The interstellar comet Borisov is so flawless that it has most likely by no means been near a star

By comparing our local comet Hale-Bopp to the interstellar visitor 2I / Borisov, a team of astronomers have concluded that the intruder is perhaps one of the most pristine comets we have ever seen.

“2I / Borisov could represent the first truly pristine comet ever observed,” says Stefano Bagnulo of the Armagh Observatory and Planetarium in Northern Ireland, UK, who led the new study recently published in Nature Communications.

Many comets pass through the inner solar system at least once in their lifetime. When they do this, they will encounter the solar wind and other random microscopic pieces of junk floating around. This contaminates them so badly that astronomers can determine how many passages a comet has made since it was formed.

The comet Hale-Bopp, which inspired stargazers in the late 1990s, was amazingly pure. Astronomers estimated that it had only come close to the Sun once before it entered in the late 20th century.

Using the FORS2 instrument on the Very Large Telescope at the European Southern Observatory in Chile, a team of astronomers carefully examined the interstellar comet 2I / Borisov. Discovered by amateur astronomer Gennady Borisov in August 2019, this visitor was the second known interstellar invader to our solar system. The research team found that Borisov and Hale-Bopp were remarkably similar.

“The fact that the two comets are remarkably similar suggests that the environment from which 2I / Borisov originated is not that different in composition from the environment in the early solar system,” says Alberto Cellino, co-author of the study the Turin Astrophysical Observatory, National Institute of Astrophysics (INAF), Italy.

Olivier Hainaut, an astronomer at ESO in Germany who studies comets and other near-Earth objects but was not involved in this new study, agrees. “The main finding – that 2I / Borisov is not like any other comet except Hale-Bopp – is very strong,” he says, adding, “it is very plausible that they formed under very similar conditions.”

2I / Borisov may never have passed near its parent star before it was expelled into interstellar space and found its way to our own solar system.

“The arrival of 2I / Borisov from interstellar space was the first opportunity to study the composition of a comet from another planetary system and to check whether the material of this comet is in any way different from our native variety,” explains Ludmilla Kolokolova from the University of Maryland in the USA, who was involved in research on nature communication.

The truth is, we don’t know much about the life of comets, especially interstellar ones. But future missions can help paint a more complete picture.

Bagnulo hopes that before the end of the decade, astronomers will have another, even better, opportunity to study a rogue comet in detail. “ESA plans to launch Comet Interceptor in 2029, which will be able to reach another interstellar visitor object if one is detected in an appropriate trajectory,” he says, referring to an upcoming European Space Agency mission.

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