In 24 hours, Euclid captured the stars of all future exoplanet discoveries
In March 2025, the European Space Agency's Euclid telescope spent a single day gazing into the ancient, star-crowded heart of the Milky Way — not as its primary purpose, but as a gift of circumstance. The resulting portrait of 60 million stars is less a scientific result than a foundation: a frozen moment in cosmic time against which future telescopes will measure the subtle bending of light that betrays hidden worlds. Humanity's search for planets it cannot yet see has always required patience; now, for the first time, it has a map.
- Euclid was never meant to hunt exoplanets, yet a 26-hour detour from its dark-matter mission produced one of the most consequential images in modern planetary science.
- The galactic bulge — a relic of the Milky Way's violent youth, packed with 10 billion ancient stars — is the only place in the sky crowded enough for gravitational microlensing to work at scale.
- Without a historical baseline showing where each star stood before any alignment occurs, microlensing detections remain incomplete; Euclid's image solves that problem in a single observation.
- The Nancy Grace Roman Space Telescope will now scan the same region in real time, cross-referencing Euclid's snapshot to confirm planets and calculate their masses with a precision previously out of reach.
- Roughly 1,400 cold exoplanets — including hundreds smaller than three Earths — are expected to emerge from this collaboration, among them worlds that no other detection method could find.
- An icy planet discovered twenty years ago and nicknamed Hoth has never had its mass precisely measured; Euclid's data may finally close that open question, illustrating how one image can reopen decades of unfinished science.
In March 2025, ESA's Euclid space telescope turned away from its primary mission of mapping dark matter and spent 26 hours staring into the Milky Way's galactic bulge. The result was a single image containing 60 million stars — nebulae and ancient star clusters included — assembled from 10 separate pointings, each wider than the full moon. The bulge is a relic of the galaxy's formation, home to roughly 10 billion old, red stars born some 10 to 11 billion years ago. Euclid was not designed for this work, but its wide-angle camera and extraordinary speed made it uniquely suited to the task.
The image's deeper purpose lies in a technique called gravitational microlensing. When two stars align from Earth's perspective, the foreground star's gravity warps spacetime and magnifies the background star's light. A planet orbiting the foreground star leaves a faint distortion in that brightening — detectable, but only if you know what the sky looked like before the alignment happened. Euclid's image is precisely that: a time-stamped reference showing where every star stood before any future encounter occurs. It already contains 51 known planetary systems.
When the Nancy Grace Roman Space Telescope begins its 15-month survey of the same region, it will catch microlensing events as they unfold. Astronomers will then reach back into Euclid's archive to reconstruct each star's prior position and motion, confirming planets and measuring their masses with new precision. The Roman mission is expected to detect around 1,400 cold exoplanets — worlds in the deep freeze of wide orbits — including roughly 300 smaller than three Earths. These are planets that transit surveys and radial-velocity measurements would likely never find.
Among the beneficiaries of this collaboration is OGLE-2005-BLG-390Lb, an icy world nicknamed Hoth after the frozen Star Wars planet, discovered two decades ago by astronomer Jean-Philippe Beaulieu — the same scientist who initiated Euclid's bulge survey. Despite twenty years of study, Hoth's mass has never been pinned down. Euclid's data may finally provide the answer. One opportunistic observation, taken during a thermal constraint that briefly redirected the telescope, has quietly become the cornerstone of a decade of exoplanet discovery.
In March 2025, the European Space Agency's Euclid space telescope pointed itself toward the crowded heart of the Milky Way and spent 26 hours collecting light. What emerged was a portrait of staggering density: 60 million stars, captured in a single image, their light traveling across space to land on Euclid's wide-angle camera. The telescope wasn't designed for this work. Its primary mission is to map the universe's dark matter and dark energy by measuring the redshift of distant galaxies. But Euclid has a gift for bonus science, and this image of the galactic bulge—that ancient, tightly-packed region at the galaxy's center—would become something far more useful than a pretty picture.
The bulge itself is a relic. It contains roughly 10 billion stars, most of them old and red, born when the Milky Way was young, around 10 or 11 billion years ago. In Euclid's image, the telescope captured not just stars but nebulae and star clusters too, all compressed into a region so dense that even a space telescope needed to break the observation into 10 separate pointings, each covering an area larger than the full moon. Yet Euclid did this faster than any ground-based telescope could manage. The speed mattered. The precision mattered more.
