Artist's impression of interstellar comet 2I / Borisov. Could there be millions of other interstellar objects like this hiding in the Oort Cloud? Credit: M. Kormesser, ESO

Solar System’s Oort Cloud Repository for Interstellar Objects — 10 Things You Need to Know

Astronomers believe that there could be millions of interstellar objects in the Solar System despite the fact that only two have been seen to date.

Astronomers at the Center for Astrophysics at Harvard and Smithsonian Institutions (CfA) present new calculations showing that the Oort Cloud could have has more interstellar objects than objects, belonging to our solar system. This controversial theory goes against all traditional understandings of planetary system formation. 

New theory: There could be more interstellar objects in the Oort Cloud than objects that formed in the Solar System

First interstellar comet

Several years ago, astronomers discovered the first interstellar comet – 2I/Borisov, and since then, scientists have been trying to determine the approximate number of interstellar objects in the Solar System.

Avi Loeb

Harvard astrophysicist Professor Avi Loeb and his ward Amir Siraj have published quite bold and high-profile works more than once. We can recall their assumption about the origin of distant objects in the Oort Cloud – a hypothetical region at the periphery of the solar system, from where long-period comets fly to us from time to time. According to the version expressed by Loeb and Siraj, these may be the remnants of a second star that once existed, the “twin” of the Sun.

Types of objects

Until now, experts logically believed that there should be significantly more objects formed in our Solar System in the Oort Cloud than interstellar visitors.

New theory

A new article has been published in Monthly Notices of the Royal Astronomical Society: Letters. In it, scientists consider an alternative option. Most of the objects in the Oort Cloud could be of interstellar origin, as indicated by the recent observation of the first such comet, 2I / Borisov.

Comet 2I/Borisov. Credit: NASA/Wikimedia Commons
Comet 2I/Borisov. Credit: NASA/Wikimedia Commons


Scientists note that the calculations made with the help of studies of comet 2I / Borisov contain significant inaccuracies. Yet, it appears that interstellar visitors dominate objects that are natural to the Solar System.

Why have we seen so few interstellar objects?

But if there are so many interstellar visitors, why have we ever seen only two (comet Borisov and Oumuamua)? According to the authors of the work, at the moment, there are no such tools that allow us to see them.

Oort Cloud

Unfortunately, this region is too far away – hundreds of trillions of kilometers away – and its features are very small and dim. It is still impossible to observe these bodies, and even more so to distinguish how many are “local” among them, and how many have come from interstellar space.


Nevertheless, the calculations of Loeb and Siraj show that quite a large amount of matter can accumulate in such regions, and they probably account for up to one percent of all carbon and oxygen in our Galaxy.

Vera C. Rubin Telescope

Observations with next-generation technology can help validate the team’s findings. For example, the observatory. Vera C. Rubin, slated to launch in 2022, will help find many more visitors, such as Borisov’s comet.

Transneptunian Automated Occultation Survey

The Transneptunian Automated Occultation Survey (TAOS II), specially designed to detect comets in the far reaches of our solar system, may also detect some of these objects.

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Center for Astrophysics. (n.d.). Interstellar comets like Borisov may not be all that rare.
Clarke, A. (2021, August 24). Over 100 trillion objects at the Oort CLOUD ICY edge of the solar system are from interstellar space Like Comet 2I/Borisov. Science Times. (2021, August 23). Interstellar comets like Borisov may not be all that rare.
Siraj, A., & Loeb, A. (2021, August 23). Interstellar objects outnumber solar system objects in the Oort cloud. OUP Academic.

Written by Vladislav Tchakarov

Hello, my name is Vladislav and I am glad to have you here on Curiosmos. My experience as a freelance writer began in 2018 but I have been part of the Curiosmos family since mid-2020. As a history student, I have a strong passion for history and science, and the opportunity to research and write in this field on a daily basis is a dream come true.

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