Astronomers have been unusually excited lately, and for good reason. A mysterious object from beyond our solar system is speeding past the Sun, offering scientists a rare opportunity to study material formed around another star. Known as comet 3I/ATLAS, it is only the third confirmed interstellar object ever discovered. Unlike planets or asteroids, it won’t stick around—think of it as a cosmic traveller briefly waving hello before vanishing forever.
On its one-time journey through the solar system, the comet made a relatively close pass by Earth, offering scientists a rare observational opportunity. On Friday, December 19, 3I/ATLAS passed our planet at a distance of about 168 million miles (270 million km). While that may sound immense, astronomically it is considered a relatively close encounter—though the comet remained nearly twice as far away as the Sun. Measuring only a few miles across, 3I/ATLAS was not visible to the naked eye, and scientists relied on telescopes to observe the flyby before it continued its journey out of the solar system.
To understand why this is significant, let’s start with the basics. Comets are often described as dirty snowballs, made of ice, dust, rock, and frozen gases. When they are far from the Sun, they are small, dark, and inactive. But as they approach sunlight, heat causes their frozen material to turn directly into gas. This process creates a glowing cloud, called a coma, around the comet’s core and often produces spectacular tails stretching millions of kilometers across space. Most comets we see come from distant reservoirs within our own solar system, but 3I/ATLAS is different. It did not form here at all.
An unexpected discovery
The story of 3I/ATLAS began when in June 2025, the NASA-funded ATLAS survey telescope in Chile captured faint images of what looked like a tiny smudge of light. At first, nothing seemed extraordinary. But when astronomers carefully tracked its movement, the surprise became impossible to ignore. This object was moving far too fast, and its path through space was far too stretched, to be gravitationally bound to the Sun. In other words, it wasn’t orbiting our star. It was just passing through.
That realisation immediately placed 3I/ATLAS into an exclusive category. Before it, astronomers had only identified two interstellar visitors: ‘Oumuamua in 2017 and Borisov in 2019. Unlike those earlier objects, however, 3I/ATLAS is unmistakably a comet, complete with a growing coma and a developing tail. And let’s get one thing clear right away — this visitor poses no danger to Earth. Its path keeps it comfortably far away, and its appearance is exciting, not alarming.
Awake before its time
As the comet plunged deeper into the inner solar system, the show truly began. By late summer, powerful ground-based telescopes such as Gemini South captured detailed images revealing a vast, fuzzy coma and a dusty tail tens of thousands of miles long. Space telescopes soon joined the effort. Hubble, TESS, NASA’s new SPHEREx observatory, and the James Webb Space Telescope all turned their attention to this fleeting target.
One of the first clues that something unusual was going on came from old data. Scientists later realised that 3I/ATLAS had already become active far beyond Jupiter’s orbit, at a distance where most comets remain frozen and quiet. This early awakening hinted that the comet contains ices that vaporise more easily than water, setting it apart from familiar solar system comets.
Built differently
The biggest surprise lies in the comet’s chemistry. When most comets warm up, water vapour dominates their atmospheres. With 3I/ATLAS, that rule seems to be broken. Observations show that it is releasing an extraordinary amount of carbon dioxide, far more than water. Measurements suggest a carbon dioxide–to–water ratio among the highest ever seen in any comet.
This comet seems to be fizzing with carbon dioxide gas. Scientists are still debating why. It may have formed in a region of its original planetary system where carbon dioxide ice was common, or its surface may be capped with CO₂-rich material that temporarily traps water ice below. Either way, this chemistry offers a rare glimpse into conditions around a star that is not our own.
There are other oddities too. Astronomers have detected an unusual amount of nickel, a lack of common carbon-based molecules, and a slightly reddish hue. That colour often signals extremely ancient, primitive material, suggesting the comet has remained largely unchanged for billions of years.
The backward tail
At one point, 3I/ATLAS seemed to defy expectations by growing a tail that pointed toward the Sun. This so-called anti-tail looks dramatic but has a straightforward explanation. Sometimes, the particles released by a comet are too heavy to be pushed away by sunlight. Instead, gravity pulls them inward, creating the illusion of a backward-pointing tail. Strange to look at, but perfectly natural.
A fossil of the galaxy
Perhaps the most astonishing thing about 3I/ATLAS is its age. Based on its speed and incoming direction, astronomers believe it originated in the thick disk of the Milky Way, a region populated by very old stars. Estimates suggest the comet could be between three and eleven billion years old. That makes it very likely older than our own solar system, which formed about 4.6 billion years ago.
This ancient traveler is also relatively large. Early measurements indicate that it is bigger than the two previously discovered interstellar objects, with a solid core several kilometers wide, wrapped in a growing cloud of gas and dust.
Interstellar objects like 3I/ATLAS are priceless scientific gifts. They carry direct evidence of how planets and comets form around other stars. In a sense, this comet is a messenger from another solar system and another era of cosmic history. Once it’s gone, it leaves behind more than just images — it leaves new ideas about how diverse and ancient our galaxy truly is.
