National Geographic : 2013 Jul
Solar System 53 planets have now been detected. Some are in tightly bunched orbits, much closer together than the planets in our solar system. Some are Jupiter- or Neptune-size worlds that race on insanely hot orbits close to their suns. Others loop deep into space on weird trajectories—on average the orbits of extrasolar planets are more eccentric than those in our solar system. Some planets even float freely in interstellar space. None of this is what you would expect from planets that were born in a spinning disk around a star and stayed quietly in their birth- place. That process should produce well-spaced, near-circular orbits, like the ones in the brass orreries. Clearly many planets had migrated, but smooth migrations didn’t seem to account for extreme orbits and late bombardments, at least not to Levison. He began to suspect that our solar system’s history had been anything but smooth—that it had somehow endured a “global gravitational instability,” as he now calls it. In 2004 he gathered with three colleagues on sab- batical in Nice, France, to try to work out how. Levison, who goes by “Hal,” is a burly man with thin, graying hair pulled back into a ponytail and an untamed Santa-style beard. He’s both seri- ous and impish; his Boulder office contains lush old illustrations of planetary orbits, an Albert Einstein action figure, and a model of Gort, the robot from The Day the Earth Stood Still. H e’s fond of giving provocative talks and will some- times wear a catcher’s mask to ward off brickbats from the audience. “What I’m going to say is really absolutely crazy,” he said at the start of a recent seminar. “If we publish this, my career might be over.” He could have made the same remark back in 2004 about what is now called the Nice model—the hypothesis that he and his colleagues, including Alessandro Morbidelli of clockwork The old models called orreries depicted an ever predictable solar system. The real one is more random: Moving a pencil across your desk today, says one astronomer, can shift jupiter halfway around its orbit a billion years from now.