Our solar system has several belts of rocky planetesimals that are essentially the crumbs left over from the formation of the planets and moons. But new research suggests some of these objects could actually be alien, captured during close flybys with other star systems. These close encounters could also explain objects like 'Oumuamua and might even provide an alternative to the "Planet Nine" hypothesis.

It's believed that our solar system formed about 4.5 billion years ago, as a disk of dust and gas swirling around the Sun. This material gradually clumped together to form the planets and moons, with the leftovers forming rings of scattered debris – the asteroid belt beyond Mars, the Kuiper belt beyond Neptune, and the Oort cloud even further out.

The gravity of the central star usually keeps these objects whizzing around in circles for billions of years, but occasionally some of them seem to go flying off on their own. The perfect example is the bizarre cigar-shaped object named 'Oumuamua, which was spotted in 2017 and identified as a brief passer-by in our neck of the woods. This interstellar interloper has escaped its own star system – but how?

To find out, researchers at the University of Zurich ran computer simulations of star systems passing close to each other to see what happens to the objects in their planetesimal belts. They found that the belt of the smaller star is drastically disrupted by the gravity of the more massive one, sending the objects flying off on weird trajectories.

"This causes a bunch of planetesimals to be ejected, flying away to become things like 'Oumuamua," says Tom Hands, lead researcher on the study. "I was surprised by the number of 'Oumuamua-like free-floating objects that can be generated in an environment like this on a relatively short time-scale."

But zipping off into interstellar space isn't the only possible fate for these objects. The team found that they can often be captured by the passing star's gravity, or stay in their home system but end up on weird new orbits.

This idea has several implications for our own solar system. For one, it's known that other stars have brushed past the Sun in the past, with the most recent event occurring just 70,000 years ago. That means there could be alien asteroids whizzing around our neighborhood. Some of these have already been identified, but perhaps they're more common than we thought.

These close calls could also provide an alternative explanation to the Planet Nine hypothesis, which has gained ground in recent years. Some astronomers speculate that there's a large ninth planet orbiting the Sun way out beyond Pluto. The main piece of evidence for its existence is the truly odd orbits of some distant trans-Neptunian objects. But, the new study says, perhaps these are the results of gravitational disturbances inflicted by a passing star.

The research was published in the journal MNRAS. An animation of the simulation can be seen in the video below.

Source: NCCR PlanetS