Why Some Hot Jupiters Orbit the Wrong Way
"As varied as the planets in our solar system are, they all have one thing in common: all revolve in the same direction as the spinning of the sun. This isnít true everywhere. In recent years, astronomers have discovered several planetary systems outside our own that contain massive, Jupiter-like planets orbiting in a direction opposite to the spin of the host star. Now, a team of researchers has performed computer simulations to show how these planets may have ended up in these funny ďretrogradeĒ orbits.
Because planets form within the whirling disk of gas and dust that extends out from a rotating star, they ought to have orbits that follow the starís rotation. Thatís why astronomers were puzzled when they first encountered a gas giant close to its parent star, also known as a "hot Jupiter," orbiting in a retrograde orbit in 2009. One early explanation was that several giant planets form together in the so-called protoplanetary disk and interact gravitationally in a way that leaves their individual orbits thrown out of whack. That puts one or more of the planets in a close-in orbit running counter to the spin of the star"