Why Some Planets Orbit the Wrong Way?

More than 500 extrasolar planets--planets that orbit stars other than the sun--have been discovered since 1995. But only in the last few years have astronomers observed that in some of these systems, the star is spinning one way and the planet is orbiting that star in the opposite direction.
"That's really weird, and it's even weirder because the planet is so close to the star," said Frederic A. Rasio, a theoretical astrophysicist at Northwestern University. "How can one be spinning one way and the other orbiting exactly the other way? It's crazy. It so obviously violates our most basic picture of planet and star formation."
The planets..

in question are typically huge planets called "hot Jupiters" that orbit in very close proximity to their central star. Figuring out how these huge planets got so close to their stars led Rasio and his research team to also explain their flipped orbits. Details of their discovery are published in the May 12th issue of the journal Nature.
And this discovery is a broader impact of NSF's MRI program support for the acquisition of a computer cluster" said Beverly Berger, an NSF Gravitational Physics Program director. Using it, and performing large-scale computer simulations, Rasio researchers became the first to model how a hot Jupiter's orbit can flip and go in the direction opposite to the star's spin. Gravitational perturbations by a much more distant planet result in the hot Jupiter having both a "wrong way" and a very close orbit.

"Once you get more than one planet, the planets perturb each other gravitationally," Rasio said. "This becomes interesting because that means whatever orbit they were formed on isn't necessarily the orbit they will stay on forever. These mutual perturbations can change the orbits, as we see in these extrasolar systems."
In explaining the peculiar configuration of an extrasolar system, the researchers also have added to our general understanding of planetary system formation and evolution and reflected on what their findings mean for the solar system.
"We had thought our solar system was typical in the universe, but from day one everything has looked weird in the extrasolar planetary systems," Rasio said. "That makes us the oddball really. Learning about these other systems provides a context for how special our system is. We certainly seem to live in a special place."

machineslikeus.com