LESSONS FROM INTERSTELLAR COMET 3I/ATLAS

Humans have only ever observed three known interstellar comets. While this latest one, 3I/ATLAS, appears to be pretty similar to comets closer to home, Elizabeth Howell discovers that there was just enough weirdness to keep the scientists on their toes.

Elizabeth Howell

Interstellar comet 3I/ATLAS was found on 1 July 2025 by a NASA-funded telescope array called ATLAS (Asteroid Terrestrial-impact Last Alert System). Then this January, the comet made its closest approach to our planet at around double the distance of the Sun. That might seem close, but it put on a spectacular show. The comet appeared relatively bright even in amateur telescopes and for professional astronomers it afforded the chance for some serious studies to take place. One of the astronomers that the comet kept very busy was Darryl Seligman, an assistant professor in the physics and astronomy department at Michigan State University.

Unlike the sensationalist content that can be found online, Seligman told Astronomy Now that, in some ways, the most surprising thing about the comet was how typical it was. “By and large, this object is behaving exactly like all the other comets that we’ve ever seen,” he says.

For example, shortly after discovery it was found that 3I/ATLAS had a short sunward-facing tail, as observed by a team using the Vera C. Rubin Observatory during the commissioning period. This anti-tail caused a lot of debate online. So much so, that you would be forgiven for thinking that it was the first time such a phenomenon had ever been seen. The truth is that astronomers have seen such features before – in Solar System comet C/2014 UN271 (Bernardinelli–Bernstein) to name just one.

According to a large team of astronomers led by Northern Arizona University’s Colin Orion Chandler, anti-tails are attributed to the “slow ejection of relatively large dust particles, predominantly from the sunlit hemisphere.” This result is published on the preprint server Arxiv (arxiv.org/pdf/2507.13409).

Nevertheless, Seligman pointed out some others features on the new comet that are “weird and unique, which make it super interesting.”

For example, 3I/ATLAS is very, very bright. Following its discovery, astronomers looked back through archival images and found that NASA’s Transiting Exoplanet Satellite Survey (TESS) spacecraft had imaged the comet in May 2025, almost two months before its discovery.

“The reason that’s so amazing is that [in May] 3I/ATLAS was around 6-ish AU,” says Seligman. An astronomical unit (AU) is the distance between the Earth and the Sun and 6 AU is extraordinarily far away to see a comet. This is because the Sun’s heat at that distance is unable to transform water ice into water vapour. Only when the comet reaches 2–3 AU does this usually happen, creating the cloud of gas known as the coma that reflects sunlight and gives the comet its brightness.

Not only does this have implications for understanding the composition of comets, it could also open the door to new discoveries. Other interstellar objects could yet be lurking in archival images. The only reason they have yet to be discovered is because astronomers assumed comets would not be that bright so far away.

As such, a potential gold rush of imagery could eventually arise, showing interstellar visitors that have passed us by in the past, unobserved. “There’s lots of interstellar objects that are probably sitting in our surveys [that] we haven’t found, so that is the most exciting aspect of it to me.”

So bright, so distant

So why was 3I/ATLAS so bright while so distant? It must have something to do with its composition, which in turn gives astronomers clues about where the comet might have come from. In our own Solar System, comets are leftovers from the early days, when gas molecules and small bits of dust clumped together. This was the process that eventually produced planets.

Studying comets, made of ice and dust, and asteroids, made mostly of dust, allow astronomers to trace the early history of our Solar System and the Sun.

Unfortunately, we cannot say which star 3I/ ATLAS came from, but we can make inferences about the star’s environment from 3I/ATLAS’s composition. For example, the James Webb Space Telescope spotted a surprising amount of carbon dioxide ‘outgassing’ or leaving 3I/ ATLAS as the comet approached our Sun. As weeks went by, more carbon monoxide and water emerged.

Seligman says it is common for emissions to change their composition as the comet gets closer to the Sun. “As the comet moves through its trajectory and it’s blowing off stuff, you’re getting into deeper and deeper parts of the nucleus, because the stuff at the very surface is going away, right?” So the changes in the emissions are telling us about the comet’s deeper composition.

That deeper composition suggests how far 3I/ATLAS likely formed from its star. In our own Solar System, carbon dioxide tends to be common in comets that form beyond Jupiter, while carbon monoxide only usually pops up in comets that formed beyond Neptune and in the Oort Cloud, the large collection of comets believed to be circling the extreme edge of our Solar System.

“We’re still trying to figure out exactly what 3I/ATLAS is made of, and how that material got processed,” says Seligman, adding that as the comet moves through the interstellar medium (the space between stars), its composition may also have been affected.

“But fundamentally,” he says, “the chemistry that we’re seeing is probably telling us something along the lines of wherever 3I/ATLAS actually formed. Whichever star it was, 3I/ATLAS must have formed in an environment that’s much colder than most of our Solar System’s comets.”

The changing composition of the comet may have had other observational consequences too.

In early January, the Gemini North telescope in Hawai’i found that the comet was changing from the reddish hue first spotted in its twin, Gemini South in Chile, to more of a greenish tinge as it approached the Sun.

Researchers attributed this to the changes in the compositional layers that were being unearthed by the increasing solar intensity as the comet sped closer. As the comet heated up, carbon molecules began to evaporate and these are known for their green glow. But what happens as the comet recedes from the Sun and cools down? Will it remain green, or go back to red?

A typical interstellar comet?

Intriguingly, the composition of 3I/ATLAS may be typical of interstellar visitors. At least one other interstellar comet, 2I/Borisov, also appeared to be highly enriched in ‘hypervolatiles’ (such as carbon monoxide), suggesting it too had formed far from its own star. This could be telling us that comet formation, maybe even planet formation, is able to take place far beyond where we usually think of these processes being possible around stars.

The only way for us to know for sure is to find and study more of these fascinating interlopers. And that is something that Seligman says we can all look forward to. “Rubin’s going to find way more of them. We should have many more, and then we’ll be able to … really get insights into exoplanet formation throughout the Galaxy.”

It is the pioneering nature of these first interstellar object observations that makes it all so enriching because, as with all emerging fields of science, there is so much to learn.

Sky survey images are therefore likely to be in high demand in the coming years, as well as AI programs to ferret out valuable information about whether other interstellar objects lie within them. If more are indeed lurking in the archives, as Seligman predicts, this will help add to our understanding of the origins, chemistry and behaviour of other comets that are visitors to our Solar System.

Maybe, by association, we can finally learn more about whether Earth and its neighbourhood are unique, or if habitable planetary systems like ours are commonplace.

Elizabeth Howell is a freelance astronomy journalist based in Canada.