How Euclid's 60-Million-Star Mosaic of the Galactic Core Sets a New Record and Prepares NASA's Roman Telescope for Planet Hunting

On March 23, 2025, ESA's Euclid space telescope pointed its optical VIS camera near the center of the Milky Way and, over about 26 cumulative hours across nine adjacent fields, produced the largest and most detailed visible-light photo ever taken of our galaxy's heart. The resulting mosaic spans 4.8 square degrees and contains more than 60 million stars . The data was released on June 24, 2026, as part of Euclid's Quick Data Release 2 (the Euclid Galactic Bulge Survey) .

Beyond the Record: A Planet-Hunting Foundation

The survey covers 51 known planetary systems detected by previous microlensing campaigns and will serve as a microlensing time-reference baseline for NASA's Nancy Grace Roman Space Telescope, which is scheduled to conduct its Galactic Bulge Time Domain Survey (GBTDS) starting in late 2026 or early 2027 . Euclid's sharp, high-resolution pre-imaging of the same fields gives Roman a 'before' snapshot that dramatically improves the mass measurement of lensing objects and the detection of exoplanets in the region .

Why This Matters: A Departure from Euclid's Primary Mission

Euclid's core mission is to map the geometry of the dark Universe: it is designed to survey billions of galaxies across 15,000 square degrees of extragalactic sky, using weak gravitational lensing and galaxy clustering to probe dark energy and dark matter . The Galactic Bulge Survey is a deliberate departure from that program. Instead of looking outward at distant galaxies, Euclid looked inward at the dense, crowded core of our own galaxy — a region so full of bright stars that it is normally avoided for cosmology. The purpose is purely exoplanet and stellar astrophysics: providing a deep, wide-field, high-resolution reference image that will enable Roman's microlensing survey to find planets more efficiently .

Technical Challenges Overcome

The team faced several significant hurdles to make this record-breaking observation happen:

• Securing approval: The observation was not part of the original Euclid mission plan. The Exoplanet Science Working Group, led by Eamonn Kerins, had to build a detailed scientific and technical case to convince ESA and the Euclid Consortium to allocate observing time away from the core dark-energy survey .

• Extensive pre-observation simulations: Preparation required 'months of exhaustive technical tests working with the Euclid instrument and operations teams' to demonstrate that imaging the ultra-dense bulge would not overwhelm the telescope's detectors or data pipeline . The team ran end-to-end simulations to verify that the spacecraft could safely point at such a bright, star-packed field without saturating the VIS camera's CCDs or degrading the mission's primary calibration .

• No compromise to the core mission: The team had to prove that the bulge observation could be slotted into a 26-hour window without disrupting Euclid's primary Wide and Deep surveys or depleting consumables needed for the main cosmology campaign .

• Data processing density: The extreme stellar density in the bulge — up to millions of stars per square degree — required specialized data reduction to avoid blending, photometric errors, and calibration drift, all while preserving the image quality needed to serve as a reliable astrometric and photometric reference for Roman .

What This Means for Exoplanet Science

Euclid's Galactic Bulge Survey is more than a stunning image. It is a strategic investment in the future of exoplanet discovery. By providing a high-resolution baseline of the same fields Roman will monitor repeatedly over five years, Euclid enables astronomers to measure the mass of exoplanets found via microlensing with up to five times greater precision . The joint Euclid-Roman effort could yield thousands of new exoplanets, from free-floating Jupiter-mass worlds to cool planets down to Earth mass .