Interstellar comet 2I/Borisov may be most pristine ever seen

An artist’s impression of comet 2I/Borisov. Image: ESO/M. Kormesser

Comet 2I/Borisov, only the second body from interstellar space to be detected passing through our solar system, is one of the most pristine comets ever observed and likely never passed close enough to a star to be affected by solar wind and radiation. As such, researchers say, it carries the original chemical signatures of the clouds of gas and dust it formed from.

And as it turns out, the comet likely formed in an environment not so different from our own.

Discovered in August 2019 by amateur astronomer Gennady Borisov, detailed observations confirmed the comet originated outside Earth’s solar system. Now, using the European Southern Observatory’s Very Large Telescope, researchers have found 2I/Borisov “could represent the first truly pristine comet ever observed,” said Stefano Bagnulo of the Armagh Observatory and Planetarium in Northern Ireland. Bagnulo led the new study published in Nature Communications.

The team used the FORS2 instrument on the VLT to study polarised light reflected from 2I/Borisov. As it turned out, the comet’s polarimetric signature was distinct from comets routinely observed in Earth’s solar system with the exception of comet Hale-Bopp, one of the most pristine comets ever seen. Prior to its visit to the inner solar system in the 1990s, Hale-Bopp is thought to have passed by the Sun only once.

This image of comet 2I/Borisov was captured by the FORS2 instrument on ESO’s Very Large Telescope in late 2019. Because of the comet’s velocity, about 175,000 kilometres per hour, background stars appear as streaks of multi-coloured light, the result of combining different wavelengths in a composite image. Image: ESO/O. Hainaut

Observations of 2I/Borisov showed it is even more pristine than the widely observed Hale-Bopp.

“The fact that the two comets are remarkably similar suggests that the environment in which 2I/Borisov originated is not so different in composition from the environment in the early solar system,” said Alberto Cellino, a co-author of the study, from the Astrophysical Observatory of Torino.

Ludmilla Kolokolova, a researcher at the University of Maryland, said the arrival of 2I/Borisov “represented the first opportunity to study the composition of a comet from another planetary system and check if the material that comes from this comet is somehow different from our native variety.”

Olivier Hainaut, an astronomer at ESO in Germany who was not involved in the study, said that appears to be the case. “The main result, that 2I/Borisov is not like any other comet except Hale–Bopp, is very strong” he said, adding “it is very plausible they formed in very similar conditions.”

2I/Borisov is the second known interstellar visitor to Earth’s solar system. The first was ‘Oumuamua, originally thought to be a comet but later reclassified as an asteroid.