Strange ingredient in interstellar comet Borisov offers a clue to its origins
Astronomers have revealed the unusual chemical composition inside 2I/ Borisov, the interstellar comet that visited our Solar System last year. A strange ingredient has provided new clues about where this travelling space rock originated from.
2I/Borisov was discovered on 30 August 2019 by amateur astronomer Gennady Borisov. Following the appearance of interstellar object ‘Oumuamua in 2017, this was the second object from another solar system ever discovered wandering through our cosmic neighbourhood. In December
2019 astronomers took a closer look at 2I/Borisov using the Atacama Large Millimeter/submillimeter Array (ALMA), a giant radio telescope in Chile.
In the new study an international team of researchers, led by planetary scientists Martin Cordiner and Stefanie Milam from NASA’s Goddard Space Flight Center, analysed 2I/Borisov’s chemical make-up. The researchers found that the gas coming from the comet contained more carbon monoxide (CO) than has been detected in any other comet this close to the Sun – less than 300 million kilometres (186 million miles). In fact, the concentration of CO in the gas coming from this comet was between 9 and 26 times higher than in the average comet in our Solar System, according to a statement from the National Radio Astronomy Observatory (NRAO) which oversees ALMA.
Using ALMA, the team detected both CO and hydrogen cyanide (HCN). However, they found a similar amount of HCN in 2I/Borisov that’s found in other comets in our Solar System, so that discovery wasn’t much of a surprise. But the unexpectedly high quantities of CO offered a major clue as to where this comet came from.
“The comet must have formed from material very rich in CO ice, which is only present at the lowest temperatures found in space, below minus 250 degrees Celsius (minus 420 degrees Fahrenheit),” said Milam.
“If the gases we observed reflect the composition of 2I/Borisov’s birthplace, then it shows that it may have formed in a different way than our own Solar System comets in an extremely cold, outer region of a distant planetary system,” said Cordiner.
Astronomers do not yet know what kind of star 2I/Borisov formed around, but these researchers suspect that 2I/Borisov came from a cold region in a larger protoplanetary disc, a rotating disc of dust and gas around a young star from which planets and planetary objects form. “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,” explained Cordiner.