Astrophile: Pinball planets get wild, deadly ride

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object type: Planet
Host: One star in a widely separated stellar pair
Fate: Doomed

Slipping through a rift in space-time ? la Doctor Who, you are dumped onto the surface of a planet very much like Earth. It is night-time, and although the landscape looks familiar, it is eerily bright. Raising your gaze, you see why ? a brilliant star blazes with the light of five full moons, giving everything around you a ghostly glow.

That's not all. A chill runs down your spine when you see it: a massive, Jupiter-like planet looms above you, as large as the Earth's moon. Bands of rippling clouds and whirling storms roil its surface. This is too close, you think, and you are right ? you have arrived on your planet's doomsday.

The gas giant is on a near-collision course with your planet, thanks to gravitational tugs between the planets in your system. Such tugs may have shifted the orbits of the outer planets in our own solar system billions of years ago ? perhaps even kicking a now-lost world into the cold depths of space in the process.

Things will not end well for the planet you find yourself on. At least you would get a spectacular naked-eye view of your tormentor, says Nick Moeckel of the University of Cambridge. "If the intruder was a bright-ringed planet like Saturn, then those rings would be clearly visible," he says.

The larger planet will hurl its smaller sibling into a much wider orbit, about 100 times as wide as Pluto's, where it will go into a deep freeze. In a single-star system, that would normally be the end of the story. In binary star systems like this one, though, the stars will play pinball with the poor planet before losing it forever, according to simulations by Moeckel and Dimitri Veras, also at Cambridge.

No direction home

In their simulations, two sun-like stars orbit one another at between 250 and 1000 times the Earth-sun distance in our solar system, with one or both stars having its own set of planets. In many cases, the alien solar system becomes unstable and one of its planets is flung into a very wide orbit where it is vulnerable to being captured by the second star's gravity.

Its new orbit around the second star tends to be very wide as well, and the first star often recaptures the planet, which can shuttle back and forth between the stars many times. The planetary pinball effect is surprisingly common in the simulations, with more than half of all ejected planets bouncing back and forth between the stars.

Could such situations actually come to pass? Observations suggest so. In recent years, evidence has grown that binary star systems can host planets, and observations suggest that planet formation is common around both tight and widely separated binary stars.

Out in the cold

The new simulations suggest the pinball planet may disturb or even eject other planets from both solar systems. Its own outlook is also bleak. In a small number of the simulations, it eventually wound up in a stable orbit around one of the stars. However, the vast majority of ejected planets eventually escape from both stars forever, voyaging into a permanent deep freeze.

"Once a planet starts transitioning back and forth, it's almost certainly at the beginning of a trip that will end in deep space," says Moeckel.

Journal reference: arxiv.org/abs/1201.6582; the work will be published in a future edition of Monthly Notices of the Royal Astronomical Society

Read previous Astrophile columns: Picture yourself on a sandboard on Titan, How to spot a dark-matter galaxy , Glimpse elusive matter in shattering star, Cool echoes from galaxy's biggest star, Stopped clocks deepen pulsar enigmas, Wounded galaxy is crux of cosmic whodunnit, Did comet killing spark Christmas light show?, Blinged-out stars were born rich, Supercritical water world does somersaults, Attack of the mystery green blobs, Undead stars rise again as supernovae, The sticky star cluster that's mostly black hole, The rebel star that broke the medieval sky.

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