Key facts
- A study in The Astrophysical Journal Letters finds the Milky Way's disk formed in two phases: a rapid early thinning ending about 10.8 billion years ago, then a much slower gradual thinning to the present.
- After that turning point, the disk's vertical scale height thinned gradually to about 0.1 kiloparsec for the youngest stellar populations.
- In light-years, the disk's thickness dropped from over 4,100 light-years to about 1,600 light-years during the rapid early phase, then to about 400 light-years for newly formed stars.
- The research supports an upside-down formation scenario in which the thick disk formed first when the galaxy's gas was highly turbulent, and the thin disk formed later as the gas calmed and settled.
Researchers led by David Mirabal at the Instituto de Astrofísica de Canarias found that the galaxy’s disk formed in two separate stages, with the disk thinning rapidly in the early period until about 10.8 billion years ago, and published the work in The Astrophysical Journal Letters in 2026.
The research describes a rapid early phase in which the disk thinned quickly, followed by a much slower gradual thinning that continues to the present. During that early stage, the disk grew thinner as its thickness dropped from above 4,100 light-years to roughly 1,600 light-years, a change that unfolded across roughly 2 billion years. Afterward the vertical scale height reached roughly 0.1 kiloparsec among the youngest stellar groups, and then fell to roughly 400 light-years for stars that formed most recently.s
The study backs an upside-down assembly in which the galaxy’s disk started thick because its star-forming gas was highly turbulent. Later, as the gas calmed and settled, a thinner disk took shape. The ancient stars of the thick disk came into existence while the whole disk was still puffy and thick, not because they were pushed upward afterward by gas clouds, spiral arms, or the central bar. Those older stars remained on their thick-disk paths as the gas settled into a thinner layer.s
Astronomers previously considered two explanations for the thick disk. One said stars formed in a thin disk and were later shaken into thicker orbits by encounters with gas clouds, spiral arms, or the central bar. The other said gas settled into progressively thinner layers, so each new generation formed in a thinner disk than the last. Telling them apart needed precise ages for individual stars, a measurement that has been very difficult to obtain, especially for the oldest stars.s
The breakthrough relied on stellar ages accurate to better than 5 per cent, with an error margin near 500 million years for the oldest stars. Those estimates come from ChronoGal, a project funded by the European Research Council and led by the Instituto de Astrofísica de Canarias. The authors say the distribution of disk stars of different ages and metallicities traces the galaxy’s dynamical evolution, and that the absence of precise ages for big, representative samples long blocked secure differentiation among scenarios. The precision provided the observational evidence needed to decide between the models.s
The researchers drew on Gaia Data Release 3 from June 2022 and fitted colour-magnitude diagrams, a method that uses stellar colour and brightness to infer age, to work out age-metallicity distributions and vertical profiles for stars in both disks. More than a billion stars have had their position, motion, and brightness measured by Gaia. The analysis ran on the Instituto de Astrofísica de Canarias’s high-performance computing resources.s
This is the first clear observational evidence favouring one of the two models for the Galaxy's early stage.
David Mirabal, Instituto de Astrofísica de Canarias • IAC
The collaboration includes scholars based at the University of La Laguna, the University of Granada, the Instituto Carlos I de Física Teórica y Computacional, INAF’s Observatorio Astronómico de Abruzzo, INFN, and the University of Victoria in Canada. Besides David Mirabal, the IAC and University of La Laguna co-authors are Carme Gallart, Emma Fernández-Alvar, Anna B. A. Queiroz, Guillem Aznar-Menargues, Chris B. A. Brook, and Pedro A. Palici. T. Ruiz-Lara is also a co-author.s
The galaxy is built from a bulge, a disk, and a halo, and its disk divides into thin and thick parts that host different stellar populations. The thin disk is home to the spiral arms that contain the Solar System, holds most of the galaxy’s gas and dust, forms young stars, and shows higher metallicity from earlier stellar generations. The thick disk today contains older, metal-poor stars and little gas remaining for new star formation.s
Back then the gas was highly turbulent, and the galaxy puffed upward until it resembled one thick disk, with stars born orbiting inside that puffy structure. Later the gas calmed, cooled, and contracted, and a thinner disk formed inside the older one around 10.8 billion years ago, not long after the Big Bang, as older stars stayed on their thick-disk paths. The study points to a sharp break close to 10.8 billion years ago, since the speed of vertical settling shifted markedly around that date.s
Earlier on the disk thinned quickly, but after that point the thinning slowed to a gradual pace, leaving ancient stars on thick-disk orbits apart from the remaining thin disk. The data do not support continuous slow thickening, even though that outcome would be expected if stars began in a thin disc and were later shifted away from the plane. That sequence produced the galaxy’s present layout once the early rapid settling ended and gradual thinning took over.s
David Mirabal made his remarks about resolving a classic problem in a press release, and said the result became possible because older stellar ages are now measured with high precision. He said, “This result resolves a classic problem in astrophysics,”s
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