They can't *all* be alien spaceships

Our Solar System may be surrounded by a halo of 10 million interstellar objects

A recent model suggests that passing interstellar objects should get pulled into our Sun's orbit fairly often.

Oumuamua asteroid, illustration

Oumuamua may not be as unusual as we thought.

As our Solar System makes its 230 million-year orbit around the outer edges of the Milky Way galaxy, it occasionally passes by bits of cosmic jetsam discarded by other stars: stray comets, asteroids, and other chunks of space rock that got kicked out of their original homes (just like our Solar System may once have booted out a whole planet). Once in a while, these objects drift through our Solar System, stirring our interest and some really wild speculation.

But our Solar System may actually have a swarm of interstellar objects trapped in its outskirts, according to a recent study by University of Edinburg astronomer and statistician Jorge Peñarrubia, published in the journal Monthly Notices of the Royal Astronomical Society.

What’s New — Wielding an amount of math that is truly terrifying to behold, Peñarrubia modeled how gravity of our Sun and the gravity of the Milky Way galaxy affect interstellar objects that come near our Solar System.

Oumuamua was the first interstellar object astronomers ever spotted in our Solar System, but it almost wasn’t the first to arrive — and it won’t be the last.


As a chunk of interstellar rock, like Oumuamua, orbits the center of our galaxy, it happens to pass close to a star system, where eight planets and a collection of smaller objects orbit a yellowish star. The tidal pull of the Milky Way’s gravity slows the stray asteroid just enough that the Sun’s gravity can capture it and pull it in for a very brief orbital dance.

Our newly-captured interstellar visitor hasn’t settled into a permanent, stable orbit, though. Instead, it will hold on for just a few laps, or maybe just part of a lap, before the Sun slings it back out into interstellar space with more energy than it had before the encounter. It will end up on a higher, faster orbit around the center of the Milky Way, in other words.

“This is exactly the same principle that we use to send ships to the outer Solar System,” Peñarrubia tells Inverse. Cassini, New Horizons, and the Voyager probes all used similar maneuvers to rich their far-flung destinations.

Essentially, the Milky Way’s gravity slows the interstellar object down enough to fall into the Sun’s orbit. And the Sun is providing a gravitational slingshot; as it pulls the asteroid into its orbit, it causes the asteroid to speed up — until that speed becomes enough for the comet to escape again.

Because the Sun is constantly capturing new interstellar objects and kicking out old ones, we keep a pretty-much constant number of them in a rocky halo around our Solar System, according to Peñarrubia. These briefly-captured interstellar objects may make up a sizeable chunk of the Oort Cloud, the hypothetical cloud of icy objects surrounding the farthest outskirts of our Solar System. And some of them end up on paths that occasionally bring them as far inward as Earth’s orbit.

It’s hard to say how many interstellar objects are part of our Solar System at any given moment, however. Astronomers aren’t actually sure exactly how many comets and other objects are in the Oort Cloud (estimates, based on the tiny number of comets that get close enough to Earth for us to see, range from hundreds of billions to trillions), let alone how many of those objects are hand-me-downs from other Solar Systems, briefly snatched up from interstellar space.

“Even if those objects are out there, we don’t have the technology yet to detect them in big numbers,” says Peñarrubia. “We only see very rare occurrences like Oumuamua and Borisov.” Based on the two or three interstellar objects astronomers have detected so far, Peñarrubia says his model predicts there should be about 10 million of them in the halo of stray space flotsam surrounding our Solar System. And a few hundred of them may even be orbiting about as close to the Sun as Neptune.

Here’s the Background — In late 2017, astronomers discovered an alien intruder in our Solar System: a long, narrow slab of rock (somewhere between 100 and 1,000 meters long), moving on a course that suggested it was just passing through our Solar System, on its way from and to somewhere out in interstellar space. And in October 2019, Comet 2I/Borisov zipped through our inner Solar System on a trajectory that suggested it had come from somewhere outside our comfy little stellar neighborhood. Astronomers branded Borisov a “rogue comet” and studied it intently.

These objects’ presence reminds us that our Solar System isn’t in a hermetic bubble, isolated from the rest of the galaxy.

“I think my work is probably telling us that there should be many more than people have thought,” says Peñarrubia.

This illustration shows, roughly, where the Oort Cloud should be in relation to the more familiar central part of our Solar System, and how far our cloud of space debris extends outward.

Ron Miller/Stocktrek Images/Stocktrek Images/Getty Images

Why It Matters — Objects like Oumuamua and Borisov are compelling to think about just because they’re from elsewhere. However, they’re scientifically interesting because they bring us evidence about the chemical makeup of other planetary systems, as well as clues about how those systems formed and evolved. And if Peñarrubia is right, then we have plenty of interstellar objects in our own Solar System, just waiting to be found and studied — if we can find them, that is.

“If you can detect a sizable population of trapped objects, then we are going to have a lot of information about how many there are in the Milky Way floating around, and what is the composition where they come in different solar systems,” says Peñarrubia. “And if so, what kind of properties those solar systems had in the past, and their age distribution, will be super interesting.”

But finding those objects is going to be a challenge. Peñarrubia’s model says they should be there, but most are too far out to see even with our most advanced telescopes — for now.

What’s Next — Even the James Webb Space Telescope can’t reveal most of the Oort Cloud to us. The small, cold objects out there are just too faint even for JWST’s powerful instruments to detect. But JWST isn’t our only chance to find and track the small objects that mostly dwell at the fringes of our Solar System. Astronomers often spot previously-unknown comets in surveys like the Sloan Digital Sky Survey and the Catalina Sky Survey. The ESA’s Gaia Observatory and the Vera Rubin Observatory will also help spot new-to-us comets, perhaps increasing our current list by several orders of magnitude.

And, according to Peñarrubia, we can tell homegrown comets from interstellar interlopers by their paths and their motion.

“If we could measure the orbit of a large number of objects — which is going to happen in the next few years, then we should start to see this kind of distinct population, which is probably also distinct in composition,” says Peñarrubia. “I wouldn’t be surprised if objects that have been trapped from the interstellar environment have this different composition than objects that were formed in the Solar System. So there will be, potentially, many different ways to see whether we can differentiate them from the rest of the comets that form in the Solar System.”

Related Tags