ESA’s Euclid captures the Milky Way’s crowded heart

For just one day, our dark universe detective, Euclid, turned his gaze toward the light: the extremely bright inner region of our Milky Way galaxy, known as the galactic bulge. This particular request came from astronomers who were looking for Euclid’s best work: capturing vast areas of the sky in vivid detail.

Designed to observe billions of distant galaxies, the space telescope’s visible light camera is so sensitive that it can tell individual stars apart in our super-crowded galaxy bulge without even looking at them. This rare ability is important for what scientists want to use this image for: studying planets around other stars using a special technique called microlensing. But before we dive in, let’s first take a closer look at this awe-inspiring image.

On March 23, 2025, Euclid captured this huge photo in about 26 hours. This is a mosaic of nine ‘pointings’ from its visible light camera [1]In which each point covers a portion of the sky larger than the full moon.

For comparison, Euclid’s sharpness and sensitivity in visible light is similar to that of the NASA/ESA Hubble Space Telescope’s Wide Field Camera. But each indicates that Euclid captured, in just a few hours, an area 270 times larger than Hubble’s field of view. To observe the same Euclid mosaic, Keck Observatory would require approximately 2000 hours. Euclid is fast, and is able to pick out details from faint stars that would otherwise be missed when viewed from the ground. This single mosaic also covers the entire area that the upcoming Roman space telescope will monitor to discover the planet.

Euclid captured more than 60 million stars along with nebulae and star clusters in this photo. This crowded region of our galaxy is the perfect place for astronomers to search for exoplanets with microlensing. The text continues after the image slider.

Finding exoplanets with gravitational microlensing

Microlensing is a form of gravitational lensing. While Euclid mostly uses lensing to detect distant massive objects, such as clusters of galaxies, this new image of the galactic center helps scientists study lensing on the smallest scales – those caused by stars and exoplanets in our own galaxy.

Microlensing depends on the coincident alignment of two stars with an observer. As one star passes in front of another, the nearest star acts like a cosmic magnifying glass, bending and brightening the background star’s light. If a planet orbits a nearby star, its gravity also bends this light in a slightly uneven manner. This small additional change in brightness reveals the presence of a planet.

“To catch microlensing, you have to observe parts of the sky that are full of stars, such as those close to the center of our galaxy,” explains Jean-Philippe Beaulieu of the Institut d’Astrophysique de Paris in France and the University of Tasmania in Australia. Jean-Philippe Euclid was the original initiator of the Galactic Bulge Survey, and co-led the Euclid Consortium’s Exoplanet Working Group.

“During the past twenty years, nearly 300 exoplanets have been discovered using this technique, all with ground-based telescopes and all toward the center of our galaxy. This image from Euclid includes 51 known planetary systems – and will help study many more to be found,” he added.

Measuring Planet Mass with Euclid

To capture the microlensing phenomenon, a telescope would need to study a star for more than twenty days. This is needed to see the unevenness of the bending of light as the planet orbits around its host star. Therefore, no new phenomenon could be found in Euclid’s one day’s observation. But what makes this image so special is that it allows scientists to measure the masses of planets that are already known, as well as those that have yet to be discovered.

“In 24 hours, Euclid has already captured the stars involved in all future microlensing events that the Roman Space Telescope will detect, but before the stars and planets involved have aligned,” says Natalia Rectacini of the Institut d’Astrophysique de Paris in France, who led the release of Euclid’s Galactic Bulge Survey data to the scientific community.

Natalia explains, “This means that anyone who detects a microlensing event in the same area, for example with Roman, will be able to use the Euclid data as a time reference from now to the past and see how the stars looked before the overlap.” “Because Euclid could clearly distinguish individual stars, one could 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 a single point in time.”

icy planets and more

With most planet-hunting techniques, it is easy to find large, hot planets orbiting massive stars. This is not the case for microlensing. “This technique is unbiased, we find whatever is out there,” says Natalia. “It is uniquely suited to the search for cool exoplanets. And we expect that every star in the galaxy will host at least one such planet.”

Euclid’s data show host stars for two known cold exoplanets, and both are special to the team.

“I led the team that discovered OGLE-2005-BLG-390Lb 20 years ago,” says Jean-Philippe. “It’s an icy planet, somewhat like Hoth from Star Wars. After all this time, I’m excited that Euclid can finally allow us to measure its exact mass.”

“OGLE-2013-BLG-341Lb is a rare and fascinating system,” says Natalia. “It consists of two stars and a planet. By combining earlier observations from Keck and Hubble with the new Euclid data, we can finally distinguish the stars and confirm the mass of the planet.”

“This result shows what a relatively small, dedicated team can achieve within a large international mission,” says Valeria Petorino, 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 visualization instrument.”

“In just 24 hours, Euclid has provided unique data on the center of the Milky Way with a large and clear view of this region. Over time, the separation between sources and lenses increases. That is why this Euclid data will be a time reference for past and future missions and will enable the study of exoplanets and their masses. This data can also be used for other scientific applications, from brown dwarfs and binary stars to stellar motions in our Galaxy and To the dust.”

Notes to editors

Explore this image at the highest resolution in ESASky.

More information on how to download the new Euclid data can be found here.

[1] For the Galactic Bulge Survey, only Euclid’s visible camera (VIS) was used to keep observations as stable as possible. That’s why the original image is in black and white. To add color to the photos for this public release, data from the ground-based Canada-France-Hawaii Telescope (CFHT) was added.

about euclid

Euclid was launched in July 2023 and began its routine science observations on 14 February 2024. The mission aims to reveal the hidden effects of dark matter and dark energy on the visible universe. Over a period of six years, Euclid will observe the sizes, distances and motions of billions of galaxies up to 10 billion light years across.

Euclid is a European mission, built and operated by ESA with contributions from NASA. The Euclid Consortium – consisting of more than 2000 scientists from 300 institutions in 15 European countries, the United States, Canada and Japan – is responsible for providing the scientific tools and scientific data analysis. ESA selected Thales Alenia Space as the main contractor to build the satellite and its service module, with Airbus Defense & Space selected to develop the payload module, including the telescope. NASA provided the detectors of the Near-Infrared Spectrometer and Photometer, NISP. Euclid is a medium-class mission in ESA’s Cosmic Vision programme.