Galactic visitor is ‘dramatically different’ from any comets we’ve seen before

21 Apr 2020

Image: © alexmak/

Researchers are now able to chemically peer inside distant comets, showing that one in particular is highly unusual.

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have found something unusual in a galactic visitor that entered our solar system last year. In a paper published to Nature Astronomy, they revealed that the gas coming out of comet 2I/Borisov contained unusually high amounts of carbon monoxide (CO).

In fact, the concentration is higher than has been detected in any comet within 300m km of the sun, equivalent to two astronomical units (AU). The levels of CO concentration in 2I/Borisov are between nine and 26 times higher than the average solar system comet.

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

In addition to CO, hydrogen cyanide (HCN) is also being ejected from the comet at levels similar to solar system comets. The astronomers believe the comet must have formed from material rich in CO ice, which is only present at the lowest temperatures of space at around minus 250 degrees Celsius.

This variation in the levels of CO among comets remains a mystery to astronomers, with theories ranging from the location where a comet formed in the solar system, to how often its orbit brings it close to the sun leading to more evaporated ices.

Peering into a planetary system

Speculating about the planetary system 2I/Borisov may have come from, Cordiner said: “Most of the protoplanetary disks observed with ALMA are around younger versions of low-mass stars like the sun.

“Many of these disks 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 disks.”

Travelling at speeds of 33km per second, the astronomers believe 2I/Borisov was kicked out of its host system, probably by interacting with a passing star or giant planet. It then spent millions or billions of years on a cold, lonely voyage through interstellar space before it was discovered on 30 August 2019 by amateur astronomer Gennadiy Borisov.

Astronomer Stefanie Milam said the discovery “gave us the first glimpse into the chemistry that shaped another planetary system”.

“But only when we can compare the object to other interstellar comets, will we learn whether 2I/Borisov is a special case, or if every interstellar object has unusually high levels of CO,” she said.

Colm Gorey was a senior journalist with Silicon Republic