Euclid's New Portrait of the Milky Way's Crowded Bulge: A Window Into Dark Worlds
The ESA Euclid Space Telescope was conceived as a cosmic cartographer of the large-scale Universe — a precision instrument engineered to measure the redshift of billions of distant galaxies and illuminate the profound mysteries of dark matter, dark energy, and the accelerating expansion of the cosmos. Yet in a stunning demonstration of its versatility, Euclid has turned its gaze inward — toward the ancient, star-choked heart of our own galaxy — and delivered one of the most remarkable astronomical portraits ever assembled.
In March 2025, the space telescope trained its wide-angle 600-megapixel VIS camera on the Milky Way's galactic bulge, capturing an image of breathtaking density and detail. The result is a sweeping mosaic containing approximately 60 million individual stars, along with a rich tapestry of nebulae, globular clusters, and interstellar dust structures — all packed into a single, coherent field of view that no ground-based observatory could hope to replicate with the same clarity.
The Galactic Bulge: A Relic of Cosmic Time
The galactic bulge is the dense, roughly spheroidal concentration of stars at the center of the Milky Way, extending roughly 10,000 light-years from the galactic core in all directions. Unlike the thin, younger disc of the galaxy — where our own Sun resides — the bulge is dominated by old, metal-rich stars that formed during the earliest epochs of galactic history, approximately 10 to 11 billion years ago. In total, the bulge harbors an estimated 10 billion stars, making it one of the most densely populated regions of the entire galaxy.
These ancient stellar populations carry in their chemical compositions a fossil record of the early Universe, offering astronomers crucial clues about the conditions that prevailed when the Milky Way was still assembling itself from primordial gas clouds. The bulge also surrounds Sagittarius A*, the supermassive black hole at the galactic center, whose gravitational influence has shaped the evolution of surrounding stars for billions of years. Understanding the structure and stellar content of the bulge is therefore inseparable from understanding the formation history of the galaxy itself.
For context, the region is so congested with stars that traditional imaging techniques — particularly from ground-based observatories hampered by atmospheric blurring — struggle to resolve individual stellar objects. Euclid's position above Earth's atmosphere, combined with its exceptional angular resolution, makes it uniquely capable of picking apart this stellar crowd and cataloguing stars as discrete, individual sources.
"In just 24 hours, Euclid has delivered unique data on the Milky Way's centre, with a large and sharp view of this region. With time, the separation between sources and lenses increases. That's why this Euclid data will be a time reference for past and future missions and enable studies of exoplanets and their masses. This data can also be used for other scientific applications, from brown dwarfs and binary stars to stellar motions and dust across our galaxy."
How Euclid Assembled the Image
Constructing this extraordinary portrait was itself a feat of engineering and operational planning. Euclid required 26 hours of observing time, spread across 10 separate telescope pointings. Each individual pointing covered an area of sky larger than the apparent diameter of the full Moon — a testament to the instrument's remarkable wide-field capability. These individual exposures were then assembled into a seamless mosaic of extraordinary depth and resolution.
Crucially, this 26-hour total represents a dramatic efficiency gain over comparable ground-based observations. Achieving the same combination of sky coverage, resolution, and depth from Earth would require significantly longer exposure times and would still be fundamentally limited by atmospheric seeing — the blurring caused by turbulence in Earth's atmosphere. Euclid's space-based vantage point eliminates this limitation entirely, allowing it to resolve stars that would otherwise blend together in a smear of light.
Euclid carries two primary scientific instruments: the VIS (Visible Instrument), a wide-field optical camera, and NISP (Near-Infrared Spectrometer and Photometer), which observes in near-infrared wavelengths. Both contributed to this galactic bulge survey, providing a multi-wavelength view that helps astronomers distinguish between different stellar populations and measure the properties of individual stars with greater precision. You can learn more about Euclid's scientific instruments and primary mission at the ESA Euclid mission page.
Gravitational Microlensing: A Technique Born From Warped Spacetime
While the image itself is visually spectacular, its greatest scientific value may lie in what it enables for gravitational microlensing — an exquisitely sensitive technique for detecting planets and other faint objects that would otherwise be entirely invisible to our telescopes.
