Johannes Sahlmann, Project Scientist at the European Space Agency (ESA), spoke with Editor Maddie Hall about the Gaia space telescope and its discovery of our Galaxy’s great wave.
Gaia, a science mission conducted by the European Space Agency, aims to deepen our understanding of how our Galaxy, the Milky Way, functions and evolves. To achieve this, Gaia maps around 2 billion stars and other objects, which, while only representing about 1% of the Milky Way’s total stellar content, creates an extensive data collection that allows us to extrapolate information about the entire Galaxy.
Even though we can’t send spacecraft beyond our Galaxy, Gaia’s exceptional ability to capture three-dimensional data is equipping scientists with the tools to craft detailed maps of our galaxy’s structure and dynamics.
Through this research, Gaia has discovered a great wave, rippling across our Galaxy from its centre, and extending for tens of thousands of light-years from our Solar System. To discuss this discovery in more detail and understand the vital role of the Gaia space telescope in understanding our Galaxy, Editor Maddie Hall spoke with Johannes Sahlmann, Project Scientist at the European Space Agency.
Can you explain the significance of the great wave discovered by Gaia? What can the existence of this great wave tell us?
Observing the properties of these stars, including their positions, distances, and motions, provides insights into the structure and dynamics of the Milky Way. It was through these measurements, published in a 2022 data release, that the Great Wave was discovered.
The existence of such waves had been anticipated, but it was Gaia’s ability to conduct both qualitative and quantitative studies, enabling precise measurements and characterisations of stellar motions, that has facilitated such discoveries. The wave identified is currently the largest known in our Galaxy and covers a vast area of the Milky Way.
This discovery is part of Gaia’s core objectives and can help to achieve that fundamental goal of understanding how our Galaxy functions and evolves. Gaia’s multi-dimensional measurements of star motions have opened up new avenues for understanding the relationship between these motions and other large-scale structures in the Milky Way.
Within the Milky Way, we have structures such as the galactic bar, a linear formation at the centre, and the spiral arms. All these components interact with one another, and none are independent because they all possess mass and gravitational influence. Gaia has introduced numerous complexities that must be considered in our theoretical models, and the data quality is so high that scientists cannot study these effects in isolation; they must be understood and modelled together.
Simulating a galaxy requires a combination of computer simulations and observational data, using our knowledge of stellar properties and interactions within the Milky Way and with nearby galaxies to inform simulated models and shed light on the formation of the wave.
The waves could potentially be due to a collision with a dwarf galaxy. Could you provide more details on how such interactions could create these ripples and how you can investigate this further? Are there other potential causes you are exploring?
The Gaia mission is ongoing, and although we have published only a fraction of the data collected by the satellite, we have already gained significant new insights into our Galaxy, suggesting that the Milky Way is much more dynamic and complex than previously thought.
One key finding is the correlation between the passage of another galaxy and increased star formation rates in our Galaxy. There have been interactions between the Milky Way and other dwarf galaxies that have resulted in similar correlations. While correlation does not imply causation, it is reasonable to consider that these interactions have an influence on star formation. The improved quality of data now available makes it possible to explore these questions, which were difficult to address before missions like Gaia.
Essentially, these gravitational interactions create a range of effects observable in the Milky Way. Large-scale perturbations can cause the galactic disk to wobble and warp, altering the distribution of stars. An analogy often used to explain this to the general public is that of a pond: if you throw a rock (representing a small galaxy) into the water (representing the Milky Way), it creates ripples on the surface, similar to the perturbations in our Milky Way.
However, the physics involved are quite complex; it’s not simply a solid rock falling into water. The dynamics are driven by the gravitational interactions among various celestial bodies. In addition to galaxy collisions, other factors influence these perturbations – for example, dark matter, which is present in our Galaxy as well.
The concept of young stars retaining a memory of the wave information is fascinating. How does this work, and why is it useful for us?
Stars can live for billions of years, and if you consider our Galaxy, it is not static; it rotates. For example, the Sun takes roughly 200 million years to complete one orbit around the centre of the Galaxy, and this is just a fraction of the lifetime of a typical star.
Consequently, older stars have made many revolutions around the Galactic Centre and have experienced interactions, for example, with the galaxy’s spiral arms. Over time, they lose information about their birthplace and the initial conditions under which they were formed.
