Rapid brightening of interstellar comet 3I/ATLAS as it nears sun surprises scientists
Paul Arnold
contributing writer
Gaby Clark
scientific editor
Robert Egan
associate editor
An interstellar comet that originated outside our solar system has just made its closest pass to the sun, brightening dramatically and rapidly as it did so. The reason for the sudden extreme activity is currently puzzling scientists.
A stranger in the neighborhood
The latest visitor to our corner of the galaxy was first spotted on July 1, 2025, by astronomers and officially named comet 3I/ATLAS. It's only the third interstellar comet ever recorded and was calculated to be on a course that would take it close to the sun without plunging into it.
As the object neared its closest point to our star (perihelion), the immense solar glare made it virtually impossible for Earth-based telescopes to see it. So astronomers turned to space-based solar observatories like SOHO, STEREO-A and GOES-19 to keep a watchful eye.
In a paper on the arXiv preprint server, researchers confirmed that as the comet closed in on the sun, it was brightening at a steeply accelerating rate. This surge was more extreme than had ever been observed in a comet before. These icy bodies usually brighten slowly as they heat up. The comet also appeared blue, which surprised scientists as comets are typically red owing to dust reflecting sunlight.
"Our cursory analysis of this data indicates the comet will likely emerge from conjunction considerably brighter than when it entered, with an extrapolated geocentric V magnitude of 鈭�9 at perihelion, perhaps driven by prominent, visible gas emission," wrote the researchers in their paper.
Searching for answers
While the causes of the comet's extreme behavior are puzzling, the study's authors propose a few possible reasons. First, the brightening could have been caused by a massive surge of gas emanating from the comet. This is supported by the fact that the gas's light was blue and strong enough to overpower the dust's reflection. The color is a tell-tale sign of molecules like carbon glowing intensely when heated by the sun.
Another reason could be 3I/ATLAS's unique composition. The researchers found that its chemical composition differs from that of comets in our solar system. For example, there is a higher carbon dioxide content, more volatile ices (which could explain gas surges), and unusual metals.
We may not have any definitive answers yet, but there will be other opportunities to study this unusual comet. As it moves away from the sun, ground-based telescopes can take over for more detailed studies.
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More information: Qicheng Zhang et al, Rapid Brightening of 3I/ATLAS Ahead of Perihelion, arXiv (2025).
Journal information: arXiv
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