ALMA Reveals Unusual Composition of Interstellar Comet 2I/Borisov

2I/Borisov likely formed in extremely cold environment, high amounts of carbon monoxide show

A galactic visitor entered our solar system last year – interstellar comet 2I/Borisov. When astronomers pointed the Atacama Large Millimeter/submillimeter Array (ALMA) toward the comet on 15 and 16 December 2019, for the first time they directly observed the chemicals stored inside an object from a planetary system other than our own. This research is published online on 20 April 2020 in the journal Nature Astronomy.

The ALMA observations from a team of international scientists led by Martin Cordiner and Stefanie Milam at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, 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 au

from the Sun (within less than 186 million miles, or 300 million kilometers) [1]. 2I/Borisov’s CO concentration was estimated to be between nine and 26 times higher than that of the average solar system comet.

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. “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.”

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 has been instrumental in transforming our understanding of the nature of cometary material in our own solar system – and now with this unique object coming from our next door neighbors. It is only because of ALMA’s unprecedented sensitivity at submillimeter wavelengths that we are able to characterize the gas coming out of such unique objects,“ said Anthony Remijan of the National Radio Astronomy Observatory in Charlottesville, Virginia and co-author of the paper.

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.”

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 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.”

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.


[1] One comet known as C/2016 R2 (PanSTARRS), which came from the Oort Cloud, had even higher levels of CO than Borisov when it was at a distance of 2.8 au from the Sun.

# # #

Media contact:
Iris Nijman
News and Public Information Manager
National Radio Astronomy Observatory (NRAO)
[email protected]
+1 (434) 249-3423

This research was presented in a paper titled “Unusually high CO abundance of the first active interstellar comet,” by M. Cordiner & S. Milam, et al., appearing in the journal Nature Astronomy (DOI: 10.1038/s41550-020-1087-2).

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

In Other News…

Cyber Expert Wins FBI Community Leadership Award

Robert R. Wells, special agent in charge of the Charlotte Division of the FBI has chosen a local cyber expert as the 2020 Director’s Community Leadership Award (DCLA) recipient for North Carolina. Torry Crass has been an invaluable partner to the FBI Charlotte field office since 2013.

2021 AUI Scholarship Recipients

Below are the fourteen winners of the 2021 AUI Scholarship conducted by International Scholarship and Tuition Services, Inc. These students will each receive an award of $3,500 per year to aid in defraying expenses at the college or university of their choice.

ITL Development Director: “We are convinced that our proposal is solid and meets all the requirements”

In an interview with Nueva Mining and Energy Magazine, Ricardo Raineri, Director of Development of the Chilean Institute of Clean Technologies (ITL) refers to the criticism that has hovered over Corfo’s decision, arguing that “it is essential to understand and emphasize that our proposal is based on an open platform model ”.

West Virginia Students Contact International Space Station LIVE

Friday, May 7th at 8:00 AM EDT, students in rural West Virginia will experience this once in a lifetime opportunity. Green Bank Elementary-Middle School (GBEMS) will be contacting astronaut Mark Vande Hei on the International Space Station (ISS).

The Universe just Became More Accessible: Free Software for Exploring the Universe Through Sound

Today free software has been released to help the blind and visually impaired (BIV) explore the universe through sound. With the support from the National Science Foundation’s STEM+C program, Innovators Developing Accessible Tools for Astronomy (IDATA) brought together nearly 200 BIV and sighted students, teachers, astronomers and programmers from across the Nation to create this innovative software called Afterglow Access.

Nueva Mineria covers the importance of ICTL’s Open Science model pioneered by AUI

The ICTL is a Chilean clean technology institute that is committed to developing innovations in the mining, power, battery, manufacturing, and related industrial sectors. The Open Science model allows a larger community to access R&D facilities based on the merit of their proposals.

VIDEO: Multi-wavelength Observations Reveal Impact of Black Hole on M87 Galaxy

In 2019, a worldwide collaboration of scientists used a global collection of radio telescopes called the Event Horizon Telescope (EHT) to make the first-ever image of a black hole — the supermassive black hole at the core of the galaxy M87, some 55 million light-years from Earth.

ACEAP Alumna Selected as Astronaut for SpaceX

Sian Procter, a participant in the Astronomy in Chile Educator Ambassadors Program (ACEAP) in 2016, has been selected as an astronaut by SpaceX. The Inspiration4 mission, scheduled to launch sometime after 15 September 2021, will orbit Earth for three days and conduct a variety of experiments.

New Images Reveal Magnetic Structures Near Supermassive Black Hole

The Event Horizon Telescope (EHT) — the worldwide collaboration that produced the first image of a black hole in 2019 — has produced a new image showing details of the magnetic fields in the region closest to the supermassive black hole at the core of the galaxy M87. The new work is providing astronomers with important clues about how powerful jets of material can be produced in that region.

After Long Shutdown, Giant Radio Telescope Array Set to Resume Observations

The Atacama Large Millimeter/submillimeter Array (ALMA), a set of 66 radio astronomy dishes perched high in the Chilean Andes, was hit hard by the pandemic. It shut down on 22 March 2020 and has remained silent ever since—far longer than most scientific facilities....

You are now leaving AUI

You will be redirected to the related partnering organization's website.

You will be redirected to
in 4 seconds...

Click the link above to continue or CANCEL