The European Space Agency’s Euclid telescope has delivered the largest and most detailed visible-light image ever taken of the Milky Way’s central bulge, a mosaic containing more than 60 million individual stars. Captured on 23 March 2025 over 26 hours, the image covers a region so densely packed with old stars that ground-based observatories struggle to separate them. The data immediately provide astronomers with a high-resolution map of 51 known planetary systems and a baseline for measuring the masses of future exoplanet discoveries in this crowded field.

The observation was a deliberate departure from Euclid’s primary mission. Launched in 2023 to survey the large-scale structure of the Universe and probe dark matter and dark energy, the spacecraft normally fixes its gaze on distant galaxies. For a single day, scientists pointed its visible-light camera toward the intensely bright galactic centre, a request driven by exoplanet researchers. The instrument’s sensitivity and wide field of view—each exposure covers an area 270 times larger than a Hubble Wide Field Camera shot—allowed it to distinguish individual stars without saturating its detectors. To observe the same mosaic, the Keck Observatory in Hawaii would require roughly 2,000 hours.

The image’s scientific value lies in microlensing, a technique that exploits the gravitational bending of light. When a foreground star passes in front of a more distant one, the nearer object acts as a cosmic magnifying glass, amplifying the background star’s brightness. If a planet orbits the foreground star, its gravity introduces a tiny additional distortion in the light curve. European astronomers note that nearly 300 exoplanets have been discovered this way over the past two decades, all toward the galactic centre. The Euclid mosaic includes the entire region that NASA’s future Roman Space Telescope will monitor for exoplanets, making it a foundational reference.

Italian researchers from the National Institute for Astrophysics (INAF) played a key role in the observation, having designed the control electronics for Euclid’s visible-light instrument and developed a dedicated data-analysis pipeline for this unusual target. The team had to mask the infrared camera to prevent overexposure that would have compromised subsequent observations. With the mosaic now public, scientists will track subtle brightness variations over time, searching for the signatures of new planets. The next milestone will be the cross-calibration of these data with Roman’s upcoming survey, which is expected to expand the census of exoplanets in the inner galaxy.