Interstellar comets like Borisov may not be so rare

In 2019, astronomers spotted something incredible in our backyard: a rebellious comet from another star system. Named Borisov, the icy snowball traveled 110,000 miles per hour and marked the first and only interstellar comet ever detected by man.

But what if these interstellar visitors – comets, meteors, asteroids, and other debris from beyond our solar system – were more common than you might think?

In a new study published Monday in the Monthly notices from the Royal Astronomical Society, astronomers Amir Siraj and Avi Loeb at the Center for Astrophysics | Harvard & Smithsonian (CfA) present new calculations showing that in the Oort Cloud – a shell of debris in the far reaches of our solar system – interstellar objects outnumber objects belonging to our solar system.

“Before the detection of the first interstellar comet, we had no idea how many interstellar objects were in our solar system, but the theory about the formation of planetary systems suggests there should be fewer visitors than permanent residents. “says Siraj, an undergraduate and graduate student competitor in the Harvard Department of Astronomy and lead author of the study. “Now we are seeing that there could be a lot more visitors.”

The calculations, made from the conclusions drawn from Borisov, involve significant uncertainties, Siraj points out. But even after taking these elements into consideration, interstellar visitors win out over objects originating in the solar system.

“Let’s say I watch a mile-long stretch of railroad for a day and observe a car cross it. I can tell that on that day the observed rate of cars crossing the stretch of road of iron was one per day per kilometer, ”Siraj said. Explain. “But if I have any reason to believe that the sighting was not a one-time event – say, noticing a pair of crossing gates built for cars – then I can take it a step further and start drawing statistical conclusions. on the whole rate of cars crossing this section of railroad.

But if there are so many interstellar visitors, why have we only ever seen one?

We just don’t have the technology to see them yet, Siraj says.

Consider, he says, that the Oort Cloud spans an area between 200 and 100,000 billion kilometers from our Sun, and unlike stars, Oort Cloud objects do not produce their own light. These two factors make the debris in the Outer Solar System incredibly difficult to see.

Senior astrophysicist Matthew Holman, who was not involved in the research, says the study’s results are exciting because they have implications for objects even closer than the Oort Cloud.

“These results suggest that the abundances of interstellar objects and Oort clouds are comparable closer to the Sun than to Saturn. This can be tested with current and future studies of the solar system,” says Holman, who is the former director of the CfA’s Minor Planet Center, which tracks comets, asteroids and other debris in the solar system.

“When you look at the data on asteroids in this region, the question is: are there any asteroids that are truly interstellar that we just didn’t recognize before? ” he asks.

Holman explains that there are asteroids that are detected but not observed or tracked year after year. “We think they’re asteroids, and then we lose them without doing a detailed examination.”

Loeb, co-author of the study and professor of astronomy at Harvard, adds that “interstellar objects in the planetary region of the solar system would be rare, but our results clearly show that they are more common than solar system materials. in the dark areas of the Oort cloud. “

Observations with next-generation technology can help confirm the team’s results.

The launch of the Vera C. Rubin observatory, scheduled for 2022, “will eliminate previous searches for interstellar objects out of the water,” Siraj said, and hopefully help detect many more visitors like Borisov. .

The Transneptunian Automated Occultation Survey (TAOS II), specially designed to detect comets at the far ends of our solar system, might also be able to detect one of these passers-by. TAOS II could go online as early as this year.

The abundance of interstellar objects in the Oort Cloud suggests that there is much more debris left from the formation of planetary systems than previously thought, Siraj says.

“Our results show that interstellar objects can impose interesting constraints on the formation processes of the planetary system, because their implicit abundance requires the ejection of a large mass of matter in the form of planetesimals,” explains Siraj. “Along with observational studies of protoplanetary disks and computational approaches to the formation of planets, the study of interstellar objects could help us unlock the secrets of the formation of our planetary system and others.”