Astronomers gather more clues about interstellar comet 3I/ATLAS

Telescopes trained on the interstellar comet 3I/ATLAS have revealed that it looks much like comets found in the outer reaches of our own solar system. However, puzzling features, such as a surprisingly large amount of water coming off it even while far from the sun, might give us clues about the ancient star system it originally came from.

Objects from other systems passing through ours are extremely rare: 3I/ATLAS, discovered in July, is only the third known interstellar visitor after the comets ‘Oumuamua in 2017 and Borisov in 2019. There is only a window of a few months to observe the newcomer, which is barrelling towards the sun at around 60 kilometres per second, before it loops around it and is flung out of the solar system forever.

So far, scientists have used its speed to infer that it might come from a star system billions of years older than our own. Early observations also suggested it was around 20 kilometres wide, but there has been little information on the vast plume of water or gases, or both, that it leaves in its wake, called its tail, which can tell us about the composition of the comet itself.

Toni Santana-Ros at the University of Barcelona in Spain and his colleagues have now used several ground-based telescopes to observe the comet and its tail, and found that it contains a relatively low to moderate amount of dust. The amount of dust in its tail also appears to be growing as it gets closer to the sun and warms up, which is similar to the behaviour of comets from the outer solar system. “It’s a regular object. There is nothing especially weird on it,” says Santana-Ros.

Astronomers have also observed the comet from space. Researchers working with the Hubble Space Telescope found it may be much smaller than it first seemed, possibly between 320 metres and 5.6 kilometres wide.

Comets often contain ice, which is vaporised as they get close to the sun, producing water vapour in the tail. Using the Neil Gehrels Swift Observatory satellite, Zexi Xing at Auburn University, Alabama, and her colleagues have detected the presence of water in the comet’s tail, at distances much further from the sun than is typical. To produce the amount of water they detected, around 20 per cent of the comet’s surface would need to be producing it, which is larger than typical solar system comets.

Producing this much water so far from the sun could fit with the idea that 3I/ATLAS comes from a star system much older than our sun, says Cyrielle Opitom at the University of Edinburgh, UK. This is because older star systems tend to have more water relative to other molecules. “It might be that it has more water than other molecules just because it was formed earlier. It could be indicative of that, but I think it’s still too early to say.”

Astronomers are also digging through older data to see whether telescopes may have accidentally picked up the comet. Adina Feinstein at Michigan State University and her colleagues found that the Transiting Exoplanet Survey Satellite (TESS) space telescope, which is looking for planets around other stars, picked up the comet by chance between 7 May and 3 June. “It just happened to be that, at this perfect time, TESS [was] staring right where 3I/ATLAS was,” says Feinstein.

They found that it was surprisingly bright during this time, implying it was releasing either water or gases from its surface at distances far from the sun. “It hadn’t quite crossed over into the region in our solar system where you would expect water to start reacting,” says Feinstein.

In this far-off zone, it is relatively unlikely that this was water and more likely to have been gases like carbon monoxide or carbon dioxide, says Opitom. “That happens all the time with comets from the solar system; they can be active very far away because of these volatiles.”

But the fact that it was active so far from the sun might suggest it is a comet that hasn’t been exposed to much starlight in its life, which is consistent with an origin in the outer reaches of its own star system, says Matthew Genge at Imperial College London.

“What it all ultimately suggests is this is a comet that was perturbed out of the outskirts of another solar system,” says Genge. It will be difficult to know exactly what caused its exit, but it may have been a passing star’s gravity that set it on a course towards us, he says.

We will soon get even more detailed observations of 3I/ATLAS, says Opitom, as the James Webb Space Telescope has just completed observations of it, which will be analysed by astronomers in the coming weeks.

The comet will reach its closest point to the sun in October, when it will be exposed to the highest intensity of sunlight, allowing astronomers to measure the gases that it produces. This could tell us exactly what molecules are in its active tail, and so also in the comet itself, but also the ratios of various molecules, which might hint at how 3I/ATLAS originally formed, says Opitom.

As with previous interstellar objects, people have speculated that it might be alien technology, but Santana-Ros sees no evidence of that. “If you take a picture on holiday and you see something which is tall and with a long neck and has four legs, you can always think this is an alien, but it’s most probable this is a giraffe,” says Santana-Ros. “Since we don’t have any evidence that this is some weird thing, and we are not seeing anything really weird, then there is no reason to think this is something abnormal.”

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