Interstellar Comet 2I/Borisov was discovered in September 2019 as it started to approach the inner Solar System from the depths of space. As it got closer to the Sun, it got hotter and started releasing more and more material from its interior. What at first looked very similar to comets in our Solar System, however, has turned out to be very different.

The observations revealed that Comet Borisov is rich in carbon monoxide (CO) ice; way richer than any comet within 300 million kilometers (180 million miles) from the Sun. Water is usually the most abundant molecule in the coma of comets in our Solar System, however, detections of CO in Borisov's coma indicated 0.7–1.7 times as much CO as water. The findings come from two independent studies both published in Nature Astronomy, one using the Hubble Space Telescope and the other using the Atacama Large Millimeter/submillimeter Array (ALMA).

The new research paints a clearer picture of the formation of Comet Borisov: It did not form close to its star but further out in a cold region, and was probably kicked out of its original star system by a passing star or giant planet, sending it on a journey for millions if not billions of years to reach us.

“This is the first time we’ve ever looked inside a comet from outside our Solar System and it is dramatically different from most other comets we’ve seen before,” co-lead author astrochemist Martin Cordiner, from the NASA’s Goddard Space Flight Center, said in a statement.

“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),” add the co-lead author planetary scientist Stefanie Milam, also from Goddard.

Its journey through the Solar System was fatal for the comet, and the latest observations show that it broke apart a few months after its closest passage to the Sun, which took place in December 2019. Observations of Comet Borisov, our first-ever interstellar comet, continue. Astronomers want to know as much as possible about this object, although we won’t know how typical it is among the newly discovered class of interstellar comets until we find more.

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