spaceSpace and Physics

An Interstellar Asteroid Might Have Been Found Hiding Inside Our Solar System – And We Could Visit It


Jonathan O'Callaghan

Senior Staff Writer


The asteroid may be one of many in the Solar System. Vadim Sadovski/Shutterstock

Astronomers have found an asteroid in our Solar System that appears to be of interstellar origin, and it's close enough that we might be able to send a spacecraft to study it.

The object is called 2015 BZ509, or BZ, and it was first spotted in 2015. Last year, scientists released a paper showing that it shared a really odd orbit with Jupiter. It was not only orbiting backwards, known as retrograde, but was also locked in a death-defying dance.


Now researchers from the Côte d'Azur Observatory in France and the São Paulo State University in Brazil have provided an explanation for these odd characteristics. They suggest the object is an interstellar asteroid, snagged by our Sun when the Solar System first formed more than 4.5 billion years ago. The results are published in the Monthly Notices of the Royal Astronomical Society.

“These two properties mean that this [asteroid] is of interstellar origin,” Fathi Namouni from the Côte d'Azur Observatory, the study’s lead author, told IFLScience.

This would not be the first interstellar visitor seen in our Solar System. That honor goes to ‘Oumuamua, revealed in October 2017, which was making a flying visit at a speed too great for us to intercept it, despite some novel ideas. BZ, however, orbits within the Solar System. It means we could study an alien asteroid that’s right here, right now.

The asteroid seen moving through our Solar SYstem by the Large Binocular Telescope Observatory (LBTO). C. Veillet / Large Binocular Telescope Observatory

The big question was how it got there. So the team in this paper performed 1 million simulations of asteroids dating back to the dawn of the Solar System. Of these, only 50 survived, and most remained in orbit around Jupiter.


“We developed a new method that can actually trace back the asteroid’s location 4.5 billion years in the past,” said Namouni. “That numerical simulation showed something very interesting, that BZ509 was in a state similar to the one it has today.”

In other words, the orbit of the asteroid has not changed in 4.5 billion years. For this to have happened, it must have arrived in its orbit early on – heavily suggesting it is of interstellar origin, although note this is not yet confirmed.

Our Sun was likely born in a nebula along with other stars, inside a relatively dense stellar cluster. It’s thought that BZ was captured from within this cluster, with lots of asteroids floating around and being tidally stripped from their parent stars.

This means there could be other asteroids like this in the Solar System, captured from other stars. Namouni said there are already a handful they think are good candidates, but there could be many more.

BZ was likely captured from a stellar nursery like this, NGC 604, where asteroid exchanges could be possible. NASA / Hubble Heritage Team (AURA/STScI)

BZ is thought to be about 3 kilometers (2 miles) across, and takes about 11.6 years to make an orbit of the Sun. It travels out to about seven times the Earth-Sun distance in its orbit, or seven astronomical units (AU), and travels as close as 3 AU.

It shares this orbit with Jupiter, in what’s called a co-orbital resonance. For every orbit of the Sun the two objects make, BZ509 passes inside and then outside the gas giant – never closer than the distance from Earth to the Sun.

BZ is also in that aforementioned retrograde orbit, which means it orbits oppositely to the orbits of the other planets and the rotation of the Sun. We know of about 80 asteroids that are on retrograde orbits, but BZ509 is the only one that’s also in a co-orbital resonance with a planet found so far.

Perhaps the most exciting thing, as mentioned earlier, is that this object is still in our Solar System today, at a position that’s very reachable. Nine missions have been sent to Jupiter or beyond before. BZ may just be our best bet to sample an object from another star.


As it's in a retrograde orbit, it might be tricky to get a spacecraft into orbit around it. But we could do something else, namely a flyby and an impactor mission, something NASA has done before with its Deep Impact mission in 2005, which sent an impactor into the comet Tempel 1.

The moment the impactor from the Deep Impact mission slammed into comet Tempel 1 in 2005. NASA

“Certainly an impactor mission would be energetically feasible,” Darryl Seligman from Yale University, who was not involved in this paper but previously studied an impactor mission to ‘Oumuamua, told IFLScience.

“Such an impact, combined with a scientific package that could record the aftermath, would tell us about the composition of something that potentially formed in another Solar System, without leaving our own.”

And that’s of huge importance for a number of reasons, perhaps most notably looking at our own origins. It’s thought that asteroids may have delivered water to Earth, and studying BZ could tell us if objects like this were the culprits.


“The holy grail is identifying its chemical composition,” Namouni told IFLScience. “If we studied this asteroid up close we could have more information about its composition, its water, and also organic material.”

So BZ will be of huge interest. If it really is an interstellar asteroid that’s inside our Solar System right now, it would give us an unprecedented opportunity to study something completely alien. Either way, this probably won't be the last you've heard of it.

spaceSpace and Physics
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  • oumuamua,

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  • BZ509