The asteroid 2015 BZ509 is weird.
At first glance it seems normal enough. It orbits the Sun roughly at the same distance as Jupiter and it's about 3 kilometers or so in diameter. But it didn't take long before its defining weird characteristic became clear: It orbits the Sun backward.
That is, most objects in the solar system orbit the Sun (or in the case of many moons, their host planet) in very close to the same plane, and they do so counterclockwise if you're above the solar system and looking down on it. We call that direction prograde.
But BZ bucks the trend. It has an orbit that's highly tipped, so much that it actually goes the other way 'round. Its orbit is retrograde.
Also weirdly, it gets close enough to Jupiter twice every orbit to get a bit of a tug from the giant planet's huge gravity. Over time, you'd think that should disturb the orbit enough to send BZ off its path, either plunging it into the Sun or ejecting it from the solar system. But that's not the case; last year it was found that, because of its unusual orbit, when it passes Jupiter one time it gets tugged first one way, and then the next pass it gets tugged the other way (think of it as left and right if that helps). The two forces balance out, and BZ's orbit is stable.
I have more details in an article I wrote in 2017 when this news was announced.
The big question is: How did it get there? There's essentially zero chance it formed in this orbit; it's overwhelmingly far more likely it started in some different orbit and got nudged into its current path by the gravity of other planets. But how? It's weird.
Before I go any farther I want to be clear: I am fairly skeptical of this claim. I'm not saying they're wrong, but the evidence they present at this time doesn't convince me. In my opinion a lot more work needs to be done before such a bold declaration can be warranted.
So what do they claim?
The astronomers looked at the orbit and used sophisticated computer modeling techniques to run it backward in time as a way to check stability. We know that over time, small perturbations (think of them as planets poking at the asteroid gravitationally) can add up, throwing a system into chaos. In fact, many small objects seen in the outer solar system are not in stable orbits in the long run; over millennia or longer they get thrown off their trajectories. So in this case, BZ's orbit looks stable, but is it really over a long time?
What they found is that BZ could have been in this same retrograde orbit quite happily for the past 4.5 billion years, since the solar system formed. That's interesting! Furthermore, they simulated a million objects like BZ but in very slightly different orbits to see how that might affect things (this is a common technique, and for example was used to see where Elon Musk's space-bound Tesla Roadster would be in the next century or two). What they found was that only a few dozen (out of a million!) were able to stay in stable orbits over billions of years. They then argue that those small odds show that BZ has been this way for a long time.
Furthermore, because the solar system settled into pretty much its present configuration not long after it formed, it's very unlikely that any object could get injected into such a tilted orbit since that time; the current arrangement of planets (they claim) makes it very difficult if not impossible to do so. Therefore, their claim goes, it must have come from another star.
They make their confidence pretty clear in the paper abstract:
We perform a high resolution statistical search for stable orbits and show that asteroid 2015 BZ509 has been in its current orbital state since the formation of the Solar System. This result indicates that 2015 BZ509 was captured from the interstellar medium 4.5 billion years in the past…
But, to be honest, I don't think their evidence supports that. I want to point out that their conclusion very well might be correct — this may indeed be an interstellar asteroid. We've seen one before, and recently, if you recall the saga of 'Oumuamua. That particular rock was just passing through, while BZ is on a stable orbit, so perhaps the two shouldn't be compared. But still, it's not impossible.
The problem here is that I don't think the astronomers eliminated every possible reason this asteroid could be in its bizarre orbit. That's a nearly impossible task, of course (it's a black swan situation), but even so, there are some likely scenarios.
First, just because their simulations show it could have been in its orbit for 4.5 billion years doesn't mean it has been. Maybe it had a planetary encounter a few thousand years ago and got lucky. In general (as they point out in their paper) many objects in this part of the solar system don't last long in their orbits before being nudged away by gravitational perturbations. But that doesn't mean there can't be exceptions. Again, I'm not saying this is one, but the onus is on them to show it isn't.
Also, it's possible we're just seeing the biggest object in a population of smaller ones in similar orbits. A good example of this is Pluto. It's also on a special orbit that brings it closer to the Sun than Neptune, but they can't interact because Pluto's orbit is shaped in such a way that every time it crosses Neptune's orbit, the big planet is one-fourth of the way around its orbit, so it's too far away to affect Pluto. There are lots of smaller objects on similar orbits just recently discovered, too. If this is the case, then it's less likely BZ is an outlier.
Now, just because I can say that doesn't mean it's true either. But there's precedent. I talked with astronomer Mike Brown (who, with Konstantin Batygin, posits the existence of a ninth planet past Neptune which has been affecting orbits of many icy bodies in its region), and he told me that objects can be sent into similar retrograde orbits as BZ, and they could even have the same weird resonating interaction Jupiter has with it. Objects like this have been seen out by Saturn for example, though, to be fair, those objects were prograde, not retrograde, and something like this hasn't been seen yet specifically for Jupiter. But he also mentioned Planet Nine could have affected BZ. I'll note that other scientists are skeptical as well.
We can make arguments like this all day long, but here's my point: There are other possibilities besides an alien origin for the asteroid that need to be checked before claiming it's interstellar. It may very well have been snatched by the Sun from some alien star, but there may also be more local explanations as well.
Either way, 2015 BZ509 is a very interesting object. And, to be very honest, I'd love it if it came from another star! That opens a door to lots of fun possibilities, and would be incredibly exciting scientifically.
Until then, no matter where it came from, it's an object well worth keeping our eyes on.