Gravitational microlensing is a direct consequence of Einstein's General Theory of Relativity, which predicts that massive objects warp the fabric of spacetime around them. When a foreground star passes almost precisely in front of a more distant background star, from the perspective of an observer such as Euclid, the gravity of the foreground star acts as a natural gravitational lens. It bends and focuses the light from the background star, causing it to appear temporarily brighter and magnified — a microlensing event.
The power of the technique for exoplanet discovery emerges when the foreground star hosts a planet. The planet's own gravitational contribution subtly distorts the lensing effect, producing a characteristic secondary brightening in the light curve — a brief, distinctive signature that can last anywhere from hours to days. By carefully analyzing these light curves, astronomers can determine the presence of a planet and estimate its mass and orbital separation from its host star.
- Sensitivity to cold, distant planets: Unlike the transit or radial velocity methods, microlensing is most sensitive to planets orbiting at distances of 1–10 astronomical units from their host stars — the so-called "snowline" region where icy, Neptune- and Earth-mass worlds are expected to be common.
- Distance independence: Microlensing works across vast cosmic distances, enabling detection of planets in the galactic bulge itself, thousands of light-years away — a regime inaccessible to other techniques.
- Statistical completeness: Because it does not rely on orbital geometry favoring edge-on systems, microlensing provides a more statistically unbiased census of the overall planet population.
- Requirement for crowded fields: The technique requires densely packed star fields — where chance stellar alignments are frequent — making the galactic bulge the ideal, and essentially unique, hunting ground within the Milky Way.
To date, ground-based observatories — including the OGLE (Optical Gravitational Lensing Experiment) survey operating from Chile, and the KMTNet (Korea Microlensing Telescope Network) — have discovered nearly 300 exoplanets via microlensing, all directed toward the galactic center. However, ground-based observations face fundamental limitations in spatial resolution and sensitivity. A space-based platform like Euclid can resolve individual stars in the densely packed bulge that ground telescopes see as unresolved blobs of light, dramatically improving both the detection efficiency and the precision of planetary mass measurements. You can explore more about current microlensing surveys at the OGLE project website.
"To catch microlensing, you need to observe parts of the sky that are crowded with stars, such as close to the centre of our galaxy. During the last twenty years, almost 300 exoplanets have been discovered using this technique, all with ground-based telescopes and all towards the centre of our galaxy. This image from Euclid includes 51 known planetary systems – and it will assist in studying many more that will be found." — Jean-Philippe Beaulieu, Institut d'Astrophysique de Paris
A Strategic Detour Within Euclid's Mission Architecture
It is worth emphasizing that gravitational microlensing and galactic bulge science were never part of Euclid's core mission objectives. The telescope's primary scientific program is focused on conducting a wide-field survey of billions of extragalactic sources to map the large-scale structure of the Universe and constrain models of dark energy and dark matter. Its core survey fields are largely away from the plane of the Milky Way, where the density of foreground stars and dust would complicate measurements of distant galaxies.
However, Euclid's observing strategy is constrained by practical engineering realities. To maintain its sensitive detectors at their required operating temperatures and minimize contamination from stray sunlight, the telescope must maintain a specific orientation relative to the Sun at all times. This means that during the spring and autumn equinoxes, its primary extragalactic survey fields become geometrically unfavorable to observe. Rather than waste this precious time, mission planners identified the galactic bulge — which happens to fall within Euclid's accessible field of view during these periods — as an ideal alternative target. Thus, twice per year, Euclid turns its attention from the distant Universe to the ancient heart of our own galaxy, transforming an operational constraint into a significant scientific bonus.
This galactic bulge survey was championed within the Euclid Consortium as part of its legacy science program — a portfolio of high-value scientific investigations that complement the primary mission without interfering with its core objectives. Jean-Philippe Beaulieu of the Institut d'Astrophysique de Paris, who co-led the exoplanet working group of the Euclid Consortium, was the original instigator of this galactic bulge survey and has been instrumental in realizing its scientific potential.
