Interstellar comets like Borisov may not be all that rare

The Borisov comet, which was discovered in 2019, was the first interstellar space comet to pass through the solar system. Credit: NASA (ESA), D. Jewitt and D. Jewitt at UCLA
Astronomers discovered something amazing in 2019: A rogue star system comet. Borisov was the name of the icy snowball that traveled 110,000 miles an hour. It was the first and only interstellar meteor ever to be detected by humans.

What if interstellar visitorscomets and meteors, asteroids, and other debris from the beyond of our solar systemare more common that we think?

Harvard & Smithsonian (CfA) present new calculations showing that in the Oort Clouda shell of debris in the farthest reaches of our solar systeminterstellar objects outnumber objects belonging to our solar system.

"Before the detection of interstellar comests, we didn't know how many interstellar objects were present in our solar system. However, theory on the formation of planet systems suggests that there should have been fewer permanent residents than visitors," said Siraj, who was concurrently a graduate student and undergraduate in Harvard's Department of Astronomy. "Now, we are finding that there could possibly be significantly more visitors."

Siraj notes that calculations based on Borisov's conclusions have significant uncertainties. These uncertainties aside, Siraj points out that interstellar visitors still prevail over objects native to the solar systems.

Let's suppose I look at a mile-long section of railroad for one day and see one car cross it. Siraj says that the observed rate at which cars crossed the railroad section was one per mile per day on that day. "But, if I have reason to believe that this observation was not an isolated event, then I can go one step further and start to draw statistical conclusions about how many cars crossed that section of railroad.

If there are so many interstellar guests, why hasn't anyone seen one?

Siraj says that we don't yet have the technology to see them.

He says that the Oort cloud is located in a region 200 billion to 100 trillion miles from the Sun. And unlike stars, objects in Oort clouds don't emit their own light. These two factors make it difficult to see debris in the outer solar systems.

Matthew Holman is a senior astrophysicist who was not involved with the research. He says that the results of the study are fascinating because they have implications on objects closer to the Oort Cloud.

These results indicate that interstellar Oort cloud objects and their abundances are similar to Saturn's. This can be verified with future and current solar system surveys," Holman says. Holman is also the former director for the CfA's Minor Planet Center which tracks comets and other debris in our solar system.

"When we look at the data on asteroid in that area, the question becomes: Are there interstellar asteroids that we didn't know about before?" He asks.

Holman explained that some asteroids are detected, but not observed or followed-up on year after year. We think they're asteroids. Then we lose them without taking a closer look."

Loeb, co-author of the study and Harvard astronomy professor adds, "Interstellar objects within the planetary area of the solar system would not be common, but our results clearly demonstrate that they are more frequent than solar system material at the dark reaches the Oort clouds."

Next-generation technology observations may confirm the team's findings.

Siraj claims that the launch of the Vera C. Rubin Observatory is slated for 2022 and will "blow out previous searches for interstellar object in the water" and help to detect more visitors such as Borisov.

Transneptunian Automated Occultation Survey II (TAOS II), which was specifically created to detect comets far from our solar system's reaches, might also be able detect one of these passersby. TAOS II could be online by this year.

Siraj believes that the Oort Cloud's high number of interstellar items suggests that there is more debris left from the formation of planetary system.

Siraj states that "our findings show interstellar objects can put interesting constraints on planetary systems formation processes." Their implied abundance requires a substantial mass of material to be ejected as planetesimals. The study of interstellar objects, together with observations of protoplanetary disks as well as computational approaches to planet formation could unlock the secrets of how the planetary system and others formed.