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How much of the solar system is made of interstellar stuff?

NASA's Hubble Space Telescope captured this view of the interstellar Comet 2I/Borisov on Oct. 12, 2019.
NASA's Hubble Space Telescope captured this view of the interstellar Comet 2I/Borisov on Oct. 12, 2019. (Image credit: NASA/ESA/D. Jewitt (UCLA))

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of "Ask a Spaceman" and "Space Radio," and author of "How to Die in Space." Sutter contributed this article to's Expert Voices: Op-Ed & Insights.

The detection of interstellar objects in the solar system has raised an interesting question: How much of the solar system is made of foreign material? New research finds that the answer is … not much at all.

Astronomers have detected a grand total of two interstellar visitors: the difficult-to-classify 'Oumuamua, in 2017, and the just-a-comet Borisov shortly thereafter. Those two objects have spent a relatively brief amount of time in the solar system — just a few years, compared with the tens of thousands of years they spent sailing through the desolate interstellar spaces between the stars.

Their arrival has fueled speculation about the number of interstellar objects flying through the galaxy. That number could easily be in the hundreds of trillions (if not more), if the ejection of unwanted debris is a common side effect of the formation of solar systems. That thought — that there could be countless tiny objects zooming around the Milky Way — spurs another question: Just how much of the solar system was made here originally, and how much of it is captured wandering space junk?

Related: 'Oumuamua: The solar system's 1st interstellar visitor explained in photos

To date, there have been no detections of any extrasolar objects currently in orbit around the sun. The best we can find are micrometeoroids, tiny specks of dust that have floated in over the eons. But that lack of detection does not necessarily mean that there are no alien rocks hiding in the shadows. We've barely mapped all of the large rocks in the asteroid belt, let alone the much more distant — and much more difficult to observe — Kuiper Belt of the outer system.

But studying rocks one by one, looking for a foreign asteroid or comet, is a painfully slow process, especially if we don't know how common those rocks are in the first place.

Should I stay or should I go?

New research, published to the preprint server arXiv and accepted for publication in The Planetary Science Journal, attempts to estimate the number of captured interstellar objects coming into the solar system and to monitor how long those objects remain here.

The researchers used lots of simulations. They studied the behavior of 276,691 objects entering the solar system at all sorts of directions and speeds, and they traced the evolution of each of those simulated objects within the solar system back a billion years. 

They found that most interstellar objects don't survive long. If they end up around the sun, within the orbit of Jupiter, they're very likely to have a close encounter with that giant planet. And when that close encounter happens, they either get eaten by the gas giant or get tossed back out of the solar system.

If the foreign object ends up in an orbit with a plane close to those of the other planets, the object likewise is likely to get tossed away by the combined gravitational influences of all the original members of the solar system.

While foreign objects tend to hang around for millions of years, it's not because they've permanently set up shop. When they're captured by the sun, they tend to have very big and very stretched-out orbits. It can take an object multiple orbits, and thus over a million years, to figure out whether it will stay for the long term.

In the end, though, foreign objects have a tough time surviving in the solar system. Of the more than 270,000 simulated objects, only 13 stuck around for more than 500 million years, and a mere three stayed for a billion. 

Wolf in sheep's clothing

OK, so foreign objects don't stay in the solar system for long. That's one piece of the puzzle. The other is estimating just how many objects are crossing into the system. If that's an overwhelmingly large number, then even with the pitiful survival rates, the solar system might be teeming with interstellar visitors.

The estimate is a bit speculative, because it relies on models of planetary formation and the little information gleaned from 'Oumuamua and Borisov.

When the sun was forming, it was embedded in a much larger star cluster. Because it was so much closer to other forming stars (and forming planetary systems), it was much more likely to capture foreign material back then. The researchers estimated that the sun captured enough objects during its birth phase to assemble 1/1000 the mass of Earth, which could be enough to craft about six asteroids the size of the dwarf planet Ceres.

In the billions of years since the solar system was born, it has also encountered a few objects like 'Oumuamua and Borisov every year.

Putting it all together, we shouldn't expect a lot of foreign material in the solar system — just a billionth of an Earth mass from foreign objects captured during our system's formation and over a thousand times less than that since then, the study authors estimated. That's barely enough material to put together a single 6-mile-wide (10 kilometers) asteroid.

This result has two important implications. One, we shouldn't bother looking for captured foreign objects, because they are exceedingly rare. Two, theories of panspermia, which posit that life may have started elsewhere and been carried to Earth later, aren't tenable. There simply isn't enough material flying through the galaxy, entering solar systems, coming into stable orbits and then impacting other planets to make the idea work.

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Paul Sutter

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe, to the earliest moments of the Big Bang, to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space," and frequently appears on TV.