'Wrong way' planetary orbits explained

Posted by Kate Taylor

Scientists believe they've worked out why some extrasolar planets orbit in the opposite direction to their star's rotation.

              
"How can one be spinning one way and the other orbiting exactly the other way?" says  Frederic A Rasio, a theoretical astrophysicist at Northwestern University.  "It's crazy. It so obviously violates our most basic picture of planet and star formation."

The planets in question are typically huge 'hot Jupiters' that orbit their central star very closely - and it was figuring out how they got so close to their stars that gave Rasio and his team the explanation of their flipped orbits.

"Once you get more than one planet, the planets perturb each other gravitationally," says Rasio. "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."

"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."

The team used orbital mechanics and computer modelling to find the answer.They assumed a star similar to the sun, and a system with two planets.

The inner planet is a gas giant similar to Jupiter, and initially is far from the star, where Jupiter-type planets are thought to form. The outer planet is also fairly large and is farther from the star. It interacts with the inner planet, perturbing it and shaking up the system.

The effects on the inner planet are weak, but build up over a very long period of time, resulting in two significant changes: the inner gas giant orbits very close to the star and in the opposite direction of the central star's spin. The changes occur because the two orbits are exchanging angular momentum, and the inner one loses energy via strong tides.

The gravitational coupling between the two planets causes the inner planet to go into an eccentric, needle-shaped orbit. It has to lose a lot of angular momentum, which it does by dumping it onto the outer planet.

The inner planet's orbit gradually shrinks because energy is dissipated through tides, pulling in close to the star and producing a hot Jupiter. In the process, the orbit of the planet can flip.