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Hubble Observations Show Interstellar Comet 2I/Borisov Rich In Carbon Monoxide

April 20, 2020

comet 2I/Borisov

Comet 2I/Borisov

Credit: NASA, ESA, K. Meech (University of Hawaii), D. Jewitt (UCLA) 


Studies of a comet from beyond our Solar System have yielded insights into how other star systems may have formed. 

Interstellar comet 2I/Borisov, observed using the Hubble Space Telescope, displayed a high amount of carbon monoxide, CO, relative to the amount of water it contains, compared to the comets from our Solar System, said Planetary Science Institute Senior Scientist Jian-Yang Li. 

“The biggest news is probably the first measurement of the CO composition in a sample from another star. This has never been possible to do due to the enormous distance to another planetary system and the extreme faintness of these small objects around other stars,” Li said. “Given the frequency of recent discoveries of such interstellar objects – two in just two years – and thanks to the advance of telescopes and survey techniques, we can expect more and more such objects discovered and characterized in the near future. This comet may represent the start of a new era in studying extrasolar planet formation.” 

Li led the acquisition and initial verification of the Hubble data and is co-author on the paper “The carbon monoxide-rich interstellar comet 2I/Borisov” ( appearing in Nature Astronomy. Dennis Bodewits of Auburn University’s Leach Science Center is lead author on the paper. 

“I led the design and planning of the observations, and validation of the data, and also contributed to the interpretations of the results – what the data are telling us about the history of the comet,” Li said. 

Observations were made in ultraviolet, which is severely blocked by Earth’s atmosphere. Hubble, above the atmosphere, was needed to make the observations, Li said. 

“The high amount of CO is an indication that it comes from a very cold place, either extremely far away from its host star or from a relatively cold star. We think it’s more likely to be the latter case – it comes from a cold red dwarf because there are far more red dwarfs in our Milky Way galaxy than other hotter stars,” Li said. “However, we are still far from saying exactly what’s going on around its host star when planets formed there.” 

2I/Borisov offers scientists a glimpse of how other planetary systems may have formed. “All other comets that we have studied so far are formed within our Solar System, whereas this comet certainly comes from around another star,” Li said. “We generally consider comets as the ‘original building blocks’ of gas giants and their major satellites in our Solar System. So this comet should represent the building blocks of exoplanets around their parent star.” 

Studying comets is important because astronomers are still trying to understand the role they play in the buildup of planets. They could also have redistributed organic material among young planets, and may have brought water to the early Earth. These activities are likely happening in other planetary systems, as demonstrated by 2I/Borisov’s makeup. 

“So far we’ve discovered thousands of extrasolar planets around other stars, but we know nothing about the formation conditions and processes. This comet is the first sample from another star that we can directly measure the composition to infer what’s going on when planets form around another star,” Li said. “However, it is still too far from knowing exactly what happened during the planetary formation process around other stars from this one single sample.” 



Alan Fischer

Public Information Officer


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Jian-Yang Li

Senior Scientist

jyli [at]

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