Scientists have discovered the first “interstellar immigrant” living among our Solar System's matter.

Unlike Oumuamua, the asteroid that made headlines for appearing to be a gigantic spliff or a potential alien spacecraft, asteroid (514107) 2015 BZ509, affectionately known as Bee-Zed, has been circling the Sun for some 4.5 billion years – so it’s probably well adjusted by now.

Oumuamua was just passing through our Solar System at high speed, but Bee-Zed is here to stay. At first glance it looks like any other space rock out there, but its weird stable retrograde orbit – meaning it's going in the opposite way of pretty much everything else – is a tell-tale sign that it is foreign to our system of worlds. Most objects, including planets and comets, revolve around the Sun in the same way. The direction comes from the early days when the Solar System was forming in a spinning protoplanetary disk around the Sun.

In a paper published today in the Monthly Notices of the Royal Astronomical Society, a team of researchers said that Bee-Zed’s orbit was “the most puzzling example of retrograde resonance” found in the Solar System so far.

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It has the same orbital period as Jupiter, and should theoretically not have a stable orbit, yet it is stable. To get into the position it is now, it would have been captured by Jupiter when it crossed into the gas giant’s territory.

Many asteroids that circle the space between the Solar System's giant planets are collectively known as Centaurs. It is believed that most retrograde Centaurs once orbited the Sun in the same direction as all the other objects, but since they are so far from the star, the galaxy’s gravitational forces nudged them somehow so that they became retrograde. This process takes a very long time, stretching to millions or billions of years. Some of these Centaurs were then pulled back into regions surrounding the gas giants.

When the team simulated statistical models of 100 asteroids with the same orbital characteristics as Bee-Zed over a million years, however, they found that they were all stable. The timescale for that stability is two orders of magnitude longer than any temporarily captured retrograde Centaur.

In other words, this thing's orbit of our system's star is definitely stable, and it's not going anywhere else soon, having waltzed in from somewhere far away.

In fact, if Jupiter was the only planet in the Solar System, Bee-Zed would still have the same orbit as it does now, and it will probably stay that way for theoretically at least 43 billion years – although the Sun burning out might change that. Instead, astroboffins reckon Bee-Zed probably existed since the Solar System began developing over four billion years ago, and was most likely captured from outer space.

"The conclusion must be that this asteroid came from outside the Solar System. It must have been cut loose from a neighboring star system and been captured by Jupiter's powerful gravitational field. Synchronism with Jupiter makes its orbit stable," said Helena Morais, co-author of the paper and an astronomer at São Paulo State University.

But where did Bee-Zed come from? Well, scientists don’t really know yet. They hope that studying it will provide key information into the environment that the Solar System formed under.

"We may possibly be able to advance more if we can determine its chemical composition," Morais said. "Given that star systems have distinct chemical compositions, immigrant asteroids like (514107) 2015 BZ509 may have enriched the Solar System with elements that didn't exist here originally. In this way, they may have contributed to the emergence of life on Earth."

It also means that there are more immigrant asteroids lurking in the Solar System waiting to be discovered. ®