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This illustration captures the orbital path and current location of Comet C/2025 D1 (Gröller) as of March 11, 2025. Notably, not all terrestrial planets are depicted in this image.
Comets are renowned for their striking tails, which form as they approach the Sun. This proximity heat causes their icy materials to vaporize, releasing clouds of dust. Typically, this activity is only seen near the Sun, as comets tend to reside in the colder regions of the outer solar system for most of their orbits.
A noteworthy addition to the catalog of celestial bodies is Comet C/2025 D1 (Gröller), discovered by Hannes Gröller of the University of Arizona and an observer with the Catalina Sky Survey. Located deep within the solar system, between the orbits of Saturn and Uranus, this comet already exhibits signs of activity, including a visible coma and a broad tail, despite its considerable distance from the Sun. It stands out as one of a select few comets to demonstrate such behavior at this range.
Exceptional Activity at Great Distances
Only four other comets have demonstrated activity from more than 20 astronomical units (AU) from the Sun, a region that extends as far as Uranus’s orbit. C/2025 D1 (Gröller) outpaces them all, with its perihelion reaching a staggering 14.1 AU from the Sun, surpassing the previous record of 11.4 AU.
“Most comets show their activity within 3 to 5 AU of the Sun,” Gröller explained to Astronomy. At these distances, the Sun’s radiation initiates the sublimation of water-ice, spurring the typical activity seen in comets. The activity of Comet Gröller at such a far distance suggests an alternative mechanism at play, according to Gröller.
Characterized by a weakly hyperbolic orbit, this comet may venture out of the solar system permanently.
Identifying the Comet
This marks Gröller’s fourth comet discovery, yet the significance of this particular find, due to its extraordinary distance from the Sun, makes it particularly thrilling. While the Catalina Sky Survey primarily focuses on locating near-Earth asteroids, it occasionally uncovers comets during its surveys. “It’s a rewarding aspect of our work to have a comet named after us,” Gröller noted.
The Catalina survey employs a method involving multiple images of the same region of sky, allowing software to detect any objects that seem to shift position. Gröller or a fellow observer then reviews the detected objects to confirm their validity. If confirmed as new, the discovery is reported to the International Astronomical Union’s Minor Planet Center, which disseminates the information for broader observation, including by amateur astronomers.
Sam Deen, a dedicated amateur astronomer focused on tracking and researching comets, located archival images of C/2025 D1 (Gröller) tracing back to 2018, aiding in the refinement of its orbit and the assessment of its exceptional perihelion distance. During that period, the comet was over 21 AU from the Sun, beyond Uranus’s orbit. Remarkably, the earliest observations stemmed from the 90-inch Bok telescope on Kitt Peak, which Gröller also utilized for his initial discovery.
“Based on what we know, objects of this composition typically wouldn’t exhibit activity from such distances,” Deen stated. Hence, the unique attributes of this new comet and its ultradistant counterparts may indicate they are vastly different from regular comets, possibly representing ancient remnants from the solar system’s formation.
Related: The science of comets
A Unique Category of Comets
Research led by Man-To Hui from the Macau University of Science and Technology and others, published last year in The Astronomical Journal, investigated the four known ultradistant comets, asserting that they are likely dynamically new comets—bodies that have never previously entered the inner solar system from the Oort Cloud.
This research suggested that their unusual levels of activity at such distances imply these comets could be some of the most primitive small bodies in the solar system. The notion that they harbor supervolatiles—like carbon monoxide and carbon dioxide, which can sublimate even under low solar illumination—adds to their scientific intrigue.
One notable item among these distant comets is C/2014 UN271 (Bernardinelli-Bernstein), which is exceptionally large, measuring at least 75 miles (120 km) in diameter. It was the farthest comet from the Sun upon its discovery, sitting at 29 AU. Gröller’s comet was discovered at about 15 AU. “We aren’t entirely certain if its activity is due to its unique nature or its sheer size that allows it to gain enough heat to become active,” Deen noted. “If Pluto were to come within 20 AU of the Sun, it would likely show similar comet-like characteristics.”
Deen also speculated that the active ultradistant comets may have originally formed far beyond the Sun, distinguishing them from typical comets from the Oort Cloud, which are theorized to have been ejected from the inner solar system. If these ultradistant comets did form in the far reaches of the solar system, it is conceivable that we are witnessing their first-ever approach to the Sun.
“These objects might be incredibly primitive, even more so than typical dynamically new comets. We could potentially be examining new forms of ices that do not exist elsewhere in the solar system,” he posited. “There are very few entities that have ever been observed at distances greater than 20 AU without facing vaporization.”
Visibility of the Comet
Currently, Comet C/2025 D1 (Gröller) shines faintly at a magnitude of approximately 20.5. As it approaches perihelion on May 19, 2028, it is expected to brighten to around magnitude 18.5. Even now, with sufficient exposure times, amateur astronomers equipped with larger telescopes can capture images of the comet. Gröller mentioned that a colleague with a 14-inch telescope successfully photographed it after a cumulative exposure of about 35 minutes. As the comet brightens, it will become observable even for smaller telescopes provided adequate exposure times are utilized.
Those interested in tracking this comet can access additional information, including an ephemeris of its position and brightness, via JPL’s Small-Body Database Lookup.
Related: How to photograph comets
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