A Stubborn Question Answered: New Research Reveals How the Milky Way's Disk Formed
The Milky Way Galaxy is one of the most studied objects in all of astrophysics, yet some of its most fundamental properties have long resisted explanation. Home to an estimated 200–400 billion stars arranged in a magnificent spiral pattern, our galaxy is far more structurally complex than it appears from within. Beyond the sweeping spiral arms visible in artist renderings, the Milky Way is physically divided into three major components: a central galactic bulge, an expansive galactic halo, and a flattened galactic disk — the last of which has been the subject of decades of intense debate among astronomers. Now, a landmark new study has finally provided clear observational evidence resolving one of the field's most stubborn questions: how, exactly, did the Milky Way's disk take the shape we see today?
Two Disks Within One Galaxy
The galactic disk itself is not a single, uniform structure. Astronomers have long recognized that it is further divided into two distinct layers: the thin disk and the thick disk. These are not merely structural curiosities — they differ profoundly in their stellar populations, chemical compositions, ages, and dynamical properties, each telling a different chapter of our galaxy's biography.
The thin disk is the more familiar of the two. It contains young and middle-aged stars, including our own Sun, which resides roughly 26,000 light-years from the galactic center within the thin disk. This region hosts the spiral arms, harbors the vast majority of the Milky Way's interstellar gas and dust, and serves as the primary site of ongoing star formation. The thin disk also exhibits relatively high metallicity — meaning its stars contain a rich abundance of elements heavier than hydrogen and helium — a direct consequence of countless generations of stellar birth, nuclear fusion, and explosive death that have progressively enriched the interstellar medium over billions of years.
The thick disk, by contrast, is a more diffuse, puffed-up structure extending further above and below the galactic plane. It is populated almost exclusively by ancient, metal-poor stars — stellar relics from the galaxy's earliest epochs — and hosts virtually no active star formation today. With its low metallicity and population of old stars, the thick disk is sometimes described by astronomers as a kind of stellar fossil record: a preserved snapshot of the conditions under which the earliest generations of the Milky Way's stars were born. Understanding the relationship between these two disks is essential to understanding galaxy formation as a whole.
A Long-Standing Debate in Galactic Astronomy
For decades, astronomers have wrestled with competing theories to explain how the Milky Way's dual-disk architecture came to be. Two principal formation models have dominated the conversation, and distinguishing between them has proven remarkably difficult.
- The Dynamical Scattering Model: In this scenario, stars are born within the thin disk, and over time, gravitational interactions — encounters with dense molecular gas clouds, perturbations from the spiral arms, or the influence of the galaxy's central bar — gradually scatter some of these stars upward and downward, away from the midplane of the thin disk. This dynamical "heating" of stellar orbits would populate the thick disk with stars that originally formed in a thinner environment.
- The Upside-Down Formation Model: This alternative scenario proposes that in the early universe, the gas from which stars form was far more turbulent and the proto-galactic disk was much thicker overall. Stars formed directly within this thick, chaotic environment. As the galaxy matured and the gas gradually cooled and settled, each successive generation of stars formed in an increasingly thin disk, born deeper and deeper within the galactic midplane.
The core difficulty in testing these models has always been the same: accurately measuring the ages of individual stars, particularly the oldest ones. Stellar ages are notoriously difficult to determine with precision, and without a reliable chronological map of the Milky Way's stellar populations, both models remained plausible. Simulations could reproduce either outcome, leaving the true history of our galaxy frustratingly ambiguous.
"The lack of precise ages for large, representative samples of stars has traditionally been a drawback for securely differentiating among formation scenarios proposed to explain the vertical properties of the Milky Way disk." — David Mirabal et al., The Astrophysical Journal Letters, 2026
Enter Gaia: Mapping the Galaxy with Unprecedented Precision
The breakthrough came courtesy of the ESA Gaia mission, one of the most ambitious astrometric surveys ever undertaken. Launched in 2013, Gaia has been meticulously measuring the positions, distances, motions, and brightness of over a billion stars with extraordinary precision. Its third major data release, Gaia Data Release 3 (DR3), published in June 2022, provided an unprecedented wealth of information on stellar properties across the Milky Way, opening new windows into galactic archaeology.
A new study published in The Astrophysical Journal Letters, titled "A Two-stage Settling of the Milky Way Disk Revealed by Precise ChronoGal Ages," harnessed the power of this dataset to tackle the disk formation problem head-on. The lead author is David Mirabal, a pre-doctoral researcher at the Instituto de Astrofísica de Canarias (IAC) in Spain, one of Europe's premier astrophysics research institutions.
This research is part of a broader collaborative effort involving multiple papers all aimed at deciphering the age structure of stellar populations across the thin and thick disks. Mirabal and colleagues employed a sophisticated technique known as colour-magnitude diagram (CMD) fitting — a method that leverages the well-understood physics of stellar evolution to infer the ages of stellar populations based on their observed brightness and color. From this analysis, the team derived age-metallicity distributions for disk populations and carefully mapped their vertical profiles — essentially, their height above or below the galactic plane — across cosmic time.
The Discovery: A Galaxy That Formed Upside-Down
The results were striking. The data revealed a clear, decisive two-phase history for the Milky Way's disk, with a sharp transition occurring approximately 10.85 billion years ago — just a couple of billion years after the Big Bang itself.
