How many of our comets come from alien solar systems?

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. Their research was published last month in the Monthly Notices of the Royal Astronomical Society . Science talked to Hands to find out more about these mysterious icy visitors. This interview has been edited for clarity and length.

Q: Where do researchers think most of our comets come from?

A: People have hypothesized that they come from something called the Oort cloud. This is a big, almost spherical cloud of objects at the very edge of our Solar System. It’s thought to have formed a very long time ago, when the giant planets scattered a bunch of cometlike materials with a lot of ice to the outskirts of the Solar System. Passing stars can scatter these things back into the Solar System, which is how we observe them today.

Q: What can we learn from these interstellar visitors?

A: I think the most the most interesting thing for me is that you’d get a chance to look at a sample of the planet formation environment around another star. We know in some detail what material is present in our Solar System, and if that differs a lot around other stars, then it tells us something about how planets are forming in other solar systems.

Q: In the paper, you ran a simulation with millions of interstellar objects to see how they might be captured by the gravity of Jupiter. What does “captured” mean?

A: Essentially, when an interstellar object approaches our Solar System, it has a very high velocity compared to the comets and asteroids that we observe every day. Once they reach their point of closest approach to the Sun, they simply start moving away again and never come back. This is similar to the way the Voyager probes are never coming back. For them to become bound, they have to lose some of this velocity, which they can do by a close interaction with a giant planet—in our case, Jupiter. This is conceptually similar to the kind of gravitational assist that spacecraft often use to increase their velocity—in our case the interstellar objects are robbed of some of their kinetic energy by the giant planet, and in a small minority of cases, they lose enough kinetic energy to become bound.

Q: How many of these interstellar objects might be in our Solar System at any given time?

A: 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. Nevertheless, we estimated that 0.33% of them should be within 6 astronomical units [about 900 million kilometers] away—a fairly typical radius for comets to “switch on”—at any given time. So, the chances of seeing one are relatively low, but it’s by no means impossible.