It’s only the second interstellar visitor a telescope on our planet has ever recorded (pipped to the post by Oumuamua in 2017), and scientists say 2I/Borisov may be the most pristine comet ever identified.
This superstar space-traveller, 2I/Borisov (named after its eponymous discoverer, astronomer Gennady Borisov), was discovered in August 2019 and has now been described in a paper published today in Nature Communications.
What’s so striking about this comet, aside from the utter rarity of interstellar comets in our solar system, is that it appears unadulterated by the usual ravages of space travel.
While most comets pass close to stars and have their clouds of gas and dust (known as comas) pulled apart or disturbed by solar wind and radiation, 2I/Borisov seems never to have experienced such a near-miss, and must have avoided coming close to any stars – so pristine is its coma.
The researchers, led by Stefano Bagnulo of the Armagh Observatory in Northern Ireland, UK, used the FORS2 instrument on the European Southern Observatory’s (ESO) Very Large Telescope (VLT), to study the comet and its coma using a technique called polarimetry.
The research found that 2I/Borisov’s properties are distinct from other Solar System comets, with the exception of Hale-Bopp, one of the most pristine comets astronomers have ever seen. Hale-Bopp is thought to have passed by our Sun only once, and so its composition is similar to the cloud of gas and dust that formed our Solar System 4.5 billion years ago.
The team found that 2I/Borisov is actually even more pristine than Hale-Bopp, meaning it carries signatures of the formation of whichever corner of the Universe it came from.
“2I/Borisov could represent the first truly pristine comet ever observed,” says Bagnulo.
The similarities between 2I/Borisov and Hale-Bopp suggest that wherever this new interloper came from, is not so dissimilar from our own small patch of the cosmos.
Atronomers can hardly believe their good fortune: “Imagine how lucky we were that a comet from a system light-years away simply took a trip to our doorstep by chance,” says Bin Yang, an astronomer at ESO in Chile, who also took advantage of 2I/Borisov’s passage through our Solar System to study the comet. Her team’s results are published in Nature Astronomy.
Yang’s team discovered that the relative amounts of carbon monoxide and water in the comet changed as it neared our Sun, suggesting 2I/Borisov is made from materials that formed in different parts of its origin planetary system.
These observations are particularly exciting, because they suggest that matter in 2I/Borisov’s planetary home was mixed from near its star to further out, perhaps because of the existence of giant planets, whose strong gravity stirs material in the system. Astronomers believe a similar process occurred early in the life of our Solar System.
Amalyah Hart is a science journalist based in Melbourne.