two-color composite image of 2I/2019 Borisov
2I/2019 Borisov is the first interstellar comet ever identified. This image was obtained using the Gemini North Multi-Object Spectrograph from Hawaii’s Mauna Kea. The image was obtained with four 60-second exposures in bands (filters) r and g. Blue and red dashes are images of background stars, which appear to streak due to the motion of the comet. Credit: Gemini Observatory/NSF/AURA

The solar system’s newest known visitor may help fill in some of the gaps in our understanding of how planets form.

At the beginning of September, astronomers realized that a newly discovered comet, 2I/2019 Borisov, originated outside our solar system, making it the second known interstellar comet. Borisov looks more like a comet than did its predecessor, 1I/2017 U1, informally known as ‘Oumuamua, which appeared more like a lump of rock. Astronomers are beginning to probe the new interstellar visitor’s composition and are finding it surprisingly like the solar system.

“It looks just like another random comet belonging to our Sun,” said Alan Fitzsimmons, a cometary scientist at Queen’s University Belfast. Fitzsimmons led an international team to use the William Herschel Telescope in the Canary Islands to study the cyanogen gas in Borisov. A molecule of carbon and nitrogen gas bound together, cyanogen is one of the first things studied in comets orbiting the Sun, in part because it is so easy to spot.

“I would be really shocked if months down the line, we didn’t find significant differences between Borisov and solar system comets.”

The prevalence of cyanogen gas in our solar system didn’t mean that its presence in Borisov was a sure thing. “This is an object from another solar system,” Fitzsimmons said. “Although in almost every single comet we’ve ever studied closely enough in the solar system we’ve seen cyanogen gas, there was no guarantee that it would be in [Borisov]—and yet, there it was.”

Fitzsimmons cautions that these results are preliminary, noting that other teams of astronomers are already probing for different types of gases in the visitor. As scientists learn more about the chemistry of Borisov, Fitzsimmons expects that it will look less and less like our friendly neighborhood comet.

“I would be really shocked if months down the line, we didn’t find significant differences between Borisov and solar system comets,” Fitzsimmons said.

Karen Meech, a cometary astronomer who was part of Fitzsimmons’s team as well as on the team that discovered ‘Oumuamua, agrees. “We wouldn’t have any reason to believe that the exact same chemical mixture should be identical from one solar system to another,” she said. “If it is, it is telling us about some fundamental process [in planetary evolution] that is the same everywhere.”

The new paper will be published in Astrophysical Journal Letters and is available on the preprint server arXiv.

Planetary Leftover

Illustration of exocomets hurled into their star in a growing planetary system.
Gas giants can toss debris into other worlds, toward their stars, or out of the system completely, where such debris could one day wander into the solar system. Credit: NASA, ESA, and A. Feild and G. Bacon (STScI)

Throughout the galaxy, gravity pulls clouds of gas and dust together to make stars. The leftover debris around a newborn star may form a disk that can birth planets. After a few tens of millions of years—an eyeblink in astronomical terms—some of the dust and rocks get thrown about, crashing into newly formed planets, tossed into the star, or hurled from the system completely.

“Being left over from planet formation is kind of the simplest explanation [for Borisov],” said Sean Raymond, who models planet formation at the Université de Bordeaux in France.

According to Raymond, most of the stuff hurled out of a young planetary system is icy because icy things tend to form farther out, where the star’s gravity has a weaker home. Giant planets are also expected to form farther from their stars, and they are often the most culpable when it comes to tossing out young comets.

“We expect most of the things floating out there in interstellar space should be kind of like comets,” Raymond said.

Like Winning the Lottery

Although the asteroid-like ‘Oumuamua left astronomers scratching their heads, Borisov looks more like what astronomers have expected to spot since they began hunting for interstellar visitors decades ago. But even Borisov has its own surprises.

“This object is just too bright,” said Robert Jedicke, a comet researcher at the University of Hawai‘i. Jedicke uses his knowledge of how asteroid surveys find asteroids to calculate how often astronomers should be able to discover new interstellar objects. Because astronomers hadn’t found any interstellar objects prior to ‘Oumuamua in roughly a decade and a half of observations, astronomers expected that ‘Oumuamua would be the last one seen for a few years.

But Borisov is shining even brighter than expected. At its brightest, around the week of Christmas, the extraterrestrial comet should shine only slightly dimmer than Pluto. That’s still too faint for most amateurs to spot, although the high-end telescopes sported by some enthusiasts could catch it. That’s already been demonstrated because the visitor was discovered by an amateur astronomer, Gennadiy Borisov, using a homemade 0.65-meter telescope.

“This object is either a complete fluke or it’s telling us something fundamental about the behavior of these kinds of objects.”

“This is like winning the lottery the first time we ever played it,” Jedicke said. For every interstellar interloper as bright as Borisov, he estimates there should be many, many fainter objects—but other than ‘Oumuamua, none has been discovered. Jedicke is confident that they haven’t been overlooked.

“This object is either a complete fluke or it’s telling us something fundamental about the behavior of these kinds of objects,” Jedicke said. Exactly what that might be will take time to learn and will probably require observing more interstellar objects.

If Borisov suddenly sheds a bulk of material in a cometary flare, it could grow even brighter than expected, allowing astronomers the opportunity to search for even more hard-to-find chemical constituents. Even without a flare, the comet will remain in sight for almost a year, although it will be visible only from the Southern Hemisphere after December. Astronomers will have far more time to study it than the scant 2 weeks they had with ‘Oumuamua.

“We’re only just beginning to explore this new population of objects,” Fitzsimmons said. “It’s the birth of a new field of scientific investigation.”

—Nola Taylor Redd (@NolaTRedd), Freelance Science Journalist


Redd, N. T. (2019), Interstellar interloper Borisov looks like a regular comet, for now, Eos, 100, Published on 04 October 2019.

Text © 2019. The authors. CC BY-NC-ND 3.0
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