April 21, 2020
Artistic rendering of comet

Credit: NRAO/AUI/NSF, S. Dagnello

Interstellar comet 2I/Borisov entered our solar system last year. Using the Atacama Large Millimeter/submillimeter Array (ALMA), a group of astronomers — including a researcher from Catholic University — directly observed for the first time the chemicals stored inside an object from a planetary system outside of our own.

This research is published online in the journal Nature Astronomy. The lead author is astrochemist Martin Cordiner, a researcher with the Institute for Astrophysics and Computational Sciences, within the Department of Physics

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

Cordiner and co-author Stephanie Milam at NASA’s Goddard Space Flight Center in Greenbelt, Md., led a team of international scientists to carry out this work. Their observations revealed that the gas coming out of the comet contained unusually high amounts of carbon monoxide (CO). The concentration of CO is higher than anyone has detected in any comet within 2 astronomical units from the sun (within less than 186 million miles, or 300 million kilometers). 2I/Borisov’s CO concentration was estimated to be between nine and 26 times higher than that of the average comet in our solar system.

Astronomers are interested to learn more about comets because these objects spend most of their time at large distances from any star in very cold environments. Unlike planets, their interior compositions have not changed significantly since they were born. Therefore, they can reveal much about the processes that occurred during their birth in protoplanetary disks (a disc of dense gas and dust surrounding a young newly formed star). 

“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,” Cordiner added. This region can be compared to the cold region of icy bodies beyond Neptune, called the Kuiper Belt. 

The team can only speculate about the kind of star that hosted 2I/Borisov’s planetary system. “Most of the protoplanetary disks observed with ALMA are around younger versions of low-mass stars like the sun,” Cordiner said. “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.”

Due to its high speed when it traveled through our solar system (33 km/s or 21 miles/s) astronomers suspect that 2I/Borisov was ejected at high velocity from its host system, probably as a result of 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 entering our inner Solar System in August 2019 by amateur astronomer Gennady Borisov. 

2I/Borisov is only the second interstellar object to be detected in our solar system. The first — 1I/’Oumuamua — was discovered in October 2017, at which point it was already on its way out, making it difficult to reveal details about whether it was a comet, asteroid, or something else. The presence of an active gas and dust coma surrounding 2I/Borisov made it the first confirmed interstellar comet. 

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