Euclid as a Time Machine for Future Discoveries
One of the most intellectually profound aspects of Euclid's galactic bulge dataset is not what it reveals today, but what it will enable in the future. The 26-hour observation was not long enough, on its own, to catch microlensing events in real time — that requires continuous monitoring campaigns lasting weeks to months. But what it did accomplish is arguably more transformative: it created a precise, high-resolution baseline snapshot of the galactic bulge that will serve as a fundamental reference for all future microlensing surveys of this region.
This is particularly significant in the context of the upcoming Nancy Grace Roman Space Telescope, NASA's next flagship observatory currently in development. Roman's mission architecture is built around several major surveys, one of the most ambitious of which is the Galactic Bulge Time-Domain Survey (GBTDS) — a dedicated 15-month monitoring campaign of the galactic center designed specifically to detect gravitational microlensing events. Based on current projections, this survey is expected to discover approximately 1,400 cold exoplanets with masses greater than that of Mars, including an unprecedented sample of roughly 300 planets with fewer than three Earth masses — a population of worlds that has remained largely beyond the reach of other detection methods.
When Roman detects a microlensing event, astronomers will need to understand the properties of both the lens star and the background source star — not just at the moment of the event, but also in the past, before the alignment occurred. This is precisely where Euclid's March 2025 dataset becomes invaluable.
"In 24 hours, Euclid has already captured the stars involved in all the future microlensing events that the Roman space telescope will detect, but before the stars and planets involved have aligned. This means that anyone who detects a microlensing event in the same region, for example with Roman, will be able from now on to use Euclid data as a time reference in the past and see how the stars looked before they overlapped." — Natalia Rektsini, Institut d'Astrophysique de Paris
Because Euclid can clearly resolve individual stars in the crowded bulge, astronomers can measure the precise positions and brightnesses of both the lens and source stars before they align. Once a microlensing event is detected — potentially years from now — researchers can look back at Euclid's data to determine how far the stars have moved, enabling a precise determination of the lens star's proper motion. This proper motion measurement is a critical ingredient in breaking the degeneracy between planetary mass, lens distance, and lens-source relative velocity that often plagues microlensing analyses. The result will be dramatically more precise planetary mass determinations than would otherwise be possible.
Resolving the Masses of Long-Known Planets
The dataset will not only benefit future discoveries — it is already poised to refine our understanding of planets found decades ago. Among the 51 known planetary systems captured within this image is the famous exoplanet OGLE-2005-BLG-390Lb, informally nicknamed "Hoth" after the icy planet in the Star Wars universe. Discovered 20 years ago, this frigid world — orbiting a dim red dwarf star at roughly three times the Sun-Earth distance — was among the first cold super-Earths ever detected via microlensing, and it remains a scientifically significant object.
Despite two decades of study, the precise mass of OGLE-2005-BLG-390Lb has never been definitively measured, because the original microlensing data did not provide sufficient constraints to break the inherent degeneracies of the lensing geometry. Euclid's new high-resolution data may finally change that.
"I led the team that discovered OGLE-2005-BLG-390Lb 20 years ago. It's an icy planet, a bit like Hoth from Star Wars. After all this time, I'm excited that Euclid might finally allow us to measure its precise mass." — Jean-Philippe Beaulieu
By measuring the current positions of the lens star and the source star — now separated after their alignment two decades ago — and combining this with the known elapsed time, astronomers can calculate the relative proper motion and use it to constrain the physical parameters of the lensing system. Similar measurements could be applied to other microlensing exoplanets whose masses remain poorly determined, potentially transforming our statistical understanding of cold planet demographics across the galaxy.
Beyond Exoplanets: A Rich Secondary Science Program
While exoplanet science may headline Euclid's galactic bulge survey, the scientific richness of the dataset extends far beyond planetary detection. The 60-million-star catalog encoded in these images represents a treasure trove for multiple branches of astrophysics:
- Brown dwarf demographics: Free-floating brown dwarfs — objects too massive to be planets but too light to sustain hydrogen fusion — can also produce microlensing events. Euclid's survey should provide new constraints on their abundance in the inner galaxy.
- Binary star systems: The precise photometry and astrometry of Euclid's dataset will enable detailed studies of binary star populations in the bulge, including the detection of binary lensing events with characteristic double-peaked light curves.
- Stellar
