How Rare Is Interstellar Comet Borisov? Possibly Not That Much, New Study Suggests

Interstellar comet Borisov marked the first interstellar comet to be detected by humans, but exactly how unique is it, really? Possibly not that much, according to the researchers of a new study.

Comet 2I/Borisov was discovered by Gennady Borisov, an amateur astronomer from Crimea, on Aug. 30, 2019. A week of observations confirmed that it is indeed a comet from outside our solar system.

According to NASA, there is only one other known visitor from another solar system, and that is the object Oumuamua, though it is still unclear if it's a comet, asteroid or perhaps a "hybrid" of both.

A pair of astronomers presented evidence Monday, just a few days shy of the second anniversary of comet Borisov's discovery, that interstellar objects such as comet Borisov may actually be more common than previously thought.

Authors Amir Siraj and Avi Loeb of the Center for Astrophysics Harvard & Smithsonian (CfA) presented their calculations in their study published in the Monthly Notices of the Royal Astronomical Society. These calculations were based on conclusions from Borisov, CfA noted in a news release. Through their work, they found that in the Oort Cloud, which is the most distant region of our solar system, there may actually be more interstellar objects than objects that are native to our own solar system.

"Here, we show that the detection of Borisov implies that interstellar objects outnumber Solar system objects in the Oort cloud, whereas the reverse is true near the Sun due to the stronger gravitational focusing of bound objects," the researchers wrote.

The Oort cloud lies some 2,000 to 10,000 astronomical units (au) from the sun, and it's a "predicted collection of icy objects farther away than everything else in the solar system," NASA noted. It is believed that the Oort Cloud is a "cometary reservoir" containing more than a trillion icy objects, and that it is where long-period comets "probably" come from.

"Before the detection of the first interstellar comet, we had no idea how many interstellar objects there were in our solar system, but theory on the formation of planetary systems suggests that there should be fewer visitors than permanent residents," Siraj said as per the news release. "Now we're finding that there could be substantially more visitors."

According to Siraj, it's possible that the reason we're not seeing more of these objects is simply that humans "don't have the technology" yet. Furthermore, the Oort cloud is so far away from the sun. And since the objects in it cannot produce their own light, it would certainly be more difficult to detect them.

"[I]nterstellar objects in the planetary region of the solar system would be rare, but our results clearly show they are more common than solar system material in the dark reaches of the Oort cloud," Loeb said as per the news release.

Further observations using more advanced instruments such as the Vera C. Rubin Observatory may help the search for other such interstellar objects, Siraj noted.