Do you remember Oumuamua? Just about six months ago our Solar System had a surprise visit from an object that came from another star. But Oumuamua was only quickly stopping by and is on its way out again. In November 2017 it came closer to the Sun than our innermost planet, Mercury. Now it is already halfway between Jupiter and Saturn and it will keep on flying away, never to be seen again.

^{Image Credit: C. Veillet / Large Binocular Telescope Observatory.}

Now scientists discovered a completely different object that also seems to come from a different star system. The object has been given the full name 2015 BZ509. The first number indicates the year in which it was discovered. Astronomers have been following the object, that they lovingly call BZ for short, for a number of years. While plotting its trajectory they found something remarkable.

From the very beginning, they saw that the rock was moving in the wrong direction. All planets and most asteroids in the solar system revolve around the sun in the same direction. This can be explained by the fact that our Solar system was created from a spinning cloud of gas. An object that moves around the Sun in the opposite direction is something special. But BZ509 is not the only such anomaly. There are a number of asteroids that once got too close to one of the bigger planets and got flung around and are now orbiting in a retrograde orbit. But their number is small. Only a few dozens of these special asteroids are known.

With the newly discovered BZ509 things seem to be different. The new asteroid moves around the sun in an orbit close to Jupiter. There are a number of other asteroids that move between the giant planets and these are known as Centaurs. The gravity of the big planets disturbs the orbits of these objects. This is the reason that most asteroids with retrograde orbit are found in this group. But this is also the reason that scientists believe that Centaurs have unstable orbits. They get pulled back and forth until, at some point, they get thrown out of the orbit altogether.

In a recent publication, Dr Fathi Namouni and Dr Helena Morais calculated the orbit of BZ509. Surprisingly, they found that the asteroid must have been around for a very, very long time. They determined that BZ509 was already in its current orbit about 4.5 billion years ago. Quite, how it was able to stick around for this long is not clear. Maybe some specific resonance is keeping the orbit stable. This time is significant because it is about as old as the planets themselves. The scientists conclude that the only possible explanation for its strange behaviour is that BZ509 is from another star.

Early on in the formation of our Solar system, BZ509 must have travelled the vast distance from some other system. It got caught by our Sun and then proceeded to find a nice cosy spot for itself. It has been with us ever since. The fact that this 3-kilometre large piece of interstellar rock has been observed suggests that there may be other, similar objects out there. Most of them will orbit the sun at some angle to the ecliptic plane or even perpendicular to it. The recent discovery has sparked interest in looking for these distant guests.