ESA's Euclid Space Telescope Discovers a New Comet

(Credit: ESA)

C/2026 S1 (Euclid) was discovered in a field of distant galaxies by the European Space Agency's (ESA) Euclid space telescope and announced on October 19, 2026, in images taken between September 16 and 24, 2026. Comets were not in the original plan for Euclid. The satellite's primary goal is to examine the universe's large-scale structure and address two of the biggest mysteries in astronomy, dark matter and dark energy. However, no other ESA space telescope had discovered a new comet in some time.

Euclid launched on July 1, 2023, and was designed to study billions of galaxies across a significant stretch of sky. With this data, scientists are aiming to learn about the distribution of galaxies throughout the universe and how that distribution has evolved over billions of years. Dark matter is an invisible substance that scientists think constitutes most of the universe's matter, and dark energy is the proposed name for whatever is causing the universe's expansion to accelerate. Mapping galaxies and measuring the extent to which they are distorted by gravity can shed light on both of those.

(Credit: ESA)

But as Euclid was doing the job it was designed for, something significantly more local showed up in the images. Mikko Pöntinen, a scientist at the University of Helsinki, had been designing software to mine Euclid's data for moving objects, and while looking at images from the Euclid Deep Field North survey, the program uncovered a tiny trail of light that streaked across several observations. Unlike the distant stars and galaxies, which hardly shifted in the images, this object was easily distinguishable against the background.

Pöntinen and his colleagues found the object in five sets of images. When zooming in, they discovered that the object was surrounded by a faint, blurry cloud. This is often called a coma and is frequently observed around active comets as gas and dust are jettisoned from the solid body into space. The motion of the object and its surrounding coma indicated that the object was a comet, not a regular asteroid or a distant galaxy.

In order to confirm that it hadn't been previously spotted, the team compared its position against those of known comets and asteroids. The searches proved fruitless, leading to the observations being filed with the Minor Planet Center, an organization that records known small objects in the Solar System and operates under the International Astronomical Union. Other astronomers then confirmed the discovery with their ground-based telescopes, and the comet was officially recognized on October 8, 2026.

Why C/2026 S1 is so intriguing is because of its distance and unconventional orbit. When the announcement was made, the comet was roughly 1.3 billion kilometers, or 800 million miles, away from Earth, a distance comparable to the region between the orbits of Saturn and Jupiter, although the comet is on a trajectory that goes way above the plane of the orbits of the planets in the Solar System.

Early orbital estimates provided by the Euclid Consortium suggest it takes about 345,000 years for the comet to orbit the Sun. If that estimate is correct, the comet last visited the inner Solar System long before recorded human history. The object may reach 9,797 AU from the Sun (1 AU is approximately the distance from Earth to the Sun), according to the Euclid Consortium. Astronomers will continue to update these estimates as they gather more observations.

While comets themselves aren't particularly uncommon within the Solar System, most are hidden from view because they are generally located so far from the Sun. They consist mainly of ice, dust, and other rocky debris left over from the formation of the Solar System some 4.6 billion years ago. When a comet nears the Sun, some of its ice can sublimate (turn directly into a gas), carrying dust particles from the surface and creating the gaseous coma and a long tail.

Scientists want to study comets because they contain pristine material from the early Solar System. While planets have undergone significant geological changes over the past billions of years, many comets have remained relatively unchanged and have become time capsules of the early days of the Solar System. Some researchers are even looking into whether comets could have brought water or organic compounds to Earth.

That said, what's been found is pretty exciting, as Euclid typically steers clear of the part of the sky where the majority of Solar System objects are located. Its pointings are aimed at distant galaxies, and moving objects tend to throw off scientists' measurements of things like galaxy brightness. Euclid usually takes a shot of any given region of the sky only once, so it's tough to follow a target through the sky over several days. But this comet's strange orbit and Euclid's pattern of observations happened to align.

While Euclid has previously taken images of known comets, C/2026 S1 is the first previously unknown comet it's uncovered through the telescope's observations. This discovery also demonstrates that its vast repository of images may be useful for studies beyond its main science goal. Moving-object detection software may find other comets and asteroids scattered among the galaxies Euclid observes.

ESA is also interested in this discovery method for one of its future missions, Comet Interceptor. The spacecraft is intended to examine a comet that has not experienced multiple close encounters with the Sun, potentially allowing scientists to take a peek at pristine material from the frigid outskirts of the Solar System. C/2026 S1 might not be a good choice for Comet Interceptor to encounter, but the way it was discovered and verified could point astronomers to future candidates.

Euclid will continue its planned six-year survey of the universe, looking at faraway galaxies and gathering data about dark energy and dark matter. Meanwhile, scientists will continue to scan through the images for objects in our Solar System. The discovery of C/2026 S1 shows that even a telescope designed to look billions of light-years away can discover something new in our own Solar System.