In contrast, young stars, which are only millions of years old, have remained close to where they formed. Therefore, they retain more information about their formative conditions, such as the movement of the gas and dust from which they originated.
Using information from both young stars and old stars can provide complementary insights that enhance our understanding of what is happening in our Galaxy. In this case, young stars were particularly useful due to the information they retain, and they are often bright, making them easier to observe with high precision. It’s an interesting example of how, in addressing a scientific question, you have to select what probe will most efficiently bring you the answer – in this case, that was young stars.
Can you tell us more about Gaia and the technology that discovered and understands the wave? What are the challenges that come with observing and mapping the Galaxy?
Gaia, launched in 2013, was designed to investigate the origin and evolution of the Milky Way. It features a 10m-diameter sunshade and optical components approximately 3m in diameter. Three onboard instruments measure the positions, brightnesses, colours, and chemical compositions of billions of stars and other objects, while the satellite spins continuously, completing a revolution every six hours.
Gaia boasts the largest focal plane ever flown in space, featuring nearly 1 billion pixels across 106 charge-coupled devices (CCDs). To deal with the satellite’s spinning motion, a new readout mode for the electronics was developed, enabling Gaia to collect over 3 trillion individual observations. Processing this immense dataset presents many challenges. The consortium responsible for data processing and analysis is composed of roughly 450 engineers, scientists, and specialists from across Europe. This team converts the satellite data into catalogues for publication. All catalogue data are publicly available free of charge and are hosted at the European Space Astronomy Centre near Madrid, where we manage the ESA Space Science archives.
The next data release, expected at the end of 2026, will encompass approximately 5.5 years of Gaia data and 500 terabytes, with the final release covering 10.5 years of data and around 1 petabyte anticipated not before the end of 2030. To date, several intermediate data releases have been made, but the upcoming release marks the first principal data compilation from Gaia’s nominal mission.
What do you hope to learn from future data releases from Gaia? What specific questions do you think need to be answered in future research regarding the great wave?
Having more data necessitates the use of more sophisticated models, and with increased and improved data, we’ll be able to employ more complex modelling approaches.
One key aspect of future study is the motion of stars. These motions are derived from individual position measurements, and with the next data release, we will have measurements spanning a longer time range, leading to more accurate results. We anticipate that this next phase of study will allow us not only to confirm the existence of the wave but also to enable more detailed investigations, including the examination of second-order signatures within it.
As we gain a clearer understanding of these features, the modelling techniques will also advance. This could enable us to identify the origin of this significant wave and disentangle the various effects, such as different perturbations, or pinpoint a specific event that contributed to its presence.
Looking ahead, what potential missions or projects do you believe could build upon the findings of the great wave? Could there be great waves in other Galaxies?
One important aspect of the Gaia mission is that it observes the visible portion of the light spectrum, which is affected by interstellar extinction. Essentially, gas and dust between us and distant stars obstruct our view and, as a result, certain parts of the Galaxy are not accessible. A proposed mission concept outlined in ESA’s scientific long-term plan, called Voyage 2050, aims to address this issue by observing in the infrared portion of the light spectrum, through which we could gain insights into the inner regions of the Galaxy.
Great Waves have been observed in other galaxies, although not with the same level of detail as in the Milky Way, as the advantage of studying our own Galaxy is that our position within it allows for a more intricate analysis. There are synergies between ESA’s science missions, in particular between Gaia and the Euclid mission, which focuses primarily on cosmology – examining large-scale structures, dark energy, and dark matter. Euclid will map billions of galaxies, including some that are relatively nearby. Together, these two missions will likely yield significant scientific insights, enhancing our understanding of our Galaxy within the broader context of the Universe.
Around 450 dedicated individuals are working hard to transform the vast Gaia data into usable products for scientists, and their efforts are communicated to the public. This European project involves collaboration across many countries and institutions, highlighting the teamwork and commitment that spans several decades.
While the Gaia satellite was switched off in March 2025 because it ran out of the gas necessary to control its position, the Gaia mission is far from over. We still have five years left to process and publish the data collected. The Gaia mission will conclude only when we have released the last of this data, which is scheduled not before the end of 2030.
