Euclid Captures 60 Million Stars in the Milky Way's Bulge, Setting a Microlensing Baseline for NASA's Roman Telescope

Overview

The European Space Agency on 24 June 2026 published a single mosaic of the crowded inner region of our galaxy, packed with more than 60 million stars, according to ESA. The image, released as part of Euclid’s second Quick Data Release (Q2), is the product of the Euclid Galactic Bulge Survey and is being framed less as a standalone result than as a reference frame for the exoplanet hunt that NASA’s Nancy Grace Roman Space Telescope will begin from the same patch of sky.

Euclid is primarily a dark-universe mission, but for this observation it turned toward the brightest, most star-dense part of the Milky Way. As Phys.org reported, the data was taken on 23 March 2025.

What We Know

Euclid captured the photo on 23 March 2025 in just about 26 hours, assembling a mosaic of nine “pointings” from its visible-light camera, with each pointing covering a patch of the sky larger than the full Moon, according to ESA. The target was what ESA describes as the extremely bright inner region of our Milky Way galaxy, known as the galactic bulge.

The Q2 paper puts precise numbers on the survey. According to the Euclid Collaboration’s Q2 release, the survey covers 4.8 deg² across nine contiguous fields, with each field built from 16 dithered exposures of 400 s each for a total of 1.8 hours per field. The same paper reports photometric catalogues containing approximately 45 million detected sources per dither down to AB magnitude 26. The 4.8-square-degree figure is corroborated by Phys.org, which notes the image is 4.8 square degrees in total.

The scientific payoff is gravitational microlensing — the brief brightening that occurs when one star passes in front of another and its gravity bends the background light. “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,” said Jean-Philippe Beaulieu of the Institut d’Astrophysique de Paris in France, and the University of Tasmania in Australia, in remarks published by ESA. Beaulieu added that “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,” and noted that the new image includes 51 known planetary systems.

The technique’s particular value is that it does not favor any one kind of planet. “This technique is unbiased, we discover whatever is out there. It is uniquely suited to discover cold exoplanets,” said Natalia Rektsini of the Institut d’Astrophysique de Paris, according to ESA.

The Roman Connection

The survey’s most concrete purpose is to prepare for NASA’s Roman Space Telescope. The Euclid mosaic encompasses the entire region that the upcoming Roman space telescope will monitor, according to ESA. In NASA’s framing, Euclid took one day out from its six-year prime mission to preview the area of sky that will be targeted by Roman’s Galactic Bulge Time-Domain Survey, which will provide one of the deepest views ever into the center of our galaxy, according to NASA.

NASA notes that although Euclid’s one-time observation is shallower and lacks some of the color detail Roman will see, it has similar resolution and covers a larger region — about 5 square degrees, or the sky area covered by about 25 full moons. Crucially, the timing matters: stitching Euclid’s observation onto the front end of Roman’s collection will essentially extend the survey by two years, since Roman’s galactic bulge observations are set to begin in spring 2027, according to NASA.

“This sneak peek gives astronomers a major jumpstart on a core Roman survey, helping scientists learn more than they could from either telescope alone,” said Jason Rhodes, Senior Research Scientist at NASA’s Jet Propulsion Laboratory in Southern California, according to NASA.

What We Don’t Know

The survey is a baseline, not a catalogue of new planets. The headline applications — measuring masses and distances for previously detected but uncharacterized exoplanets — depend on years of follow-up. Researchers expect the data to help characterize roughly 60 previously detected but uncharacterized exoplanets, according to LSU, whose Matthew Penny, an assistant professor in the Department of Physics & Astronomy, co-leads Euclid’s exoplanet science working group. But those mass measurements will largely arrive once Roman is observing the same field and the two datasets can be combined.

The precision target is set in the Q2 paper itself rather than in the press materials: the survey is designed to enable microlensing lens-mass measurements to better than 10%, according to the Euclid Collaboration. Whether that precision holds across the crowded bulge, where blending of overlapping stars complicates photometry, will be tested as the catalogues are analyzed.

Analysis

The Euclid bulge image is a useful illustration of how modern space astronomy increasingly works as a relay rather than a sprint. A mission built to chart dark energy spent a single day pointed at the galactic center, and the resulting data’s primary value is to a different telescope, run by a different agency, that has not yet started its survey. For microlensing in particular, where a planet’s signature is a one-time, non-repeating brightening, having an earlier high-resolution snapshot of exactly which stars sit where is not a luxury — it is the only way to reconstruct what a star looked like before a future event overlaps it.

For one of the discoveries Beaulieu helped make two decades ago — the icy world OGLE-2005-BLG-390Lb, which he likened to Hoth from Star Wars — the new image carries a personal stake: the chance, finally, to pin down a precise mass, as he told ESA. It is a reminder that the planets this survey targets are not hypothetical. Many were detected years ago from the ground; what has been missing is the resolution and the temporal baseline to weigh them.