"Comet - C/2019 Q4 (Borisov) - Artist Concept" by NASA / Public Domain

Astronomers Find Our Second Interstellar Visitor Looks like the Locals

Oct 4, 2019

By Jonathan O’Callaghan

Last month astronomers were thrilled by the confirmation that a second known interstellar object is flying through our solar system. Named 2I/Borisov—after its discoverer, Crimean amateur astronomer Gennady Borisov—it has already attracted huge attention. Countless observatories, from the Very Large Telescope in Chile to NASA’s Hubble Space Telescope, are studying the object, and plenty more science is on the way as 2I/Borisov approaches its peak brightness in December. “It’s been a rapid assembly of telescopes around the world,” says Michele Bannister of Queen’s University Belfast. “This, basically, is establishing a new subfield of astronomy.”

Whereas the first interstellar object, ‘Oumuamua, was found in 2017 as it was already leaving our solar system, 2I/Borisov was caught on the way in. The latter object exhibits traits of cometary activity, with dust and gas surrounding it, whereas ‘Oumuamua was more sedate, like an asteroid (although its classification is still unclear). The question now is whether or not 2I/Borisov resembles comets in our solar system, with either answer being equally thrilling. “I’m torn both ways,” Bannister says. “If it’s like the things that we have in our solar system, the processes that we see taking place are more typical than we realized. If it’s really different, then that tells us this chemistry takes place in quite a different way—in the diversity of exoplanetary systems—than we see.”

Gennady Borisov spotted the object with a homemade 0.65-meter telescope in late August. And almost immediately, other astronomers, both professional and amateur, began training their own telescopes on it. The trajectory of the comet confirmed it was unbound from our sun and therefore hailed from another star system. Some of the earliest results came from Julia de León of the Institute of Astrophysics of the Canary Islands and her colleagues, who used the Great Telescope of the Canary Islands on September 12. (They presented their findings in the non-peer-reviewed works-in-progress journal Research Notes of the AAS on September 19.) “As soon as we heard about this potential interstellar object, we decided to go for it,” de León says. By studying the light reflected by the dust emitted by the object, she and her team discovered the dust was similar in composition to that of comets in our own solar system.

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