Caught between a star and a hot place (Image: Detlev van Ravenswaay/SPL)

Helter skelter! It turns out that in some rare cases, a planet in a binary system may spiral around the axis that connects its two stars – although how such planets come to be is unclear.

We normally think of planets orbiting sedately around their star, like Earth does. Binary systems are more complicated, but astronomers usually assume that a planet will stay confined to a single plane of motion, tracing a disc either around both its parent stars or just one.

Eugene Oks, a theoretical physicist at Auburn University in Alabama, wondered what would happen without that assumption. His model shows that, if you imagine a line connecting the two stars, a planet could trace a corkscrew around that line, travelling back and forth between the stars.


As it moves closer to one star, the spirals get closer and closer together as the planet moves more slowly, until it turns and moves back toward the other star. In the middle, it traces wild, fast curves around the axis.

Life – if it could survive – would be very different to that on Earth. Sandwiched between two stars, only a small slice of the planet would ever experience night. If the planet was tilted on its own axis, then mini seasons would come and go quickly, with every turn of the spiral.

Quasimolecules

Oks was inspired by a rare class of molecules called one-electron Rydberg quasimolecules that display the same corkscrew orbit of their electrons under electromagnetism that Oks’s hypothetical planets do.

If such planetary systems do exist, Oks says they should show up using the same techniques that we already use to find exoplanets, but they would be indistinguishable from other orbits.

Oks admits that his expertise is in molecular physics, not planetary science, but exoplanet experts greet the idea warmly.

“The idea that one can have a stable corkscrew orbit in a binary star system is quite cool,” says Gregory Laughlin of the University of California, Santa Cruz. “There is, in fact, a veritable zoo of unusual orbital trajectories.”

“I’m delighted at the likelihood that somewhere in our vast universe there surely must be some screwball planets like these in binary star systems,” says Geoff Marcy of the University of California, Berkeley. “What a discovery – wacky but undoubtedly true!”

Sara Seager, an astrophysicist at the Massachusetts Institute of Technology, is more sceptical. While the corkscrew planet is mathematically plausible, it is less clear how such an orbit could come to be through the evolution of a real stellar system. “It’s hard to imagine planets forming or being captured in such an orbit,” she says. “But for exoplanets, never say never!”

Journal reference: Astrophysical Journal, DOI: 10.1088/0004-637X/804/2/106