Recent discoveries from the European Space Agency’s Gaia mission have revealed that the Milky Way’s history includes a major merger with a smaller dwarf galaxy about 8 to 11 billion years ago. This event, now known as the Gaia-Enceladus merger (also called Gaia-Sausage-Enceladus), added a significant amount of stars and gas to our galaxy and left clear traces in the motions and chemical makeup of stars we see today. Data from Gaia’s precise measurements show that this collision helped build the inner stellar halo and the thick disk, two major parts of the Milky Way’s structure.
This ancient merger is just one chapter in the galaxy’s long history of interactions, but it stands out because of its size and lasting effects. Simulations and observations together paint a picture of a young Milky Way absorbing a dwarf galaxy roughly one-quarter its mass, mixing materials in ways that changed how stars formed for billions of years afterward. These findings come from the most detailed star catalog ever created and are changing how we understand galaxy growth. Could this long-ago collision have played a role in creating the galactic environment that later allowed our Solar System to form?
What Is the Gaia-Enceladus Merger in Galactic History?
The Gaia-Enceladus merger was a major collision between the young Milky Way and a dwarf galaxy that occurred roughly 8 to 11 billion years ago. According to ESA’s 2018 announcement of the Gaia-Enceladus discovery, the incoming galaxy had a mass estimated at about 20–25% of the Milky Way at that time. When the two galaxies met, their stars and gas mixed, creating new populations that still orbit in the Milky Way today.
The merger is named after the Gaia satellite that found the evidence and the Greek myth of Enceladus, reflecting the “buried” nature of the merged galaxy’s stars inside the Milky Way. Models suggest that this event added roughly 50 billion solar masses of material, including stars, gas, and dark matter. Many of the stars that once belonged to the dwarf galaxy now move in opposite directions to the Milky Way’s normal rotation, a clear sign of their foreign origin.
Fun fact: some of these stars are still passing through our local neighborhood today, moving at speeds that stand out from the rest of the stars around us. This merger is part of the broader galactic collision history that shaped the Milky Way from a smaller, simpler galaxy into the large spiral we know.
How Did Ancient Galactic Collisions Occur in the Milky Way?
Galactic collisions happen when two galaxies come close enough for gravity to pull them together. In the case of Gaia-Enceladus, the dwarf galaxy approached the Milky Way on a nearly straight, radial path, diving toward the center rather than circling slowly. Simulations show that this kind of trajectory caused strong tidal forces that stripped stars from the dwarf galaxy and scattered them into the Milky Way’s halo.
The process unfolded over hundreds of millions of years. As the dwarf galaxy got closer, its outer stars were pulled away and formed long streams around the Milky Way. When the cores collided, gas clouds compressed and triggered bursts of star formation. Unlike car crashes, stars rarely hit each other because space is so empty, but gas collisions created new stars and heated the disk.
According to the original 2018 Nature paper by Helmi and colleagues, this heating increased the vertical motion of stars (called velocity dispersion) by roughly 20–30 km/s, which explains why the thick disk is puffier than the thin disk. Estimates indicate the merger occurred between 8 and 11 billion years ago, with most studies agreeing on a central value around 10 billion years.
What Evidence Supports the Ancient Galactic Collision Theory?
The strongest evidence comes from the Gaia satellite’s detailed measurements of star positions, speeds, and distances. In 2018, researchers identified about 30,000 stars moving in retrograde orbits (opposite to the Milky Way’s rotation), which is a hallmark of the merged galaxy. These stars also show distinct chemical patterns, with lower amounts of heavy elements compared to stars born inside the Milky Way.
When combined with ground-based surveys that measure chemical composition, the data reveal two clear groups: one with high alpha-element abundance (oxygen, magnesium) and one with low, matching the signatures of the incoming dwarf galaxy. Simulations reproduce these patterns best when a large, retrograde merger is included. Many globular clusters and variable stars also share these orbits, supporting the merger story.
Recent work confirms that the merger contributed significantly to the inner halo, with estimates suggesting it accounts for a large fraction of its mass. As Gaia continues to release higher-precision data, interpretations of these events may evolve slightly, but the core evidence remains strong.
How Did the Collision Shape the Milky Way’s Structure?
The merger helped build the Milky Way’s inner stellar halo and thick disk. The incoming stars populated the halo, a spherical region of old, metal-poor stars that surrounds the galaxy center. The collision also heated the disk, making it thicker in the vertical direction.
Models suggest that the merger increased the disk’s scale height (vertical thickness) to roughly 3,000 light-years in places, compared to about 1,000 light-years for the thin disk. The event also mixed chemical elements, creating a noticeable bimodality in the disk’s abundance patterns (two distinct tracks in element ratios). This mixing likely altered how stars formed in later generations.
Fun fact: without this merger, the Milky Way might have remained a smaller, less massive galaxy with a simpler structure. The added material and energy helped create a more stable, diverse galaxy over time.
Did This Ancient Collision Influence the Solar System?
The Gaia-Enceladus merger happened about 5 billion years before the Sun formed, so it did not directly cause the Solar System. However, it did contribute to the broader galactic conditions under which the Solar System later formed. The merger mixed gas and stars, helping create the chemical environment of the thin disk where the Sun was born 4.6 billion years ago.
Some researchers suggest that the event may have increased the likelihood of forming metal-rich regions suitable for planet-forming disks. Models show that gas flows and chemical enrichment from the merger are consistent with the metallicity seen around the Sun. While the connection is indirect, it highlights how ancient events helped set the stage for our Solar System’s birth.
What Are the Ongoing Effects of Galactic Collision History?
The merger’s effects are still visible today. Stars from the original dwarf galaxy continue to move through the Milky Way, and their motions create subtle ripples in the disk (called phase-space wrinkles). These ripples can influence star orbits over millions of years.
The chemical bimodality in the disk also persists, guiding current star formation patterns. The merger helped make the Milky Way more massive and stable, preparing it for future interactions, including its eventual merger with the Andromeda galaxy in about 4.5 billion years.
This ancient galactic collision history shows how dynamic and interconnected the universe is.
The Gaia-Enceladus merger transformed the young Milky Way, adding stars, building its halo and thick disk, and contributing to the conditions that later allowed the Solar System to form. As Gaia continues to deliver new data, our understanding of these ancient events may evolve. How many other hidden mergers are still waiting to be discovered in our galaxy’s past?
(This article summarizes findings from peer-reviewed astrophysics research and ESA Gaia mission data for a general audience.)
Sources
European Space Agency. (2018, October 31). Galactic ghosts: Gaia uncovers major event in the formation of the Milky Way. European Space Agency. https://www.esa.int/Science_Exploration/Space_Science/Gaia/Galactic_ghosts_Gaia_uncovers_major_event_in_the_formation_of_the_Milky_Way
Helmi, A., Babusiaux, C., Koppelman, H. H., Massari, D., Veljanoski, J., & Brown, A. G. A. (2018, October 31). The merger that led to the formation of the Milky Way’s inner stellar halo and thick disk. Nature. https://www.nature.com/articles/s41586-018-0625-x
Johnson, J. W., Feuillet, D. K., Bonaca, A., & de Brito Silva, D. (2025, October 9). That’s so Retro: The Gaia-Sausage-Enceladus Merger Trajectory as the Origin of the Chemical Abundance Bimodality in the Milky Way Disk. arXiv. https://arxiv.org/abs/2510.08688