The European Space Agency's Euclid space telescope has discovered a comet that appears in no catalogue. The images were taken between 16 and 24 September 2026. The Minor Planet Center confirmed the discovery on 8 October, and the object now carries the designation C/2026 S1 (Euclid).

For the mission, this is an unexpected result. Euclid was designed to take deep, steady images of distant galaxies. Fast-moving bodies such as comets are not part of its main job.

How software for cleaning images found a comet

The discovery was made by Mikko Pöntinen, a PhD student at the University of Helsinki, together with the Solar System Objects Science Working Group of the Euclid Consortium. He developed software that monitors small Solar System objects in Euclid's images. It flags "streaks", the trails of bodies that shift between exposures.

I was excited to see my software flag a "streak" smeared across several images – the result of a fast-moving object.Mikko Pöntinen, University of Helsinki

By predicting the object's position, Pöntinen then identified it in other images. The comet appeared in five image sets in the Euclid Deep Field North. Each field was imaged two to four times, giving 16 exposures in total.

Further inspection showed a fuzzy cloud around the object. That is a sign of an active body: comets shed dust and gas as their ice turns to vapour.

From suspicion to an official comet

First, the object was checked against catalogues of known comets and asteroids. No match turned up, so Pöntinen reported the observations to the International Astronomical Union's Minor Planet Center. The center collects asteroid and comet observations from around the world, confirms new discoveries, calculates orbits and assigns official designations.

The report alerted ground-based astronomers, who contributed follow-up observations. The object was then confirmed as a comet. Its orbital period will be refined once more data has been collected.

The comet is now about 1.3 billion km from Earth, roughly between the orbits of Jupiter and Saturn. It does not lie in the plane of the planets. It is "straight up" from the Sun's perspective, on an orbit that arches over the Solar System and will take thousands of years to complete.

Why it was hard to find

Euclid deliberately avoids the band of sky along the plane of the Solar System. That region is rich in asteroids and comets, and its dust scatters light that would hinder sensitive deep-sky imaging. As a result, the telescope rarely finds new Solar System objects.

On top of that, Euclid uses a "step-and-stare" strategy. It typically visits each section of the sky only once. A comet cannot simply be revisited to confirm its characteristics.

Bruno Altieri, Euclid Archive Scientist and part of the discovery team, says very few comets are expected from Euclid. He calls it a happy coincidence that the comet's orbit and the telescope's scanning over those days captured the object in multiple images.

What's new

Euclid has detected comets before, but only ones that were already known. C/2026 S1 (Euclid) is the first new comet the telescope has discovered itself.

Tracking Solar System objects also serves the main mission. Euclid has to identify everything in its field of view and remove "noise" such as comet trails and extra light. Otherwise it would interfere with precise galaxy mapping. Over six years, the mission will observe the shapes and motions of billions of galaxies out to 10 billion light-years. Its goal is to reveal the hidden influence of dark matter and dark energy on the visible Universe.

What comes next

This comet is not a suitable candidate for ESA's future Comet Interceptor mission. But the quick turnaround between identification and validation shows that Euclid can detect new comets while they are still relatively far from the Sun. Such objects could potentially become targets for Comet Interceptor.

Valeria Pettorino, Euclid Project Scientist, calls the case a real test of how the two mission teams will coordinate in the future. The Solar System community also gains a previously unknown comet to follow up.

Comets are icy, dusty leftovers from the birth of the Solar System about 4.6 billion years ago. They have changed very little since, so they help scientists understand the raw ingredients of the planets. Among other things, they may offer clues about where Earth's water and the building blocks of life came from.