2I/Borisov is not your average comet

The interstellar comet 2I/Borisov contains an abundant amount of carbon monoxide and is likely to have formed in a cold region of its home planetary system far from the star, according to two papers in the journal Nature Astronomy. 

2I/Borisov was first identified in August 2019 and is the second interstellar visitor to be reported in our planetary system. The first was ‘Oumuamua, about which we learned more just last week.

Early observations suggested Borisov is compositionally similar to the vast majority of comets that exist in our Solar System, but new observations from both the Hubble Space Telescope (HST) and the Atacama Large Millimetre/submillimetre Array (ALMA) suggest otherwise.

Both were taken between December and January, when the comet was at a distance of roughly two au from the Sun (approximately 300 million kilometres), just outside the orbit of Mars. 

They reveal, the researchers say, that Borisov has a large abundance of carbon monoxide (CO) in its coma — a gas cloud that forms around the comet’s nucleus as it nears the Sun. 

Water is typically the most abundant molecule found in the coma of comets in the Solar System; however, both papers indicate that Borisov has more CO than expected for a comet found at a similar distance from the Sun, with 0.7–1.7 times as much CO as water.

The corresponding author for the first is Martin Cordiner from NASA Goddard Space Flight Centre. For the second it is Dennis Bodewits from Auburn University in the US.

“This is the first time we’ve ever looked inside a comet from outside our solar system,” says Cordiner, “and it is dramatically different from most other comets we’ve seen before.”

As the comet is so rich in CO, the authors suggest it originated in a very cold region far from its home star where CO could be frozen onto the comet’s nucleus. 

Following the ejection of Borisov from its original environment, they say, the cold temperatures of interstellar space would have preserved its chemical make-up for millions or even billions of years until the warmth of the Sun eventually caused the ice to vaporise.

Cordiner says his team can only speculate about the kind of star that hosted 2I/Borisov’s planetary system. 

“Most of the protoplanetary discs observed with ALMA are around younger versions of low-mass stars like the Sun,” said Cordiner. 

“Many of these discs extend well beyond the region where our own comets are believed to have formed and contain large amounts of extremely cold gas and dust. It is possible that 2I/Borisov came from one of these larger discs.”