Astroboffins dig into the weird backwards orbit of the Bee-Zed asteroid

Tiny body moves similar to Jupiter – but travels in the opposite direction

Asteroid 2015 BZ509 – also known as Bee-Zed – is the only asteroid in our solar system with a confirmed retrograde orbit lasting 12 years; the same orbital period as Jupiter.

The discovery was published in Nature in March by a team of physicists led by Paul Wiegert, a researcher at the University of Western Ontario in Canada. It was predicted two years earlier by Helena Morais, an astronomy professor at São Paulo State University in Brazil.

Now Morais is working in partnership with Fathi Namouni, an astronomer at the Côte d'Azur Observatory in France. They're working on a general theory to explain retrograde co‑orbitals and retrograde orbital resonance.

Bee-Zed is an odd asteroid:

  • It's rare in that only 82 of more than 726,000 asteroids orbit in the opposite direction of the incoming traffic of planetary bodies.
  • Its orbit is similar to that of a planet – it has the same orbital period as Jupiter.
  • Its configuration has been stable for millions of years, baffling scientists.

"Instead of being ejected from orbit by Jupiter, as one would expect, the asteroid is in a configuration that assures stability thanks to co‑orbital resonance, meaning its motion is synchronized with the planet's, avoiding collisions," Morais said in a statement from São Paulo Research Foundation on Wednesday.

Co-orbital resonance describes two nearby bodies that exert regular gravitational influences on one another when their orbital periods reach a certain ratio. For example, Bee-Zed crosses Jupiter's path every six years, but their motion is locked and they never come within 176 million kilometers (109 million miles) of each other. Jupiter keeps the asterioid's orbit stable.

Most objects orbit the Sun in the same direction as when the Solar System was born from a swirling cocktail of dust and gas known as the protoplanetary disk. But in the final stages, objects like Bee-Zed were kicked far away into the outer edges in the Oort cloud, a shell of floating icy debris.

"At these distances, the Milky Way's gravitational effects disturb small bodies. To begin with, they orbited close to the plane of the ecliptic in the same direction as the planets, but their orbits were deformed by the galaxy's tidal force and by interactions with nearby stars, gradually becoming more inclined and forming a more-or-less spherical reservoir," Morais said.

If these bodies pass a star like the Sun and their orbits are disturbed, they can veer off their paths and become active comets. Their icy exteriors are warmed by the Sun and they sublime, forming a tail of gas, cloud and dust particles. But it's a situation that is not likely to happen with Bee-Zed any time soon, as according to Morais and Namouni, its orbit will go undisturbed for another million years or so.

Talking to The Register, Morais added: "We don't know exactly how they [asteroids from the Oort cloud] can come back to orbits such as that of Bee-Zed. The Halley family comets also include high inclination and retrograde orbits and we are not sure if they came from the Oort cloud or if there is another source or evolution path." ®

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