How many of our comets come from alien solar systems?
https://www.sciencemag.org/news/2020/01/how-many-our-comets-come-alien-solar-systems
Source: By Eva Frederick, Science Magazine.
Excerpt: Comets are generally thought to originate in our Solar System, made up of the leftover gas and rocks thrown out as the planets formed. The recent arrival of two interstellar objects—a rock named ‘Oumuamua and a flashy comet called Borisov—have challenged that assumption. Tom Hands, an astrophysicist at the University of Zurich’s Institute for Computational Science and his co-author Walter Dehnen at Ludwig Maximilian University of Munich used mathematical models to estimate just how many long-period comets—those that take 200 years or longer to circle the Sun—could be interstellar visitors. ...We estimated from the study that there should be 100,000 ‘Oumuamua-style small rocks and 100 Borisov-style comets in the Solar System. Making far more conservative estimates for how long these objects would survive in the Solar System [shorter than 10 million years], we would expect 20,000 ‘Oumuamuas or 20 comets. The majority of these things would have highly eccentric orbits with periods of a few hundred thousand years, meaning they spend the vast majority of their time far, far out beyond the orbit of Pluto....
Source: By Eva Frederick, Science Magazine.
Excerpt: Comets are generally thought to originate in our Solar System, made up of the leftover gas and rocks thrown out as the planets formed. The recent arrival of two interstellar objects—a rock named ‘Oumuamua and a flashy comet called Borisov—have challenged that assumption. Tom Hands, an astrophysicist at the University of Zurich’s Institute for Computational Science and his co-author Walter Dehnen at Ludwig Maximilian University of Munich used mathematical models to estimate just how many long-period comets—those that take 200 years or longer to circle the Sun—could be interstellar visitors. ...We estimated from the study that there should be 100,000 ‘Oumuamua-style small rocks and 100 Borisov-style comets in the Solar System. Making far more conservative estimates for how long these objects would survive in the Solar System [shorter than 10 million years], we would expect 20,000 ‘Oumuamuas or 20 comets. The majority of these things would have highly eccentric orbits with periods of a few hundred thousand years, meaning they spend the vast majority of their time far, far out beyond the orbit of Pluto....