Euclid Space Telescope Maps 60M Stars in Milky Way Core for Exoplanet Discovery

For a single day in June, the Euclid Space Telescope broke from its usual mission and pointed itself at something almost too bright to study: the dense, luminous heart of the Milky Way. The galactic bulge—that crowded region of stars and dust surrounding the black hole at our galaxy's center—is not where Euclid was designed to look. But astronomers had a specific reason for the request, and what the telescope captured in those 24 hours has already begun reshaping how scientists hunt for planets beyond our solar system.

Euclid's primary job is to map the distant universe, to peer at billions of galaxies so far away that their light has traveled for billions of years to reach us. Its visible-light camera was built with the sensitivity to do this without being overwhelmed by glare. That same precision, it turned out, could work on a crowded stellar neighborhood much closer to home. In a single pointing, Euclid recorded more than 60 million individual stars, along with nebulae and star clusters, all rendered with a sharpness that rivals the Hubble Space Telescope. But here is what makes Euclid remarkable: while Hubble's wide-field camera sees a certain patch of sky, Euclid's view of the same region is 270 times larger. To capture what Euclid did in one day, the ground-based Keck Observatory would need roughly 2,000 hours of observation time. Euclid is faster, and it can detect fainter stars that would vanish into noise from Earth's atmosphere.

The reason astronomers wanted this image has to do with a technique called gravitational microlensing, a method for finding exoplanets that works in a way almost no other technique does. Imagine two stars aligned so that one passes directly in front of the other from our vantage point. The nearer star's gravity acts as a cosmic lens, bending and magnifying the light from the distant star. If a planet orbits that nearer star, its gravity creates a small additional distortion in the light pattern—a tiny, telltale wobble in brightness. That wobble is the fingerprint of a world. Over the past two decades, ground-based telescopes have used this method to discover nearly 300 exoplanets, nearly all of them in the direction of the galactic center, where stars are densely packed enough to make such alignments reasonably common.

What makes Euclid's image extraordinary is not that it will immediately reveal new planets—detecting a microlensing event requires watching a star for more than 20 days, far longer than Euclid's single observation. Instead, the image serves as a time reference, a snapshot of the galactic bulge before future alignments occur. When the Roman Space Telescope or other observatories detect a microlensing event in the coming years, astronomers can compare those observations to Euclid's data from the past. By measuring how the stars have moved between the two observations, they can calculate the precise mass of any planet involved. This would be impossible with data from a single moment in time. Jean-Philippe Beaulieu, an astronomer at the Institut d'Astrophysique de Paris who originally proposed the Euclid observation, noted that the image already contains 51 known planetary systems and will help scientists study many more yet to be discovered.

Microlensing has a unique advantage over other exoplanet-hunting methods. Most techniques favor finding large, hot planets orbiting massive stars—worlds that are easy to spot because they block starlight or create strong signals. Microlensing is unbiased. It reveals whatever is actually there. This makes it exceptionally good at finding cold, distant planets orbiting their stars in the outer regions where Earth-like worlds might exist. Scientists expect that nearly every star in the Milky Way hosts at least one such planet, yet most remain undiscovered. Two known cold exoplanets appear in Euclid's data. One, discovered 20 years ago, is an icy world somewhat like the fictional planet Hoth from Star Wars. The other is a rare binary system—two stars with one planet—that Euclid's data has finally allowed astronomers to fully resolve and confirm. These are not abstract achievements. They represent the first concrete payoff from a space telescope that was built to study the universe's largest structures, now being repurposed to measure the smallest ones. As more observations accumulate and as Roman and other missions detect new microlensing events, Euclid's single day of observation will become an increasingly valuable reference point, a fixed moment in time against which the motion of stars and the presence of hidden worlds can be measured.