NCSA NEWS

In the three years since the discovery of 51 Peg, astronomers have been puzzling over how to explain what they've seen. Conventional theories on how planets coalesce out of a spinning disk of debris can't explain how a planet ends up in a highly elliptical orbit. Because the stuff that forms planets is moving in a circular path, it follows that the planet it forms should also continue moving in a circular path.

Conventional theories also can't explain how a large planet, such as the one orbiting 51 Peg, could form so close to the star. Douglas Lin of University of California, Santa Cruz, suggests that the planet was born farther out in the usual fashion and then gravitational interactions with the remaining disk of dust and gas caused it to migrate inward toward its current star-hugging position.

Rasio's simulations offer an alternate explanation for these astronomical surprises. Apparently, we are lucky that our solar system has only one Jupiter.

If the wanderings of one planet confuse astrophysicists, throw two planets around a star, and the dynamics become infinitely more complicated. If two Jupiter-sized planets pass close to each other, their mutual gravitational attractions distort their orbits. Sometimes -- rarely -- the orbits remain stable despite the mutual tuggings of gravity. At other times, the orbits become more and more distorted until gravity literally crashes the two planets together. The most common occurrence is a sort of planetary do-si-do -- swing your partner and throw 'em out of the solar system. The planets don't collide, but they pass so close together that the gravitational pull slings one out into deep space.

That leaves the remaining planet in a highly elliptical, "wacky" orbit like that of 16 Cygni B's planet. If the elliptical orbit passes close enough to the star, tidal forces -- just as the moon raises ocean tides on Earth -- would gradually erode the elliptical orbit into a small, tight circle -- the 51 Peg orbit. (In our solar system, such planet-wandering catastrophes are much less likely because Saturn, the second largest planet, has less than one-third the mass of Jupiter.)

Although this picture is pretty simple and the dynamical equations easy to write down -- just Newtonian F = ma, not Einsteinian general relativity -- it takes a supercomputer to calculate and sort out the details. That's because Rasio's two-Jupiters-around-a-star scenario is just a variation of what physicists call the "three-body problem." It's impossible to write down an exact solution, and what happens depends critically on the initial positions, masses, and speeds of the planets.

"The key is that all these systems are chaotic," Rasio says. "Even if you make a minuscule change in the initial conditions, you can get a completely different outcome. For example, in one case you can find that the two planets will keep going around happily forever." Nudge one of the planets and either they will collide or one will be ejected.