and to the oldest disk stars.

The next test arrives on 2 December 2026, when ESA publishes Gaia Data Release 4, covering 5.5 years of observations. Sharper distances and motions for more distant stars should let teams check whether the bend at 10.85 billion years ago shows up beyond the Sun’s neighbourhood, or whether the galaxy settled on a different clock elsewhere.

Reference: Mirabal et al., “A Two-stage Settling of the Milky Way Disk Revealed by Precise ChronoGal Ages,” The Astrophysical Journal Letters, 2026. DOI: 10.3847/2041-8213/aeb0a3

Tags: astronomy, Milky Way, galaxy formation, ChronoGal, Gaia, Instituto de Astrofísica de Canarias