In the galaxy's earliest epochs, the primordial gas was highly turbulent and dynamically hot, and the proto-galactic disk was correspondingly thick and puffed up — more like a single, massive, vertically extended structure than the elegant, flattened disk we see today. Within this turbulent environment, the first generations of Milky Way stars were born on wide, looping orbits that carried them far above and below the galactic midplane — the stars that would eventually become the thick disk population.
Then, roughly 10.85 billion years ago, something dramatic happened. The turbulence in the interstellar gas subsided. As the gas cooled and lost its kinetic energy, it settled gravitationally into an increasingly flat, thin layer. New star formation began occurring in this progressively thinner environment, with each successive generation of stars born closer and closer to the galactic midplane. The rapid vertical collapse of the disk during this transition phase was extraordinary in its speed.
"We find evidence that the scale height decrease rate changed drastically (~10.85) Gyr ago: at older ages, the evolution is very steep, with hZ (vertical scale height) dropping from over 1.26 to ∼0.5 kpc in merely 2 Gyr; subsequently, the thinning is very gradual, with hZ ∼ 0.1 kpc for the youngest populations." — Mirabal et al., 2026
To put those numbers in more intuitive terms: before 10.85 billion years ago, the disk collapsed from a thickness of roughly 4,100 light-years down to approximately 1,600 light-years in just 2 billion years — an astonishing rate of vertical compression. After this transition, the thinning slowed dramatically, continuing gradually until the thin disk reached its present thickness of roughly 400 light-years. For context, our entire Solar System, including the Sun and all its planets, fits comfortably within this final, razor-thin structure on the cosmic scale.
Two Phases, One Comprehensive Picture
What emerges from this analysis is a coherent, evidence-backed narrative of galactic disk formation that the authors describe as a "two-stage settling" process:
- Phase One — The Turbulent Epoch (~13.5 to ~10.85 billion years ago): The young Milky Way was a hot, chaotic, vertically extended structure. Rapid star formation occurred in a thick, turbulent gas disk. Stars born during this era inherited wide, high-inclination orbits, forming what we now call the thick disk. Metallicity was low, as few previous stellar generations had yet enriched the interstellar medium with heavy elements.
- Phase Two — The Quiescent Epoch (~10.85 billion years ago to present): As the galaxy matured and intergalactic gas accretion calmed, turbulence subsided. The gas disk cooled and settled into an ever-thinner plane. Successive generations of stars formed in this increasingly thin environment, producing the metal-rich, dynamically cold stellar populations of the modern thin disk, including stars like our Sun.
Crucially, this picture strongly favors the upside-down formation model over the dynamical scattering scenario. The thick disk's ancient stars were not born in the thin disk and later scattered to high orbits — they were born in situ, in a time when the entire galaxy was thick. The thin disk grew from within the thick disk over billions of years, not the other way around.
"The extremely rapid early vertical settling of the disk points to a fundamentally upside-down formation process at early times, when the thick disk component is basically formed." — Mirabal et al., 2026
Why This Matters: Implications for Galaxy Formation
The significance of this finding extends well beyond the Milky Way. Spiral galaxies are among the most common large-scale structures in the universe, and understanding how their disk components form is a central challenge of modern observational cosmology and galaxy evolution theory. Studies of external galaxies have long hinted at a two-phase disk formation process in galaxies across cosmic time, but direct observational confirmation within our own galaxy — where individual stars can be resolved and characterized — provides a level of detail impossible to achieve at extragalactic distances.
The result also has implications for our understanding of the chemical evolution of the Milky Way. The two-phase history neatly explains the observed metallicity bimodality — the clear separation between the metal-poor thick disk and the metal-rich thin disk — as a natural consequence of the transition between a turbulent, chemically primitive early epoch and a more quiescent, chemically enriched later period. Theoretical models of galaxy formation and evolution will need to account for this sharp transition and its timescale.
Furthermore, this research underscores the transformative power of the Gaia mission as a tool for galactic archaeology. The ability to derive precise ages for large, statistically representative samples of stars — including the oldest and most metal-poor — is what made this breakthrough possible. As future data releases from Gaia and next-generation surveys such as the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) come online, the chronological resolution of our galaxy's history will only improve.
A Classic Problem, Finally Resolved
For lead author David Mirabal, the personal significance of the result is clear. Speaking in a press release accompanying the publication, he reflected on what this finding represents for a field that has grappled with this question for generations:
"This result resolves a classic problem in astrophysics. In the simulations, both disc formation models were possible, but there was a lack of observational evidence to determine which one occurred in the Milky Way. This is the first clear observational evidence favouring one of the two models for the Galaxy's early stage. And it has been possible thanks to the high precision we achieved for the older ages." — David Mirabal, Instituto de Astrofísica de Canarias
The Milky Way we inhabit today — with its thin, star-forming spiral disk nestled within a broader, ancient thick disk of stellar fossils — is the end product of a process that began in the chaos of the early universe and unfolded over more than 10 billion years. Thanks to the extraordinary precision of modern astronomical surveys and the ingenuity of researchers willing to take on decades-old problems, we now have a far clearer picture of how our cosmic home came to be. The galaxy's disk did not grow from the inside out in the conventional sense — it settled, quite literally, from the top down, forming an increasingly thin and orderly structure from an originally thick and turbulent one. In a very real sense, the Milky Way built itself from the outside in, and the stars of the thick disk — ancient, metal-poor, and quietly orbiting far from the galactic plane — are the silent witnesses to that primordial era.
For more on the science of galactic structure and the Gaia mission's contributions to our understanding of the Milky Way, visit the ESA Gaia Science Portal.
