Oort Cloud news: How many comets from elsewhere?

Oort cloud news: Fuzzy blue light with dimmer fuzzy, gaseous cone around and trailing it.
Comet Borisov, spotted in 2019, was a visitor from another solar system. It’s the 2nd known interstellar object, and 1st known interstellar comet. But could there be billions more interstellar comets in the Oort Cloud? Image via NASA/ ESA/ D. Jewitt (UCLA).

Oort Cloud news

Astronomers picture the Oort Cloud as a cloud of comets on the farthest outskirts of our solar system. Dutch astronomer Jan Oort theorized its existence in 1950. He said long-period comets are sometimes knocked from their distant orbits in the Oort Cloud (perhaps by passing stars). That’s how they end up in orbits that bring them near our sun. If it exists, Oort thought, this comet cloud is made of material left over from our solar system’s formation 4.5 billion years ago. But is it? Scientists now generally agree that billions of comets must reside in the Oort Cloud. But what fraction of these comets might have originated in other star systems? This week (August 22, 2021), two scientists said the answer might be … most of them.

The two scientists are Amir Siraj and Avi Loeb, both of Harvard. Loeb is also author of Extraterrestrial, the First Sign of Intelligent Life Beyond Earth, which proposes that the first known interstellar visitor (1I/ ‘Oumuamua), might have been an artificial object, made by an advanced extraterrestrial civilization. The peer-reviewed journal Monthly Notices of the Royal Astronomical Society published these scientists’ new study about the Oort Cloud on August 23, 2021.

The study focuses on a realm of space that lies between 1,000 and 100,000 times Earth’s distance from our sun. In fact, some astronomers estimate the Oort Cloud may extend out nearly as far as a light-year from our sun. By contrast, the nearest star to our sun is about 4 light-years away.

Comet Borisov, the first interstellar comet

Astronomers spotted the first known interstellar object, ‘Oumuamua, in 2017. In 2019, they spotted a second object from interstellar space. This one looked more distinctly comet-like. Astronomers call it 2I/ Borisov, or sometimes just Comet Borisov. Traveling at 110,000 miles per hour (177,000 kph), Comet Borisov passed closest to the sun and Earth in December 2019, displaying a tail 14 times the size of Earth. Then it headed back toward interstellar space.

It’s the information drawn from Comet Borisov that enabled Siraj and Loeb to speculate that the Oort Cloud consists mostly of interstellar visitors. The scientists admit the information on Comet Borisov still holds a degree of uncertainty. But, even with these uncertainties, the scientists said their calculations show that interstellar objects are more numerous than solar system objects in the Oort Cloud. Siraj commented in a statement:

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. Now we’re finding that there could be substantially more visitors.

Interstellar objects are dark and distant

If the Oort Cloud contains perhaps billions of interstellar objects, why haven’t we seen more of them? Siraj said it’s because we don’t yet have the technology to see them. Consider first how far away the Oort Cloud is. The Earth-sun distance (93 million miles, or 150 million km) is called an astronomical unit (AU). Tiny Pluto is about 40 AU from the sun. The even smaller comets in the Oort Cloud are some 1,000 to 100,000 AU away. But remember that the comets don’t shine by their own light. They only reflect our sun’s light. So a comet in the Oort Cloud, interstellar or otherwise, is simply too far from the sun, too dim and too small for us to see directly. Thus we see comets – and speculate about an Oort Cloud – only thanks to those comets that are dislodged from the Oort Cloud and come hurtling in to our part of the solar system.

Diagram of solar system with Oort Cloud large and far right.
The Oort Cloud is so far out from the sun that its existence is still hypothetical (though it’s logical to believe it does exist). The cloud is visualized as stretching from 1,000 times the Earth-sun distance to about 100,000 times that distance. Image via NASA.

Could there be interstellar objects closer to Earth?

Matthew Holman, former director of the Center for Astrophysics Minor Planet Center, who did not participate in the research, wondered if the abundance of interstellar visitors in the farthest regions of the solar system could translate to some interstellar visitors closer to the sun. He said:

These results suggest that the abundances of interstellar and Oort cloud objects are comparable closer to the sun than Saturn. This can be tested with current and future solar system surveys. When looking at the asteroid data in that region, the question is: Are there asteroids that really are interstellar that we just didn’t recognize before?

There are asteroids that scientists have detected but not tracked over the years. Holman mused:

We think they are asteroids, then we lose them without doing a detailed look.

Co-author Loeb added:

Interstellar 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.

Future searches

The Vera C. Rubin Observatory, currently under contruction in Chile, should help astronomers find more visitors from outside our solar system. Siraj commented that the new observatory will:

… blow previous searches for interstellar objects out of the water.

First light for the Vera Rubin’s engineering camera is expected in October 2022, and full survey operations expected perhaps a year later.

Also slated for 2022, the Transneptunian Automated Occultation Survey will operate three medium-sized telescopes at the Observatorio Astronómico Nacional at San Pedro Mártir, a mountain range in Baja California, México. This system is specifically designed to find small bodies on the outer edges of our solar system. Perhaps it will unlock the door to more interstellar objects. Siraj said:

Our findings show that interstellar objects can place interesting constraints on planetary system formation processes. [A large number of interstellar objects in the Oort Cloud] requires a significant mass of material to be ejected [from our solar system] in the form of planetesimals. Together with observational studies of protoplanetary disks [disks around newly forming stars] and computational approaches to planet formation, the study of interstellar objects could help us unlock the secrets of how our planetary system – and others – formed.

Bottom line: Scientists’ calculations show that the majority of comets in the Oort Cloud may be interstellar objects, or visitors from beyond our solar system.

Source: Interstellar objects outnumber Solar system objects in the Oort cloud

Preprint at arXiv: Interstellar Objects Outnumber Solar System Objects in the Oort Cloud

Via Harvard and Smithsonian Center for Astrophysics

August 26, 2021

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