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Borisov was not an isolated case –

Only a few years ago, astronomers discovered the first asteroids and comets of interstellar origin in our solar system. A model study now suggests that these exoplanets are by no means rare—on the contrary: the Oort cloud, the distant reservoir of icy comets at the outer edge of our solar system, can contain even more interstellar bodies than those that formed in our own system. In general, about one percent of all the carbon and oxygen in the Milky Way can also be contained in interstellar clumps.

It was only a few years ago that our solar system received regular visitors from the outside: in the fall of 2017, astronomers noticed a cigar-shaped object that passed the Sun at high speed and then disappeared again into the depths of space. Evaluation of observational data allowed only one conclusion: the part called “Oumuamua” was of interstellar origin and only passed through our solar system. In 2019, researchers discovered a second interstellar guest. But this time, it was a comet made up largely of ice, as evidenced by its tail-like outgassing and piece detachment. More detailed analyzes of the interstellar comet, Baptist 2I/Borisov, also showed that its material showed no traces of the passing of a nearby star – so the Sun’s flyby could be its first voyage through a star’s ocean.

Oort cloud on the horizon

The observation of these two “voyagers” raises the question of how frequently such interstellar visitors are, and also how many of them got into the sphere of influence of our star in the early days of the solar system. According to the popular theory, our sun formed with other stars in a common gas cloud. It is possible that the density of young stars and the gravitational interactions they cause will further deviate celestial bodies from the outer region of accretion disks around young stars and eject them into interstellar space. As a result, there could be many more than was long believed. Based on these considerations, Amir Siraj and Avi Loeb of the Center for Astrophysics | Harvard & Smithsonian (CfA) counts the number of interstellar visitors and regular guests who can hide in the vastness of the Oort cloud.

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“Before the discovery of the first interstellar comet, we had no idea how many interstellar objects there were in our solar system,” says Siraj. “But theories about the formation of planetary systems state that there will in any case be fewer visitors than permanent residents.” For their model, Siraj and Loeb started from the features and trajectory of Comet 2I / Borisov as well as from what about the Oort cloud and the behavior of its “inhabitants” are known. This reservoir of long-period comets and other ice masses begins at more than 2,000 times the distance between the Sun and Earth and can extend up to 1.6 light-years. This means that this bowl-shaped region is not only huge, but also lies in a region where the solar gravity is much weaker than it is inland.

More interstellar guests from local articles

From their model calculations, the astronomers concluded that this distant region could be distinguished more strongly by interstellar transmissions than previously assumed. “There could be a lot more visitors,” Siraj said. Specifically, they determined that the number of interstellar masses probably exceeds the number of native objects in the outer part of the Oort cloud. “Interstellar objects are rare in the solar system’s planetary region, but our results clearly show that they are more common in the dark depths of the Oort cloud than material in the Solar System,” says Loeb. Because these interstellar guests are too small and dark to be seen through our telescopes at this distance, they remain hidden from us, astronomers explain.

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Also interesting: based on their calculations, Siraj and Loeb hypothesize that a relatively high percentage of the heavy elements in our Milky Way are bound to interstellar material. “Roughly one percent of the carbon and oxygen can be found in interstellar bodies,” the team says. This could mean that during the formation of stars and planetary systems, much more material is dumped into space than current models of planet formation take into account. “Interstellar objects can impose interesting constraints on the formation processes of planetary systems,” says Siraj. Conversely, a large loss of materials means that initially there must be more “stocks of building materials” than previously calculated.

Quelle: Amir Siraj und Avi Loeb (Center for Astrophysics | Harvard & Smithsonian, Cambridge), Monthly Notices of Letters of the Royal Astronomical Society, doi: 10.1093/mnrasl/slab084