NASA has found a Jupiter-like planet that orbits in the opposite direction of its sun. No solar system would generate a planet orbiting like this on its own. So, what could make an entire planet reverse directions?
Our solar system formed out of the collapse of a giant, spinning cloud of particles. The pull of gravity caused the cloud to spin (much the way two people who clasp hands and run in opposite directions spin around their center of gravity). Extremely dense parts of the cloud formed solids. The largest area became the sun. Other parts condensed into smaller fragments which collided with each other and became the planets.
All of this happened within the spin of the larger cloud, so all the materials wound up orbiting the same directions. The sun spins one direction, and the planets orbit the same direction. To have one planet moving the opposite direction would be a little like seeing water spin down a drain, except for a certain section, which goes the opposite way. Impossible. And yet we have found an exo-planet that does exactly that. Astronomers have been puzzling over what could have reversed the orbit of the planet. At last, they believe that they have an answer.
It takes a planet to flip a planet. Scientists believe that the orbit-flipped exoplanet originally orbited far from the sun, though in the same direction as it. It was close to another planet, also a giant, that was slightly farther from the sun. The outer planet and the inner planet gravitationally interacted. Each time the inner planet passed the outer one, the outer one exerted a gravitational pull on all that gas. Like the tides of earth move toward the moon, the gas was pulled towards the nearby planetary object. This sapped angular momentum from the inner planet, the same way someone lightly clutching at a person's sleeve would slightly slow them down as they moved.
The major thing keeping the planets from falling into the sun is the fact that they're orbiting. Their momentum is pushing them away from the sun, while the gravity of the sun keeps them from flying off altogether. The combination of push and pull makes them spin around the sun without falling in. When the inner planet lost its outward momentum, its orbit plunged inwards towards the planet's sun. This is what scientists call an "eccentric" orbit. The eccentric orbit didn't result in a firey death for the planet only because its plunge gave it angular momentum in the oppoisite direction. This momentum caused it establish a new orbit, albeit a much smaller one, close to the sun and in the opposite direction as the rest of its solar system.
Image Credit: Lynette Cook