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 isn’t 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 “retrograde” 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 star’s rotation. That’s 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.
In the new study, Smadar Naoz of Northwestern University in Evanston, Illinois, and colleagues test a different scenario. In their simulation, a gas-giant planet starts out orbiting in the same direction as the star’s rotation. But then the gravitational effect of another planetary body or a brown dwarf farther out from the host star yanks the planet out of its original orbit and into a new one tilted at an angle to the star’s equatorial plane. The orbit gets tilted further and further, until at some point it flips. That’s how a retrograde orbit is born.
“The effects on the inner planet are weak but build up over a very long period of time,” says Naoz, whose team reports its findings online today in Nature. She says the simulations show that this kind of orbit flipping may explain why nearly half of the hot Jupiters seen so far have retrograde orbits.
Joshua Winn, an astrophysicist at the Massachusetts Institute of Technology in Cambridge, says a way to confirm this mechanism is to look for the planetary bodies or stellar objects that might have caused the orbital flipping. “Several groups are now undertaking more sensitive searches for additional stars or planets in systems with close-in giant planets,” he says. “We should search for the 'guilty hand' that flipped the orbits of these retrograde Jupiters.”
The headline has been changed for accuracy.