Images taken in ESA’s Euclid mission of the bright center of our Milky Way on 23 March 2025. It produced the largest and most detailed visible-light image ever made of the Milky Way’s core – in only 26 hours. The nine exposures that made this mosaic are wider than the full Moon and overlap by about 1/3. The results are then impressive: More than 60 million stars within the galactic bulge, entwined nebulae, and crowded star clusters.
The image is not just a spectacle. It is a scientific tool, a map of crowded starlight where astronomers hunt for planets using one of the most subtle tricks in astrophysics: microlensing.
Euclid’s visible-light camera rivals the sharpness of Hubble’s wide-field instruments, but its reach is far broader. Each pointing covers an area 270 times larger than Hubble’s field of view. To match Euclid’s mosaic, the Keck Observatory in Hawaii would need nearly 2000 hours of observing time. Euclid did it in barely more than a day, capturing faint stars invisible from Earth’s surface.
The mosaic also spans the exact region that NASA’s upcoming Roman Space Telescope will monitor for planet hunting, making Euclid’s snapshot a kind of cosmic prologue.
Microlensing occurs when a foreground star passes in front of a background star, bending and magnifying its light. If the foreground star hosts a planet, the planet’s gravity adds a tiny, uneven flicker to the magnification. That flicker is the planet’s fingerprint.
“To catch microlensing, you need to observe parts of the sky that are crowded with stars, such as close to the center of our galaxy,” explains Jean-Philippe Beaulieu of the Institut d’Astrophysique de Paris and the University of Tasmania. Beaulieu, who first proposed Euclid’s galactic bulge survey, co-led the exoplanet working group of the Euclid Consortium.
“During the last twenty years, almost 300 exoplanets have been discovered using this technique, all with ground-based telescopes and all towards the center of our galaxy. This image from Euclid includes 51 known planetary systems, and it will assist in studying many more that will be found,” he adds.
Normally, telescopes have to watch for weeks to confirm a microlensing event. Euclid’s single-day mosaic did not capture such an event as it happened. Still, it did something equally valuable. It froze the stars in place before their alignments, giving a reference frame for future detections.
“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,” says Natalia Rektsini of the Institut d’Astrophysique de Paris, who led the release of Euclid’s bulge survey data.
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“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,” she explains.
“Since Euclid can clearly separate individual stars, one can then measure how fast they move over time and use that information to confirm the existence of a planet and determine its mass. This would not be possible with data from one point in time.”
Unlike transit or radial velocity methods, microlensing does not favor giant, hot planets. “This technique is unbiased; we discover whatever is out there,” says Natalia. “It is uniquely suited to discover cold exoplanets. And we expect every star in the Milky Way to host at least one such planet.”
Euclid’s data already highlights two known cold exoplanets. For Beaulieu, one is personal.
“I led the team that discovered OGLE-2005-BLG-390Lb 20 years ago,” he recalls. “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.”
Natalia points to another: “OGLE-2013-BLG-341Lb is a rare and fascinating system. It consists of two stars and one planet. By combining earlier observations from Keck and Hubble with new Euclid data, we can finally separate the stars and confirm the planet’s mass.”
“This result shows what a relatively small, dedicated team can achieve within a large international mission,” says Valeria Pettorino, Euclid Project Scientist at ESA. “The exoplanet team included strong contributions from early-career researchers and was supported by the Science Ground Segment unit working on the visible instrument.”
She emphasizes the broader impact: “In just 24 hours, Euclid has delivered unique data on the Milky Way’s center, with a large and sharp view of this region. With time, the separation between sources and lenses increases.
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That’s why this Euclid data will serve as a 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.”
More than a photograph, this is Euclid’s mosaic. The future discoveries in the world of inevitable stars, and a camera recording that becomes motionless between moments. Astronomers, however, see not only the possibility of finding new planets in its detailed view but also a way to understand movements among stars within AGGLOMERATION1, using dust blocking light, which will give further insight into changes that occurred at our Milky Way center over time.
For now, the image stands as a testament: in one day, Euclid captured the crowded heart of our galaxy, and in doing so, gave planet hunters a time machine.
The Milky Way’s center has never looked so crowded, or so full of promise.
