Astronomers believed that binary star systems were too volatile to support many planets, because the overlapping gravitational forces of the two stars would pull most planets apart. But one exoplanet 69 light-years away survives just orbiting the wrong way.

The Nu Octanis system is a binary star system that can be seen in the southern skies, particularly in the higher latitudes around Antarctica. Manfred Cuntz and Jason Eberle, both researchers at the University of Texas-Arlington, propose there's a planet orbiting the primary star in the Nu Octanis system based on slight wobbles in the star's movement, which are known to be caused by orbiting planets.

Planets around binary stars aren't impossible, but they were thought to exist only in very small zones right around one of the two stars, so that the planets were safely away from the destructive influence of the other star. If there is a planet in Nu Octanis, then it's got to be well outside this safe zone, which raises the very real question of how it is surviving the gravitational pummeling of the other star.


According to their simulations, Cuntz and Eberle have come up with an answer - the planet could indeed exist outside the safety zone, as long as it's in a retrograde orbit. This means that it's revolving around the main star in the opposite direction of the second star. So if, say, the second star orbited the primary star in a clockwise direction, then the planet would orbit the primary star counterclockwise. This increases the possible range of orbital stability, allowing the planet to escape gravitational destruction.

Retrograde orbits are rare, and we've never seen anything quite like this before in our exploration of extrasolar planets. Still, they're not unknown, at least in this solar system's moons - most famously, Neptune's major moon Triton is in a retrograde orbit, a likely artifact of its previous existence as a massive asteroid in the Kuiper Belt.


If the researchers are right, this greatly expands the amount of planets we could expect to find in binary systems, particularly within potentially habitable zones. That's particularly good news for our neck of the galactic woods, because most of the star systems in our immediate vicinity are binary stars.

[Astrophysical Journal Letters]