An international team of astronomers led by the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences has reported the first comprehensive study of C/2025 D1 (Groeller), a comet that will make the most distant closest approach to the Sun of any known comet. 

The findings, published online on December 5 in Astronomy & Astrophysics, show that this icy wanderer, provides a unique opportunity to study the ancient materials that formed our solar system billions of years ago.

Discovered in February 2025 by Hannes Gröller of the Catalina Sky Survey, C/2025 D1 (Groeller) will swing as close as roughly 14 astronomical units (AU), or about two billion kilometers away from the Sun. This places its closest approach beyond the orbits of all planets except Neptune and Uranus, making it the farthest "inbound" comet ever documented.

By examining archival images from telescopes in Hawaii and Arizona, the researchers traced the comet back to a time when it was over 20 AU (more than 3 billion kilometers) from the Sun. Even at that extreme distance, the comet had already begun to glow as sunlight turned its frozen gases, such as carbon monoxide, into vapor and pushed dust away. This formed a tenuous atmosphere and a hazy glow, called a coma, around the comet.

Unexpectedly, the comet began to fade starting in late 2023, despite still heading inward to the Sun, where it should have been growing brighter. The researchers speculated that this slowdown could be linked to changes in the types of ice driving the activity, such as a transition from carbon monoxide to carbon dioxide. Alternatively, the comet's water ice could have shifted from an amorphous to a crystalline form. Such transitions tend to quickly release heat and would heat up trapped gases inside the comet, causing sudden bursts in the comet's activity. It is also possible that the comet's supply of easily released gases was running low. Astronomers will continue searching for other comets exhibiting similar behaviors to better understand what is happening.

The comet also stands out for its reddish hue, which makes it appear warmer than most known comets. By analyzing its activity and how its dust scatters sunlight, the researchers estimated that the comet's solid core is at least 0.4 kilometers long—roughly the length of four football fields.

Most excitingly, detailed path calculations confirm that C/2025 D1 (Groeller) is a "newcomer" to the inner solar system. It hails from the Oort Cloud, a distant shell of icy objects surrounding our solar system like a cosmic bubble wrap, about six million years ago. Its last close pass by the Sun was well over the planetary region, and after this visit, the gravity of giant planets such as Jupiter is likely to fling it out of the solar system forever.

"This comet is a frozen time capsule from the dawn of our solar system," said lead author Dr. Man-To Hui (XU Wentao) of SHAO. "Watching it wakes up so far out from the Sun helps us understand how these ancient relics behave when they finally feel the warmth of the Sun after millions of years in the cosmic deep freeze."

With powerful new surveys such as the Chinese Space Station Telescope and the Vera C. Rubin Observatory come online, astronomers anticipate to find more of these distant travelers, shedding light on how our planetary family formed and evolved.

Selected archival observations of comet C/2025 D1 (Groeller). In some of the panels where the comet appears faint, a dotted white circle marks its location. The white and magenta scale bars represent an angular distance of 10′′ and a linear distance of 105 km projected at the observer centric distance of the comet, respectively. J2000 equatorial north is up, and east is to the left. In each panel the antisolar direction (yellow arrow) and the projected negative heliocentric velocity of the comet in the observer's sky plane (dark orange arrow) are also indicated. (Image by SHAO)