Last fall, an asteroid we named ʻOumuamua passed through the Solar System. Its visit marked the first time we've identified an object inside the Solar System that definitely came from outside. However, a new study argues that we might have a more permanent interstellar guest: a weird asteroid called 2015 BZ509.
Thousands of asteroids swarm around Jupiter's orbit, but they all orbit the same direction as the giant planet except 2015 BZ509. This weirdo orbits the opposite direction — "retrograde" in technical terms — at a highly tilted angle. Fathi Namouni and Helena Morais performed a computer simulation which demonstrates that 2015 BZ509 could stay in its orbit for billions of years, but it's unlikely it formed there when the rest of the Solar System was born. Instead, the authors argue, it probably originated outside the Solar System and drifted in, where it was captured by gravity.
Two Large Binocular Telescope images of asteroid 2015 BZ509 (circled in yellow), showing its motion... [+]
That's a tricky thing to prove, but the simulations are definitely thought-provoking. The authors of the study simulated a million "clones" of 2015 BZ509: asteroids that orbit under the gravitational influence of the Sun, Jupiter, and the other giant planets. They also accounted for effects from the rest of the galaxy, which are small, but can build up over billions of years. The simulation looked for stable orbits within those million clones, which are rare: many asteroids get kicked around over the Solar System's 4.5 billion-year history. The changes in gravity from the motion of the big planets can force asteroids to shift orbit dramatically, or even boot them into interstellar space. (Some might even crash into the Sun or planets, as the dinosaurs learned to their chagrin.)
Namouni and Morais found that some of the stable orbits in their simulation were very similar to the orbit followed by 2015 BZ509. And extremely stable at that: they estimated 2015 BZ509 could maintain its orbit for roughly 45 billion years, much longer than the Sun's total lifetime.
More importantly, though, they compared this simulation with the theory of how the Solar System formed, and determined 2015 BZ509's orbit couldn't have happened during planet formation. In other words, their model doesn't show a way for the natural process of making asteroids to produce something like 2015 BZ509. As a result, Namouni and Morais concluded 2015 BZ509 must have been formed in another star system entirely, and captured by Jupiter's gravity early in the history of the Solar System.
ʻOumuamua formed around another star, and was set adrift through interstellar space. It just happened to drift through the Solar System as the Sun orbits the galactic center, and astronomers expect to see more like it as we build bigger telescopes. If 2015 BZ509 is another alien, it's one that has stuck around: a now-permanent resident, rather than a vagabond. The authors concluded that, based on their simulation, there should be other alien-born asteroids orbiting the Sun, with interstellar origins betrayed by their weird orbits.
Chaos And Stability
The Solar System is mostly an orderly place, at least for big stuff. All the planets like Earth and Jupiter, all the dwarf planets like Pluto and Ceres, most asteroids, and a good fraction of comets all orbit the Sun in the same direction. The larger objects also orbit in the same plane (or close enough). The best model we have tells us they all formed from a single rotating disk of matter that surrounded the infant Sun.
However, some asteroids and many comets don't stick to the plane, including a few that are nearly "polar": looping almost perpendicular to the plane of Earth's orbit. A few others orbit in retrograde: opposite to Earth and the other planets. That's because the smaller objects get buffetted by gravity from the giant planets, kicking them into more unpredictable orbits.
But 2015 BZ509's orbit doesn't make sense even in that context. It orbits about 5.14 times farther from the Sun than Earth does, which means its orbit brings it very close to Jupiter. About 6000 other asteroids also flock around Jupiter, but they mostly stick to predictable spots, shepherded by Jupiter's gravity. 2015 BZ509 is the only known Jupiter-neighbor asteroid that orbits in retrograde.
Plot of the orbit of 2015 BZ509, with markers showing the asteroid's position every 100 days. Note... [+]
Since its discovery, astronomers have proposed various explanations for 2015 BZ509's orbit. Many of those conclude the asteroid's orbit isn't stable. In those models, we're seeing it during a period where it follows a certain path, but skip back or forward a few million years, and it would be in a completely different place. (For example, many "Centaurs", which are small bodies orbiting among the giant planets, probably came from the Oort cloud in very outer reaches of the Solar System. Maybe 2015 BZ509 came from there too.) This new paper is the first to claim both that 2015 BZ509's orbit is stable and that it must be alien-born.
It's very difficult to predict things like long-term stability because of all the ways the planets kick asteroids around. For that reason, we probably haven't heard the last about 2015 BZ509. But the good thing about this new paper is how it predicts the existence of other alien asteroids in addition to explaining where 2015 BZ509 came from. We'll learn if this weird asteroid was born here, or if it truly came from another star.
