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Planets Prefer Wacky Orbits


In extrasolar systems, planets don't necessarily orbit in simple Earthlike paths.

In our solar system, nine planets orbit the sun in nearly circular orbits, seemingly in perpetuity.

Planet Elsewhere in the universe, it looks like a different story. Newly discovered planets around other stars rarely orbit in circles and often get into gravitational tussles with their sibling planets. Massachusetts Institute of Technology physics professor Frederic Rasio is trying to understand why.

Essentially just one force -- gravity -- governs the paths of planets, but because everything pulls on everything else -- the gravity of a star pulls on its planets, the gravity of the planets pull on the star, the gravity of the planets pull on each other -- it's not easy to figure out precisely what's going on over the course of millions of years. It's such a complex computation that Rasio and his students require the power of the Alliance supercomputers.

The orbit of the first extrasolar planet discovered by astronomers seemed to be an absurdity. The planet circling around the star 51 Pegasi B, or 51 Peg as astronomers call it for short, hugged the star in an almost impossibly close orbit, completing its circuit in a mere four days. Astronomers soon found a couple of other star-hugging planets.

Frederic Rasio Astronomers also found planets, like the ones around 16 Cygni B and 70 Virginis, that travel in long, elliptical orbits, more like the paths of comets than of planets. "Far-Off Planet Discovered with Wacky Oval Orbit," reported the San Francisco Chronicle on October 24, 1996, about the discovery of 16 Cygni B's planet. That planet takes 2.2 years to complete an orbit. Were it in our solar system, its orbit would swing out farther than Mars, then sweep in as close as Venus.

But wacky orbits may be common. Of the 17 extrasolar planets discovered so far, only the one orbiting 47 Ursae Majoris B has a relatively circular orbit fairly far out from the star, the kind of orbit astronomers initially thought was typical. "It's possible we live in a remarkable and unusually stable system," Rasio says. "And there is a strong suggestion that a typical planetary system is more like the ones we've detected now. That is telling you that these eccentric orbits are representative of the whole sample of extrasolar planetary systems."

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