Why do some planets orbit the wrong way

The transiting giant planet orbits very close to the star and in a direction opposite to the stellar rotation. This peculiar configuration results from gravitational perturbations by another much more distant planet (upper left). Credit: Credit: Lynette Cook

In the last few years astronomers have observed that in some extrasolar systems the star is spinning one way and the planet, a “hot Jupiter,” is orbiting the star in the opposite direction. A Northwestern University research team is the first to model how these huge planets got so close to their stars — thanks to gravitational perturbations by a much more distant planet — and how the planets’ orbits can flip in the process.

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, a “hot Jupiter,” is orbiting the 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.”

Figuring out how these huge planets got so close to their stars led Rasio and his research team to also explain their flipped orbits. Using large-scale computer simulations, they are 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. (A hot Jupiter is a huge Jupiter-like planet in very close proximity to the central star.)

“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.”

via Flipping hot Jupiters: Why some planets orbit the wrong way.

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