Interstellar comet Borisov is the most pristine space object ever seen

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Borisov

An artist’s impression of the interstellar comet Borisov

ESO/M. Kormesser

The interstellar comet Borisov is unlike anything from our own solar system. Two studies of the light passing from the comet’s coma – the cloud of dust and gas that envelopes a comet as it hurtles through space – have revealed that it is more pristine than any other object we have seen before.

Borisov was spotted in August 2019, and its trajectory through the solar system indicated that it must have come from another star. It was the second interstellar object ever discovered after the asteroid ‘Oumuamua, which was spotted on its way out of the solar system, giving astronomers very little time to study it. Because Borisov was spotted earlier in its journey through the solar system, we were able to observe it in more detail.

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Bin Yang at the European Southern Observatory in Chile and her colleagues used the Atacama Large Millimeter/submillimeter Array (ALMA) and the Very Large Telescope (VLT) to examine the properties of the comet’s dust. They found that it throws off about 200 kilograms of dust per second.

The researchers also discovered that Borisov has far more carbon monoxide than comets in our solar system usually do, but that the amount isn’t consistent across the entire object. “This comet is actually a bunch of small snowballs squeezed together, and those snowballs formed in a lot of different places,” says Yang. The differing amounts of carbon monoxide indicate that Borisov probably started forming relatively close to its home star before moving outwards, perhaps due to the influence of giant planets in the system.

Stefano Bagnulo at Armagh Observatory and Planetarium in the UK and his team used the VLT to analyse the light reflecting off Borisov’s coma. They found that it was different from any object in our solar system except for the comet Hale-Bopp, a relic from the early solar system.

The light was far more polarised than light bouncing off any other comet we have seen, and the coma was also remarkably smooth. “Interaction with a star creates jets and structure in the coma that we don’t see in the comet Borisov,” says Bagnulo. “The comet is pristine – it has never interacted with another star.”

Most of the differences between Borisov and comets from our solar system can be explained by this pristine nature, he says. Its similarities with Hale-Bopp hint that its home stellar system and our own early solar system were probably rather similar, not that Hale-Bopp is actually from another stellar system. “I cannot say Hale-Bopp is definitely not an interstellar comet that was captured by our solar system, but the most likely explanation is that the origin of our solar system is not so different from this other stellar system,” says Bagnulo.

Studying objects like this can let us understand the diversity of other stellar systems. “Comets and asteroids around other stars are so far away that there’s no way we can study them individually,” says Yang. “This interstellar object is the link we’ve been searching for between our solar system and other systems.”

While Borisov is too far away now for astronomers to keep studying it, a series of new telescopes that are currently under construction are expected to find many interstellar objects once they begin observing. Those objects will help us put our solar system into context.

Journal references: Nature Communications, DOI:10.1038/s41467-021-22000-x; Nature Astronomy, DOI:10.1038/s41550-021-01336-w

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