New Gaia-based study says the Milky Way’s disk began thick and flattened over billions of years

New measurements from the ChronoGal project at the Instituto de Astrofísica de Canarias suggest that the Milky Way’s disk began as a thick, turbulent layer and then flattened over roughly 3 billion years. The study, based on Gaia data, was published in The Astrophysical Journal Letters on 16 October 2026 and announced by the IAC on 19 October 2026.

What the study says about the young Milky Way

For decades, astronomers have debated whether the Milky Way’s thick disk formed from stars born high above the galactic plane or whether a thin disk was later stirred upward by internal processes. According to the new work, the evidence favors the first idea for the galaxy’s early history: the oldest stars in the thick disk appear to have been born in a puffy, unsettled layer rather than lifted there later.

The researchers say the disk’s thickness changed in two stages. In the early phase, the typical height of stars above the midplane dropped quickly, from more than 4,100 light-years to about 1,600 light-years in around 2 billion years. After a turning point around 10.85 billion years ago, the flattening slowed sharply but did not stop.

• Project: ChronoGal, led by the Instituto de Astrofísica de Canarias
• Data source: Gaia mission measurements
• Published: 16 October 2026
• IAC announcement: 19 October 2026
• Next major test: Gaia Data Release 4 on 2 December 2026

How Gaia data turned ages into a timeline

Gaia measures the positions, motions and brightness of huge numbers of stars, but it does not give each star an age directly. ChronoGal estimates ages statistically by comparing observed stars with synthetic stellar populations built from evolution models. The team used colour-magnitude diagrams and matched them against stars with known age and chemistry to recover when different groups formed.

According to the paper, that approach gives age precision better than 5 percent, or about 500 million years for stars older than 10 billion years. The authors say that level of detail matters because older studies often used chemistry as a substitute for age, which blended stars of very different generations and blurred the earliest changes in the disk.

The team analyzed two regions centered on the Sun and split the stars into several layers above and below the galactic plane. They also corrected for stars hidden by dust or too faint for Gaia to detect. From there, they measured how far each population spread vertically and compared that with age.

Why the result matters

The study argues against the idea that the Milky Way’s thick disk was mainly built by later heating of a thin disk. Heating by spiral arms, gas clouds and the galactic bar can raise stars over time, but the authors say that process does not easily explain a very rapid early drop in thickness in a young galaxy that was still interacting with smaller ones.

They note that the end of the fast-thinning phase lines up with the period around the last major collision with a dwarf galaxy known as Gaia-Sausage-Enceladus, about 10 billion years ago. In the authors’ view, the first few billion years of the Milky Way’s life were a turbulent, upside-down stage in which stars formed in a thick layer of gas before the disk settled into a thinner shape.

The study also fits with some galaxy simulations, including FIRE-2 runs, which predict a chaotic early period followed by a collapse into a rotating thin disk. Still, the authors limit their claim to the Sun’s neighborhood, since the work does not yet test whether the same timeline applies across the rest of the galaxy.

What remains uncertain

According to the paper, the slow settling phase could be explained by either continued cooling or some level of heating, so the later evolution is not fully settled. The authors also say the early fast phase rests on relatively few points, and the uncertainty on its slope is large. In addition, the analysis excludes some stars from the inner galaxy that migrated outward and would not follow the local pattern.

The next opportunity to test the result comes with Gaia Data Release 4, scheduled for 2 December 2026. That release should provide sharper distances and motions for more distant stars, allowing astronomers to see whether the bend in the disk’s evolution appears outside the solar neighborhood as well.

For ordinary readers, the practical takeaway is that the Milky Way likely did not start out looking like the neat, thin disk seen today. Instead, its early structure appears to have been messy and thick, with the galaxy spending billions of years settling into the shape it has now.

Source: Peter Finch