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

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Paul M. Sutter is an astrophysicist at SUN 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 The expert voices of Space.com: Op-Ed & Insights.

The interstellar object detection in the solar system raised an interesting question: to what extent is the solar system made up of foreign matter? New research shows 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 after. These two objects spent a relatively short time in the solar system – just a few years, compared to the tens of thousands of years they spent navigating the desolate interstellar space between the stars.

Their arrival has fueled speculation about the number of interstellar objects flying across the galaxy. That number could easily run into the hundreds of billions (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 tiny objects zooming around the Milky Way – prompts another question: what part of the solar system was originally created here and what part is stray space debris?

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

To date, there has been no detection of extrasolar objects currently orbiting the sun. The best we can find are micrometeoroids, tiny grains of dust that have been floating 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 belt, not 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 a painfully slow process, especially if we don’t know how common these 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 to monitor how long those objects stay here.

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

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

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

While foreign objects tend to hang around for millions of years, it is not because they have settled permanently. When captured by the sun, they tend to have very large and very elongated eye sockets. It can take an object several orbits, and therefore more than a million years, to determine if it will stay in the long term.

Ultimately, however, foreign objects find it difficult to survive in the solar system. Of the more than 270,000 simulated objects, only 13 have remained for more than 500 million years, and only three have remained for a billion.

Wolf disguised as lamb

OK, so foreign objects don’t stay in the solar system for long. It’s a piece of the puzzle. The other is to estimate the number of objects entering the system. If this is an extremely high number, then even with 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 integrated into 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 matter at the time. Researchers estimated that the sun captured enough objects during its birth phase to assemble 1/1000 of Earth’s mass, 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, he has also encountered a few objects like ‘Oumuamua and Borisov every year.

Overall, we shouldn’t expect a lot of foreign matter in the solar system – just a billionth of Earth’s mass from foreign objects captured during the formation of our system and more than a thousand times less than that since then. , estimated the authors of the study. . That’s barely enough material to assemble a single 10-kilometer-wide asteroid.

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

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