A publication in Nature Astronomy

Large amounts of carbon monoxide in the nucleus of the interstellar comet 2I/Borisov

Comet 2I/Borisov, an interstellar object that entered our solar system last year, continues to fascinate the scientific community. By pointing the antennas of the ALMA observatory and other telescopes such as ULiège's TRAPPIST-Sud, researchers were able to directly observe the chemical compounds stored inside the comet, revealing large quantities of carbon monoxide (CO), which was quite unexpected. This study was published in Nature Astronomy.


omet 2I/Borisov, discovered on 30 August 2019 by amateur astronomer Gennady Borisov, is only the second interstellar object to have been detected in our solar system. Discovered relatively quickly, scientists were able to make various observations to learn a little more about this object from elsewhere. This time, using the antennas of the Atacama Large Millimeter Array (ALMA) Observatory's giant radio telescope and the TRAPPIST-South telescope, located in the Atacama Desert in Chile, researchers were able to characterize more precisely the comet's composition.

Observations made by the international team led by Martin Cordiner and Stefanie Milam of NASA's Goddard Space Flight Center - and including astronomer Emmanuël Jehin, from the STAR Research Unit (Faculty of Sciences) of the University of Liège - revealed that the gas escaping from comet 2I/Borisov contained unusually high amounts of carbon monoxide (CO). The concentration of CO is higher than anyone had previously measured in a comet less than 2 astronomical units from the Sun (i.e. less than 300 million kilometres away).


The small TRAPPIST-South telescope in La Silla Observatory joined the Atacama Large Millimeter/submillimeter Array (ALMA), the largest observatory in the world, to study the composition of the interstellar comet 2I/Borisov. - Credit: E. Jehin and ALMA (ESO/NAOJ/NRAO)

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 could reveal much about the processes that occurred during their birth in protoplanetary disks.

ALMA detected two molecules in the gas ejected by the comet: hydrogen cyanide (HCN) and carbon monoxide (CO). While the team expected to see HCN, which is present in 2I/Borisov at similar amounts to that found in solar system comets, they were surprised to see large amounts of CO. “The comet must have formed from material very rich in CO ice, which is only present at the lowest temperatures found in space, below -420 degrees Fahrenheit (-250 degrees Celsius),” said planetary scientist Stefanie Milam.


ALMA observed hydrogen cyanide gas (HCN, left) and carbon monoxide gas (CO, right) coming out of interstellar comet 2I/Borisov. The ALMA images show that the comet contains an unusually large amount of CO gas. -Credit: ALMA (ESO/NAOJ/NRAO), M. Cordiner & S. Milam; NRAO/AUI/NSF, S. Dagnello.

Carbon monoxide is one of the most common molecules in space and is found inside most comets. Yet, there’s a huge variation in the concentration of CO in comets and no one quite knows why. Some of this might be related to where in the solar system a comet was formed; some has to do with how often a comet’s orbit brings it closer to the Sun and leads it to release its more easily evaporated ices.

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,” added Cordiner. 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,” said Cordiner. “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.”

The small robotic telescope TRAPPIST-South at the la Silla Observatory, further North in the chilean Atacama desert, joined the ALMA giant radio antennas to observe the comet in the visible light. "In the optical we detect smaller molecules that are photo-dissociated products of the larger parent molecules released directly from the ices of the nucleus, and that are observed with ALMA" said Emmanuël Jehin from the Liège University and co-author of the paper. "In the course of our regular monitoring of the comet with TRAPPIST, we have detected in 2I/Borisov's atmosphere the CN radical simultaneously to the ALMA observations of HCN, allowing to check the link between those molecules and found, contrary to CO, a normal abundance for those species".

Due to its high speed when it travelled through our solar system (33 km/s) astronomers suspect that 2I/Borisov was kicked out from 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 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.

Until other interstellar comets are observed, the unusual composition of 2I/Borisov cannot easily be explained and raises more questions than it answers. Is its composition typical of interstellar comets? Will we see more interstellar comets in the coming years with peculiar chemical compositions? What will they reveal about how planets form in other star systems?

“2I/Borisov gave us the first glimpse into the chemistry that shaped another planetary system,” said Milam. “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.”

Référence Scientifique

M. Cordiner & S. Milam, et al., “Unusually high CO abundance of the first active interstellar comet", Nature Astronomy, Avril 2020 


Université de Liège

Emmanuel JEHIN | STAR Research Institute | Faculté des Sciences

NASA Goddard Space Flight Center


Stefanie MILAM


Illustration : artist impression of 2I/Borisov - Credit: NRAO/AUI/NSF, S. Dagnello

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