A comet can appear in observations collected for another purpose. Recognising one depends on reading the images as a record of movement as well as a view of the sky.
What Did Euclid Find?
Comet C/2026 S1 is a previously unknown comet that Euclid recorded in images taken between 16 and 24 September 2026. The International Astronomical Union’s Minor Planet Center confirmed it as a comet on 8 October 2026.
When observed, C/2026 S1 was approximately 1.3 billion kilometres from Earth. Its orbit arches above and below the plane in which the planets orbit the Sun, rather than following that broad, relatively flat region of the Solar System.
How Did Euclid Spot the Comet?
Euclid used software developed by Mikko Pöntinen of the University of Helsinki to identify streaks made by moving objects in its images. The software flagged a streak, and Pöntinen found the comet in five image sets by predicting where it would appear next.
Euclid captured the object in 16 exposures across five fields of view in the Euclid Deep Field North. A fuzzy cloud around the streak indicated that the object was shedding gas and dust, a sign of an active comet rather than just a moving point of light.
Before reporting the object to the Minor Planet Center, the team compared it with catalogues of known comets and asteroids. That check helped establish that the object was both comet-like and previously unknown.
Why Was a Galaxy Survey Looking at a Comet?
Euclid’s main science mission is to map distant galaxies and investigate dark matter and dark energy.
Euclid observes the sky with a visible-light camera and a near-infrared spectrometer and photometer. Its VIS instrument images 0.5 square degrees of sky at a time, an area equivalent to about 2.5 full Moons.
Comets are icy, dusty remnants from the formation of the Solar System about 4.6 billion years ago. As a comet approaches the Sun, its ice turns into gas, forming a glowing cloud called a coma and a tail.
Why Does the Discovery Matter?
Euclid’s discovery showed that its survey data can reveal previously unknown comets, even though the telescope was designed to make deep, steady images of distant galaxies. Before C/2026 S1, Euclid had detected comets, but those objects were already known.
Confirmation by the Minor Planet Center alerted ground-based astronomers, who could then make follow-up observations. ESA said the discovery and alert process provided a real test of how the Euclid and Comet Interceptor mission teams could coordinate.
What Is Still Unknown About C/2026 S1?
The Minor Planet Center’s orbital period for C/2026 S1 is expected to be refined as additional observations are collected. ESA describes the comet as following an orbit that will take thousands of years to complete.
The discovery does not make C/2026 S1 a target for every future mission: ESA said it is not suitable for the Comet Interceptor mission.
ESA said astronomers will continue to search Euclid’s data for more comets.