The real value of this image lies in a technique called gravitational microlensing, a method for finding exoplanets that works only in crowded star fields. When two stars align from Earth's perspective, the gravity of the foreground star warps spacetime itself, acting as a lens that magnifies and brightens the background star. If a planet orbits that foreground star, it creates a tiny, detectable distortion in the light pattern. Ground-based telescopes have found nearly 300 exoplanets this way over the past two decades, all of them toward the galactic center. But they work blind, observing the sky without knowing which stars will align. Euclid's image changes that equation entirely.
Jean-Philippe Beaulieu, an astronomer at the Institut d'Astrophysique de Paris who initiated Euclid's bulge survey, explained the logic: "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." The 60-million-star image already contains 51 known planetary systems. More importantly, it serves as a time reference—a snapshot of where every star was before any future alignments occur. When the Nancy Grace Roman Space Telescope begins its own 15-month survey of the galactic bulge, it will detect gravitational microlensing events in real time. Astronomers will then look back at Euclid's data to see how those same stars appeared months or years earlier, measuring their motion and using that information to confirm planets and calculate their masses with precision.
Natalia Rektsini, who led the release of Euclid's data, described the power of this approach: "In 24 hours, Euclid has already captured the stars involved in all the future microlensing events that the Roman space telescope will detect, but before the stars and planets involved have aligned." This matters because gravitational microlensing is unbiased in a way other exoplanet-detection methods are not. The transit method finds large planets on close orbits more easily than small planets on distant ones. Microlensing finds whatever is there. Astronomers expect every star in the Milky Way to host at least one cold exoplanet—worlds orbiting far from their host stars, in the deep freeze. The Roman mission is expected to find about 1,400 of them, including roughly 300 with masses smaller than three Earths.
For some exoplanets, Euclid's data will finally answer questions left hanging for years. Jean-Philippe Beaulieu led the team that discovered OGLE-2005-BLG-390Lb two decades ago, an icy world nicknamed Hoth after the frozen planet in Star Wars. Despite two decades of study, its mass has never been precisely measured. Euclid may change that. The telescope's ability to resolve individual stars clearly, combined with its time-reference data, will allow astronomers to track stellar motion and use that motion to constrain the masses of orbiting planets. It is a gift that keeps giving: one 26-hour observation, taken during a window when Euclid's thermal constraints forced it away from its primary mission, has become the foundation for a decade of exoplanet science.
Citas Notables
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.— Jean-Philippe Beaulieu, Institut d'Astrophysique de Paris
This technique is unbiased, we discover whatever is out there. It is uniquely suited to discover cold exoplanets.— Natalia Rektsini, Institut d'Astrophysique de Paris
La Conversación del Hearth Otra perspectiva de la historia
Why does Euclid need 26 hours just to photograph stars? Isn't it in space, above the atmosphere?
It's not about atmospheric distortion—Euclid is above that. The 26 hours comes from the telescope's design. It had to break the observation into 10 separate pointings, each one covering an area the size of the full moon. That's how you build a detailed, high-resolution image of such a vast region.
But the real question is: why does this particular image matter so much? It's just stars.
It's a time machine. When the Roman Space Telescope finds exoplanets through gravitational microlensing in the coming years, astronomers will look back at Euclid's image to see where those same stars were before they aligned. That historical snapshot lets them measure how fast the stars moved, which tells them the planet's mass.
So Euclid is doing work it wasn't designed to do.
Exactly. Its main job is studying dark matter and dark energy in distant galaxies. But Euclid can only observe the galactic bulge twice a year, during the equinoxes, when pointing at its primary targets wouldn't be productive. So it looks at the bulge instead—and that happens to be perfect for exoplanet hunting.
How many exoplanets could this lead to?
The Roman mission alone expects to find about 1,400 cold exoplanets using Euclid's data as reference. These are worlds that other detection methods would miss—planets in the deep freeze, far from their stars. Gravitational microlensing is unbiased. It finds what's actually there.
And Euclid captured 60 million stars in one image?
Yes. The galactic bulge is ancient and dense—10 billion stars born when the galaxy was young. Euclid's wide-angle camera caught 60 million of them, along with nebulae and star clusters. It's crowded enough that chance alignments happen regularly, which is exactly what you need for microlensing to work.