A mosaic taken by the European Space Telescope over about 26 hours is the largest and most detailed visible-light image of the center of the galaxy. The image is not intended to immediately discover new planets, but to provide a reference point that will allow future discoveries to be confirmed and their masses to be measured using tiny gravitational lensing.
The space telescope Euclid של European Space Agency photographed the largest and most detailed visible-light mosaic ever created of the dense central region ofThe Milky WayThe image shows more than 60 million stars, along with nebulae and star clusters.
Euclid was primarily designed to study billions of distant galaxies and examine the influence of dark matter and dark energy on the evolution of the universe. However, for one day, the telescope turned its gaze to one of the brightest and busiest areas in the sky – the galactic bulge at the center of the Milky Way.
The mosaic was taken on March 23, 2025, over a period of about 26 hours, and consists of nine fields of view from the VIS camera. Each field covers an area larger than the full moon in the sky.
Despite the enormous density of stars, the camera was able to separate a large number of individual light sources without being blinded by the glow of the entire region. This ability will make the image an important reference point in the search for exoplanets using Tiny gravitational lens.
Sharpness that resembles mourning in a field hundreds of times wider
The sharpness and sensitivity of the VIS camera in visible light are similar to those of the wide-field camera on the Hubble Space Telescope. The difference is in the size of the imaged area: each Euclid field covers an area 270 times larger than the corresponding Hubble field of view.
According to the European Space Agency, it would have taken the Keck Observatory in Hawaii about 2,000 hours of observation to cover the entire area included in the Euclid mosaic from the ground. Euclid completed the mission in a little over 24 hours, and in space it is unaffected by the disturbances of the Earth's atmosphere.
The mosaic also includes the entire area thatNancy Grace Roman Space Telescope NASA's Euclid is scheduled to follow it as part of its search for planets. This means that Euclid has already imaged the stars that will participate in future lensing events that Roman may detect – even before the stars happened to line up along that line of sight.
How a star becomes a magnifying glass
Tiny gravitational lensing is a small-scale version of the gravitational lensing phenomenon that Euclid typically uses to study galaxy clusters and dark matter.
When a relatively close star passes almost exactly in front of a more distant star, its gravity warps the space around it. As a result, the light from the distant star is bent and focused, making it appear brighter for a limited time.
If a planet orbits the nearby star, the planet's gravity also affects the light. It creates a small, brief change in the brightening pattern, from which we can conclude that there is a planet in the system.
Such lensing events depend on rare geometric coincidences and do not usually recur. Therefore, astronomers need to continuously monitor regions where the density of stars is particularly high. The central bulge of the Milky Way is one of the most suitable areas for this.
According to Jean-Philippe Beaulieu of the Institut d'Astrophysique de Paris and the University of Tasmania, almost 300 planets have been discovered in the past 20 years using tiny gravitational lensing. All were found in ground-based observations, and most searches focused towards the center of the galaxy.
Euclid's new mosaic contains 51 planetary systems that were already known to astronomers.
The photo did not reveal any new planets.
To detect a tiny gravitational lensing event, the star must be monitored for more than twenty days. This allows us to measure the gradual change in brightness and look for the small deviation caused by a planet.
Because Euclid observed the region for only one day, the new mosaic cannot by itself reveal new lensing events. Its importance lies in the fact that it records the region at an early moment and clearly separates the dense stars.
In the future, when Roman or other telescopes detect a lensing event in the same region, it will be possible to compare the new observations to the image taken by Euclid in 2025. The time difference will allow us to measure how the foreground and background stars moved relative to each other.
Measuring the relative motion can help astronomers separate the two stars, confirm that a planet is causing the change in light, and calculate its mass. Data from just one point in time is usually not enough to make this measurement.
As the years pass, the angular distance between the star that acted as a lens and the background star will increase, and therefore the value of Euclid's image as a reference point is expected to increase.
Advantage in searching for cold worlds
Common methods for planet detection, including measuring transits in front of the star or its radial velocity, tend to favor large planets that orbit their star in short orbits.
Tiny gravitational lensing is also sensitive to cold planets that are far from their star. It can even reveal free-floating planets that are not orbiting a star, because the detection is based on the object's gravity rather than the light it emits or its periodic passage in front of a star.
Two known cold planets appear in the new Euclid data.
One of them is OGLE-2005-BLG-390Lb, an icy world discovered about 20 years ago. Boulier, who led the team that discovered it, likened it to the icy planet Hoth from the Star Wars series. The Euclid data may finally allow for a more precise measurement of its mass.
Another system is OGLE-2013-BLG-341Lb, which includes two stars and one planet. Combining Euclid data with previous observations from the Keck Observatory and Hubble may allow us to separate the stars and confirm the mass of the planet.
Not just planets
The mosaic of the center of the Milky Way will have other scientific uses. It will be possible to search for brown dwarfs, study binary star systems, and measure the motions of stars in the central region of the galaxy.
The data will also help map interstellar dust, which obscures much of the Milky Way's center in visible light. Comparing the colors and brightnesses of millions of stars can reveal how the dust is scattered and how it alters the light passing through it.
The original VIS image is in black and white. To create the color image released to the public, mission personnel combined data from the Canada-France-Hawaii Telescope on Mauna Kea.
Euclid was launched in July 2023 and began its routine scientific observations on February 14, 2024. During six years of planned operation, it is expected to measure the shapes, distances, and motions of billions of galaxies at distances of up to about ten billion light-years.
The extraordinary observation at the heart of the Milky Way shows that even a telescope designed to study the distant universe can provide a new perspective on the galaxy in which we live – and help consider worlds that are not directly visible.
FAQ
Did Euclid discover new planets in the picture?
No. The observation lasted about 26 hours, while detecting a tiny gravitational lensing event typically requires more than twenty days of observation. The image will serve as a reference point for confirming and measuring future planet discoveries.
How can you measure the mass of a planet using an old photo?
After the lensing event, the lensing star and the background star continue to move relative to each other. Comparing their positions in the early Euclid image with future observations allows us to measure the relative motion and reduce the uncertainty in calculating the planet's mass.
Why look for lensing events near the center of the Milky Way?
For a lens to form, two stars must align almost exactly along the same line of sight. The central region of the galaxy contains millions of stars in a small area, so the chance of a coincidental alignment is higher.
What is special about gravitational lensing compared to other methods?
The method is particularly sensitive to cold planets far from their star, and sometimes even free planets. It does not depend on the planet emitting light or repeatedly passing in front of the star.
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