Joe Yates
A mysterious object hurtling through our solar system has once again left experts scratching their heads.
The interstellar visitor known as Comet 3I/ATLAS - also dubbed the 'interstellar invader' - has suddenly begun to brighten at a much faster rate than scientists ever expected, and no one can quite explain why.
3I/ATLAS is only the third known object to have entered our solar system from another planetary system - joining the bizarre, cigar-shaped ‘Oumuamua (discovered in 2017) and comet 2I/Borisov (spotted in 2019) - and these rare cosmic visitors offer scientists a glimpse into the materials that make up planets and stars far beyond our own corner of the galaxy.
Researchers had predicted that 3I/ATLAS would glow more brightly as it neared its closest approach to the Sun - an event called perihelion - on Wednesday (October 29). That’s standard behaviour for comets that originate in the distant, icy region known as the Oort Cloud.
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As a comet heats up, its frozen surface turns directly into gas - a process called sublimation - which releases dust and ice that form a glowing halo and tail, and the reflected sunlight makes the comet appear brighter.
But still, 3I/ATLAS is brightening far faster than any scientist predicted.
"The reason for 3I’s rapid brightening, which far exceeds the brightening rate of most Oort cloud comets at similar r [radial distance], remains unclear," wrote Qicheng Zhang, of Lowell Observatory, and Karl Battams, of the US Naval Research Laboratory, in a paper posted to arXiv.
Observations from NASA’s STEREO spacecraft, the Solar and Heliospheric Observatory (SOHO), and the GOES-19 weather satellite confirmed the surprising surge in brightness, while ground-based telescopes won’t get another look until the comet re-emerges from behind the Sun in mid- to late-November 2025.
The researchers have proposed a few theories - the unusual flare-up could be caused by the comet’s speed and trajectory, or perhaps something more exotic about its chemical makeup.
"Oddities in nucleus properties like composition, shape, or structure - which might have been acquired from its host system or over its long interstellar journey - may likewise contribute," they continued.
"Without an established physical explanation, the outlook for 3I’s postperihelion behavior remains uncertain, and a plateau in brightness - or even a brief continuation of its preperihelion brightening - appears as plausible as rapid fading past perihelion.
"Continued observations may help provide a more definitive explanation for the comet’s behavior."
For now, scientists remain baffled. As the interstellar object speeds away, Zhang and Battams are hopeful further research will uncover whether this mysterious comet is just an extreme outlier - or if it is something truly not natural, as Harvard astrophysicist Avi Loeb boldly claimed.
"It could indicate that it was an alien probe sent to do recon on Earth," he told the New York Post.