Jen Mills
Published December 3, 2025 7:20am
Updated December 3, 2025 7:38am
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Previous Page Next PageA mysterious space rock from another star system has started brightening and dimming again in a regular pattern.
Interstellar visitor 3I/ATLAS could now be the most famous comet ever (you’ve probably already forgotten 2024 YR4, right?) and the latest thing making headlines is that it’s apparently ‘pulsing’.
Dr Matthew Genge, a micrometeorite and cosmic dust specialist at Imperial College London, told Metro: ‘There’s a periodic brightening of the comet. That’s actually nothing particularly unusual.
‘It happens once every 16 hours, so if it’s a heartbeat, then those aliens are really, really, super chill, because that’s incredibly slow.’
The idea of it potentially being evidence of an alien craft manouvering around the solar system comes from Harvard astrophysicist Dr Avi Loeb, whose blog post on Sunday called for more observations to work out why jets are coming from the comet in what seem to be a pattern like a ‘heartbeat’ once every 16.16 hours.
A Nasa image of the comet taken on November 19, annotated with its trajectory and scale bar (Picture: NASA/JPL-Caltech/University of Arizona)
He did acknowledge that a natural explanation was possible, but said it also might be a way for the object to deliberately manouvre.
But Dr Genge said although he was happy we’re all thinking so much about thc comet, the ‘media frenzy’ around it was ‘ludicrous’, and there’s still no reason to think it’s anything more than a dirty snowball in space.
So what is causing the pulsing?
The main theory is that there could be a particularly volatile patch of ice on its surface, which turns to gas at a low temperature. When the comet rotates bringing this patch into the sunlight, it heats up, and blasts out dust and gas, also known as jets.
Dr Genge said we can’t actually see the nucleus, the solid part of the comet, because it’s producing so much gas: ‘It’s like trying to see a white cat in fog.’
Even if we can’t see the icy patch to prove it, it’s the most likely explanation, as many comets have had this periodic brightening before.
Interstellar comet 3I/Atlas streaking through space on November 19, seen from Italy (Picture: Gianluca Masi via AP)
Dr Genge joked: ‘I would be more impressed if it was brightening periodically and beeping while it did it, because then it might be about to go into reverse.’
He added: ‘It just reinforces that it’s behaving like a comet. It looks like a comet, it behaves like a comet… therefore, it’s most likely a comet.’
Are interstellar comets like buses?
Even if 3I/ATLAS is ‘just’ a comet, it’s still one thought to be billions of years older than our own star.
Scientists are excited about the possibility of one day being able to land on a interstellar comet and examine part of it in a lab, but so far we have only ever seen three.
Until the first one, Oumuamua, was spotted in 2017, they were thought to be incredibly rare.
3I/ATLAS seen from the Hubble Telescope on July 21, 2025, when the comet was 277 million miles from Earth, with a teardrop-shaped cocoon of dust (Picture: NASA, ESA, David Jewitt)
Since then, another two have appeared, indicating they are more common than we realised, and we are just getting better at spotting them.
Dr Genge said three is still not enough to give reliable statistics on how frequently we should expect them to appear: ‘We’ve all sat waiting for buses, haven’t we? Three can come at once.’
But he said it does suggest more rocks are thrown out of other planetary systems than we previously assumed.
At least four more months of speculation
Once comets pass further from the Sun than the orbit of Jupiter, they tend to become less active, and less bright.
This one is expected to get there around March 16 next year, and until then it’s a safe bet that it will be eagerly tracked both by astronomers, and by those who believe the truth is out there.
Dr Loeb has claimed that 3I/ATLAS will pass so close to the gas giant that it could ‘release technological devices as artificial satellites of Jupiter’.
Comet 3I/ATLAS seen on November 22 from an observatory in Extremadura, Spain (Picture: Facebook/Peter Carson Leigh)
But it won’t be captured within its orbit, becoming a permanent visitor: it is expected to pass back on our of our solar system and into outer space.
It’s potentially an interaction with another giant planet which initially threw the rock out of its own part of the galaxy, however.
‘Jupiter is a bit of a bully, and if asteroids or comets encounter it, it can do a gravitational slingshot and literally shoot them out of the solar system,’ Dr Genge said.
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‘Maybe that [type of interaction] happens more commonly than we previously thought, which would fit with there being quite a few giant planets around other stars.’
The comet will pass its closes to Earth on December 19, but it will still be very far away: around 170 million miles, or twice the distance of the Sun.
And after that?
‘When it actually leaves and nothing happens, we’ll stop talking about it,’ Dr Genge said.
Get in touch with our news team by emailing us at [email protected].
For more stories like this, check our news page.
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