The First Interstellar Comet Ever Detected Was Lonely

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Astronomers had already assumed that, given the vast distances between stars, objects like Borisov can travel for eons without running into another one. The last time Borisov felt the warmth of a star was probably in the bounds of its own system. Astronomers can retrace Borisov’s journey only so far, and we’ll likely never know where the comet came from. But the shimmery cloud of particles surrounding Borisov, known as a coma, can tell us something. “Dust carries rich information about the planetary system,” Bin Yang, an astronomer at the European Southern Observatory, in Chile, told me.

In another recent analysis of Borisov’s dust particles, Yang and her team found evidence that suggests the comet formed close to its parent star before spinning into the outer parts of its system, gathering up different kinds of cosmic material as it went along. Yang says Borisov might owe its composition to the presence of giant planets, which are known for stirring things up with their gravity. Perhaps Borisov once shared a home with its own versions of Jupiter and Saturn.

Though Borisov’s arrival was a surprise, the comet is less mysterious than ‘Oumuamua, the first interstellar visitor ever detected, seen in 2017. At the time, astronomers had been expecting something that resembled Borisov; current theories suggest that icy comets near the edges of a planetary system can be jostled by big planets and flung out into an untethered existence in the space between stars. Our solar system, in its early, more chaotic days, probably kicked out a few comets of its own. But ‘Oumuamua looked more like an asteroid, and astronomers are still debating its exact nature, including its shape. Given two very different interstellar visitors, the astronomy community is eager to see what’s next, and it won’t have to wait long. A new observatory in Chile expected to excel at spotting interstellar objects will begin operations in 2022, and the European Space Agency plans to launch a set of spacecraft in 2029 that will idle in space until they’re commanded to chase after a newly found interstellar object.

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Borisov has now left us behind, traveling beyond the view of any telescopes. It will not leave our solar system in the condition it arrived. Last spring, as Borisov neared Jupiter, the Hubble space telescope captured imagery that showed a piece of the comet breaking off. “It is not pristine anymore,” Ludmilla Kolokolova, an astronomer at the University of Maryland who worked with Bagnulo, told me. Borisov now bears a mark of its visit through the solar system. Which presents an interesting question: Hypothetically, if Borisov were to pass by another star, where a set of alien astronomers could observe it, would they notice any evidence of its encounter with our sun?