Interstellar Comet Borisov "Indistinguishable" from Solar System Comets

Ground-based observations of interstellar comet 2I/Borisov show that it looks just like comets in the solar system.

The solar system's newest interstellar visitor, Comet 2I/Borisov, looks remarkably familiar. A new study characterizing the comet revealed details, such as its nucleus size and gas distribution, that bear a striking resemblance to the icy population of the outer solar system.

At the end of August, amateur astronomer Gennady Borisov spotted an object moving through the sky. Subsequent observations revealed that the object’s speed and trajectory indicate an origin outside the solar system. Now officially known as Comet 2I/Borisov (C/2019 Q4), it is the second interstellar interloper ever identified by astronomers. The first interstellar visitor is called ‘Oumuamua (1I/2017 U1), but that one looked more like an asteroid than a comet. With its fuzz of a tail, Borisov takes a more familiar appearance.

"So far, Comet Borisov is indistinguishable from comets observed in our backyard, except for its orbit," says Michał Drahus (Jagiellonian University, Poland). "With its familiar properties, [Comet Borisov] tells us that comets similar to the ones we know from this solar system also form around other stars."

Drahus worked with Piotr Guzik, also at Jagiellonian, to observe the object in early September with the 8.2-meter Gemini North Telescope in Hawaii and the 4.2-meter William Herschel Telescope in the Canary Islands. The pair of telescopes proved ideal for studying Borisov, as they can observe at low elevations while the comet remains close to the horizon. Drahus also noted that the instruments are "extremely well managed," which enabled their technicians to rapidly respond to the unexpected observing request issued by the astronomers.

“Remarkably Similar”

For decades, astronomers have scoured the sky for comets from outside the solar system. In the violent early years of planet formation, a significant amount of material is expected to be ejected from the growing system. Most ejected material would likely start off on the outskirts of a system, in icy objects weakly held by the gravity of their star. As young planets migrate to different orbits, they would scatter this material, ejecting most of it from the system.

Yet for decades, the only comets we observed came from our own system. Then, in 2017, astronomers around the world turned their telescope towards the strange object eventually called ‘Oumuamua, a Hawaiian name meaning “messenger from afar arriving first.” Unlike Borisov, ‘Oumuamua showed no obvious signs of cometary activity. Its appearance defied expectations of what interstellar visitors should look like.

The arrival of ‘Oumuamua caused astronomers to revise their expectations of how frequently extrasolar objects should visit the solar system. Even with the updated numbers, astronomers didn’t anticipate the next visitor until after 2022. This is when the Large Synoptic Survey Telescope (LSST), a wide field-of-view telescope that will survey the whole sky every three days, is scheduled to begin science operations. Certainly, no one expected a second visitor only two years after the first.

Comet Borisov is everything that ‘Oumuamua wasn't. While the first scout seemed more like an asteroid, with little to no activity, Borisov has a fuzzy tail that marks it as cometary. Because it is active and coming closer to the Sun and to Earth, Borisov is brightening (currently at 16th magnitude) where ‘Oumuamua was dim (never brighter than about 20th magnitude). Most importantly, while ‘Oumuamua could only be studied for a handful of weeks before it became too faint to observe even with the Hubble Space Telescope, Comet Borisov will be visible to astronomers for months, allowing more in-depth observations.

Based on the new observations, Borisov's nucleus has a radius of about 1 kilometer, a common size for solar system comets. The nucleus is dominated by dust with traces of gas. The material it ejects travels at speeds similar to ejecta from solar system comets, suggesting a similar process. All in all, it "appears remarkably similar to solar system comets," Draghus says.

Future Study

Guzik's team is not the only one turning electronic eyes to the new visitor. Astronomers around the world have been observing it and will continue to do so as long as it is visible. Many of those observations have been posted on the preprint server arXiv, where Guzik's paper was originally posted in early September before its publication this week in the journal Nature Astronomy. All observations suggest that Borisov bears a striking resemblance to its solar system siblings.

"We have been happy to see subsequent submissions confirming our initial results and conclusions," Drahus says.

As Borisov moves toward its closest approach to the Sun on December 8th, and its closest approach to the Earth on December 28th, astronomers around the world — amateurs and professionals alike — will be ready. Drahus and his colleagues plan to continue monitoring the interloper to trace brightness as it receives increasing amounts of solar radiation. They also hope to continue to investigate the comet's composition.

As castoffs from other planetary systems, ‘Oumuamua, Borisov, and other interstellar scouts can reveal insights into the era of planet formation, providing context for our own solar system’s history. While ‘Oumuamua's unusual properties suggested that some young systems might be wildly different from the solar system, Borisov's similarity reassures us that other systems are more like our own.

"It seems that the population of interstellar objects is more diverse than we thought," Drahus says.