Researchers show Milky Way’s disk formed upside-down in two stages

David Mirabal of the Instituto de Astrofísica de Canarias led research reported Oct. 19 showing the Milky Way’s disk assembled in an upside-down sequence. Ancient stars formed in a thick layer, and younger stars formed in progressively thinner layers as time passed. The work appeared in The Astrophysical Journal Letters and describes the assembly in two distinct stages.

The research identifies two separate phases. An early period of rapid settling built the thick disk, and a later period of slow gradual thinning built the thin disk. Before 10.8 billion years ago, the disk’s vertical thickness shrank from more than 1.26 kiloparsecs, or roughly 4,100 light-years, down to 0.5 kiloparsecs, or about 1,600 light-years, over two billion years. Once the break passed, the thinning slowed to a gradual pace, and the youngest stellar populations ended up at a scale height — the disk’s thickness above and below the galactic midplane — of roughly 0.1 kiloparsecs, about 400 light-years.

The disk’s thinning rate changed sharply about 10.8 billion years ago, a couple of billion years after the Big Bang, marking when the thin disk formed. The ancient stars of the thick disk originated in that puffy layer while the galaxy was turbulent in its early years. They were not lifted there later by gas clouds, spiral arms or the central bar. Older stars remained in their thick-disk orbits after the transition.

The researchers built on Gaia Data Release 3, which the European Space Agency released in June 2022 after charting more than a billion stars. Gaia is an ESA mission measuring stellar position, motion and brightness. The ChronoGal project produced stellar ages with accuracy better than 5 percent, and even for the oldest stars the margin of error comes close to 500 million years.

Researchers applied color-magnitude diagram fitting to complete samples from the solar neighborhood, building age-metallicity distributions and grouping stars by nearly identical age and metal content. For each quasi-mono-age-metallicity population, the team measured the group’s scale height above and below the galactic midplane. Accurate ages for large, representative samples had previously been difficult to obtain, especially for old stars, leaving astronomers unable to choose between competing formation scenarios.

One explanation holds that stars are born in the thin disk and are later pushed away from the plane by encounters with gas clouds, spiral arms or the central bar, a process called dynamical heating. The alternative says the gas itself settles into a thinner disk over time, so each successive generation of stars forms in an increasingly thin layer. The new ages favor the second picture: turbulent gas thinned as it calmed and settled. That fast change cannot be explained by slow gradual settling in a young galaxy that often collided with other galaxies.

David Mirabal, the lead author and a predoctoral researcher at the Instituto de Astrofísica de Canarias, said simulations had room for both models but no observational proof, so the result settles a classic problem. “This is the first clear observational evidence favoring one of the two models for the galaxy’s early stage.” Mirabal added that the breakthrough was possible thanks to the high precision achieved for older ages. “This result resolves a classic problem in astrophysics.”

The paper, A Two-stage Settling of the Milky Way Disk Revealed by Precise ChronoGal Ages, appeared in The Astrophysical Journal Letters with DOI 10.3847/2041-8213/aeb0a3. The arXiv preprint arXiv:2610.03164, submitted Oct. 2, 2026, puts the break between thinning regimes at 10.85 billion years ago, with uncertainty of roughly +0.13 and -0.12 billion years.

Co-authors come from the University of La Laguna, University of Granada, Instituto Carlos I de Física Teórica y Computacional, INAF, INFN, and the University of Victoria in Canada. Besides David Mirabal, co-authors include Carme Gallart, Emma Fernández-Alvar, Anna B. A. Queiroz, Guillem Aznar-Menargues, Chris B. A. Brook and Pedro A. Palici.

The European Research Council funds ChronoGal, led by the Instituto de Astrofísica de Canarias, and the analysis made use of the institute’s high-performance computing resources.

The Milky Way consists of a galactic bulge, a disk and a halo. Its disk divides into a thin disk and a thick disk, and these two layers differ in structure and in their stellar populations.

The thin disk contains young and middle-aged stars such as the Sun. It includes the spiral arms where the solar system lives, holds most of the galaxy’s gas and dust, and serves as the site where young stars form. Earlier generations of stars that have lived and died gave it higher metallicity.

By contrast, the thick disk is puffier and more diffuse than the thin disk. It holds ancient stars, shows almost no new star formation, and has much lower metallicity. It is sometimes called a stellar fossil record.

At present, the thin disk contains younger stars, most star formation, and most of the gas available for new stars. The thick disk holds older, metal-poor stars and almost no gas for new star formation. Long ago, the galaxy’s gas was more turbulent, and the Milky Way puffed up vertically as a single thick disk, with stars born orbiting in the thick disk. Around 10.8 billion years ago, a couple of billion years after the Big Bang, the thin disk formed even as older stars remained in their thick-disk orbits.

Because the age samples come from the solar neighborhood, researchers do not know whether the same two-stage history applies across the whole disk. The real test will come from independent analyses that use different age-dating methods. This question about the thick and thin disks tests how disk galaxies generally assemble. If the thick disk records a turbulent early phase, its stars hold information about the galaxy’s first few billion years.

Stellar ages accurate to better than 5 percent, drawn from complete local samples, let the team spot a break in the disk’s history. That precision may help transform Gaia data from a snapshot into a timeline. This study forms part of a collaboration that has produced multiple papers on stellar ages in thin and thick disk populations.