Interstellar comet Borisov came from a cold and distant home star


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Two views of the Borisov comet, taken by NASA’s Hubble Space Telescope

NASA, ESA and D. Jewitt (UCLA)

The interstellar comet Borisov came in from the cold. Two sets of observations have revealed the composition of this strange comet, which may have formed in a cold, dark stellar system.

Borisov was spotted in September, flying towards Earth on a trajectory that indicated it must have come from another star. This was only the second interstellar object astronomers have definitively identified, and the first interstellar comet.

As the comet reached its closest point to Earth in December and January, passing just outside the orbit of Mars, astronomers around the world pointed their telescopes in its direction in hopes of finding out more about its composition and where it came from.

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Martin Cordiner at NASA’s Goddard Space Flight Center in Maryland led a team that observed Borisov with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, and Dennis Bodewits at Auburn University in Alabama and his colleagues used the Hubble Space Telescope to take a glimpse.

Both groups used their observations to analyse the chemical composition of Borisov’s coma – the cloud of gas that forms around a comet as heat from a star warms it up. While most comets in our solar system have comas that are mostly water, Borisov’s appears to be mostly carbon monoxide.

“It was quite shocking to look at the data and see all this carbon monoxide,” says Cordiner. “This thing basically looks like a regular comet, but it has these different chemical signatures.” Those signatures indicate that Borisov formed in a stellar system that wasn’t quite like our own.

“Carbon monoxide ice disappears very easily when you heat it, so we think that Borisov formed in a system that was colder than ours,” says Bodewits. “It’s a sort of snowman from a cold and dark place.” The more of these interstellar objects we find, the more we’ll be able to learn about the conditions in distant stellar systems.

Journal references: Nature Astronomy, DOI: 10.1038/s41550-020-1087-2 and DOI: 10.1038/s41550-020-1095-2

Article amended on
20 April 2020

Correction: We have corrected which instruments each team used.  

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