How much of the solar system is made up of interstellar matter?

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of “ask an astronaut” and “space radio” and author of “how to die in space.” Sutter contributed this article to Expert Voices: Op-Ed & Insights.

the detection of interstellar objects in the solar system raised an interesting question: how much of the solar system is made up of foreign matter? New research reveals the answer is… not much at all.

Astronomers have detected a grand total of two interstellar visitors: the hard-to-classify ‘Oumuamuain 2017, and the just-comet Borisov shortly after. Both of these objects have spent relatively little time in the solar system – just a few years, compared to the tens of thousands of years they spent navigating the desolate interstellar spaces between stars.

Their arrival has fueled speculation about the number of interstellar objects flying through the galaxy. This number could easily be in the hundreds of trillions (if not more), if the ejection of unwanted debris is a common side effect of solar system formation. This thought – that there could be countless small objects zooming around the Milky Way – raises another question: what part of the solar system was originally made here, and what part is captured by errant space junk?

Related: Oumuamua: the 1st interstellar visitor to the solar system explained in photos

To date, there have been no detections of extrasolar objects currently orbiting the sun. The best we can find are micrometeoroids, tiny specks of dust that have floated around over eons. But this lack of detection doesn’t necessarily mean there aren’t alien rocks lurking in the shadows. We barely mapped all the big rocks in the asteroid beltnot to mention the much more distant — and much more difficult to observe — Kuiper Belt of the external system.

But studying rocks one by one, looking for an alien asteroid or comet, is an extremely slow process, especially if we don’t know how common those rocks are.

Should I stay or should I go?

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

The researchers used many simulations. They studied the behavior of 276,691 objects entering the solar system in all kinds of directions and speeds, and they traced the evolution of each of these simulated objects in the solar system over a billion years.

They discovered that most interstellar objects do not survive long. If they find themselves around the sun, in the orbit of Jupiter, they are very likely to have a close encounter with this giant planet. And when that close encounter happens, they’re either eaten by the gas giant or thrown out of the solar system.

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

If foreign objects tend to hang around for millions of years, it’s not because they’ve settled in permanently. When captured by the sun, they tend to have very large and very elongated orbits. It may take an object several orbits, and therefore more than a million years, to determine if it will stick around for the long term.

Ultimately, however, alien objects struggle to survive in the solar system. Of more than 270,000 simulated objects, only 13 have survived for more than 500 million years, and just three have survived for a billion.

wolf in lamb’s clothing

OK, so foreign objects don’t stay long in the solar system. It’s a piece of the puzzle. The other is to estimate how many objects pass through the system. If that’s an extremely high number, then even with the pitiful survival rates, the solar system could be teeming with interstellar visitors.

The estimate is a bit speculative, as 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 pick up foreign matter at the time. 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 make about six asteroids the size of the dwarf planet. Ceres.

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

All in all, we shouldn’t expect many foreign bodies in the solar system – just a billionth of Earth’s mass from foreign objects captured when our system was formed and more than a thousand times less than that since then. , the study authors said. . That’s barely enough material to assemble a single 10 kilometer wide asteroid.

This result has two important implications. First, we shouldn’t bother searching for captured foreign objects, as they are extremely rare. Two, the theories of panspermia, which posit that life may have originated elsewhere and was transported to Earth later, are untenable. There just isn’t enough matter flying through the galaxy, entering solar systems, arriving in stable orbits and then impacting other planets for the idea to work.

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Arline J. Mercier