Posted by
Kelly Kizer Whitt
August 26, 2021
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.
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) from the sun. 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.
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 Astronomico 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.
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
Interstellar comets visit our solar system more frequently than thought
Comets from other star systems, such as 2019 Borisov, visit the sun's neighborhood more frequently than scientists had thought, a new study suggests.
The study, based on data gathered as Borisov zipped by Earth at a distance of about 185 million miles (300 million kilometers) in late 2019, suggests that the comet repository in the far outer solar system known as the Oort Cloud might be full of objects that were born around other stars. In fact, the authors of the study suggest that the Oort Cloud might contain more interstellar material than domestic stuff.
Named after famous Dutch astronomer Jan Oort, who first proved its existence in the 1950s, the Oort Cloud is a spherical shell of small objects — asteroids, comets and fragments — far beyond the orbit of Neptune. The cloud's inner edge is thought to begin about 2,000 astronomical units (AU) from the sun, and its outer edge lies about 200,000 AU away. (One AU is the average Earth-sun distance — about 93 million miles, or 150 million kilometers.)
No spacecraft has ever visited the Oort Cloud, and it will take 300 years for NASA's farflung Voyager 1 probe to even glimpse the cloud's closest portion.
Related: Interstellar Comet Borisov Shines in New Photo
Astronomers have very limited tools to study this intriguing world, as objects in the Oort Cloud don't produce their own light. At the same time, these objects are too far away to reflect much of the sun's light.
So how exactly did the scientists figure out that there must be so many interstellar objects in the Oort Cloud, and what did Borisov have to do with it?
Amir Siraj, a graduate student at Harvard University's Department of Astronomy and lead author of the study, told Space.com in an email that he could calculate the probability of foreign comets visiting the solar system simply based on the fact that the Borisov comet had been discovered.
"Based on the distance that Borisov was detected at, we estimated the implied local abundance of interstellar comets, just like the abundance of 'Oumuamua-like objects was calibrated by the detection of 'Oumuamua," Siraj said.
The mysterious 'Oumuamua, first spotted by astronomers in Hawaii in October 2017, was the first interstellar body ever detected within our own solar system. The object passed Earth at a distance of 15 million miles (24 million km), about one-sixth of the distance between our planet and the sun. An intense debate about 'Oumuamua's nature ensued, as it wasn't clear at first whether the object was a comet or an asteroid.
Even the detection of a single object can be used for statistical analysis, Siraj said. The so-called Poisson method, which the astronomers used, calculates the probability of an event happening in a fixed interval of time and space since the last event.
Taking into consideration the gravitational force of the sun, Siraj and co-author Avi Loeb, an astronomer at Harvard, were able to estimate the probability of an interstellar comet making its way to Earth's vicinity. They found that the number of interstellar comets passing through the solar system increases with the distance from the sun.
"We concluded that, in the outer reaches of the solar system, and even considering the large uncertainties associated with the abundance of Borisov-like objects, transitory interstellar comets should outnumber Oort Cloud objects (comets from our own solar system)," Siraj added.
So why have astronomers seen just one interstellar comet so far? The answer is technology. Telescopes have only recently gotten powerful enough to be able to spot those small but extremely fast-travelling bodies, let alone study them in detail.
"Before the detection of the first interstellar comet, we had no idea how many interstellar objects there were in our solar system," said Siraj. "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."
The astronomers hope that with the arrival of next-generation telescopes, such as the Vera C. Rubin Observatory, currently under construction in Chile, the study of extrasolar comets and asteroids will truly take off.
The new study was published in the journal Monthly Notices of the Royal Astronomical Society on Monday (Aug 24).
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