National Geographic : 2013 Jul
56 national geographic • JULY 2013 rock are lofted high into the atmosphere, and they later rain out as solid, glassy beads called spherules. Deposits of spherules from the six- mile-wide asteroid that hit the Yucatán some 65 million years ago, wiping out the dinosaurs, have been discovered all over the world. So far at least a dozen comparable spherule beds have been found that date from a series of impacts between 1.8 billion and 3.7 billion years ago. The computer simulations by Bottke’s team trace the source of those impacts to a now van- ished inner rim of the asteroid belt, which shed asteroids for two billion years after Jupiter dis- turbed it. According to Bottke, as many as 70 may have struck Earth, each comparable to the one that extinguished the dinosaurs. “Solar system evolution is dynamic,” Levison says. “It’s violent. Our solar system is probably on the mild side compared with what happens elsewhere. You probably need that mildness in order to have a habitable planet.” The Nice model is a hypothesis, and not all scientists are convinced it’s true. Everyone now agrees that at least some planets migrated, but whether that set off a violent solar-system-wide paroxysm is up for debate. “It’s a fascinating con- cept,” says Donald Brownlee. “It must happen in places, around other stars. Whether or not it happened here, we don’t know for sure.” It’s clear that comet particles like Inti were blasted out- ward from near the sun, he says, but the planets may have shifted more gently. The key to testing the Nice model is map- making. Charting the composition and orbits of distant objects should reveal whether and how the planets cast them there. Stern is leading a NASA mission called New Horizons that will send an unmanned probe past Pluto and its five known moons in July 2015. From there Stern hopes to redirect New Horizons to examine at least one other body in the Kuiper belt. Powerful new telescopes planned for the next decade will expose far more objects in the Kuiper belt. They may also peek into the Oort cloud, which Stern calls the solar system’s attic. The debris cast there by Jupiter may include some lost planets. “I think the Oort cloud will blow our minds,” says Stern. “It will be littered with planets. I think we’ll find lots of Marses and Earths out there.” What about the future of the planets we know? Forecasting the solar system is like forecasting the weather. There’s so much ran- domness in the system, says theorist Greg Laughlin of the University of California, Santa Cruz, that the forecast—as well as any historical reconstruction—has to be given in probabilities. Scientists are as certain as they can be that the four giant planets have finished wandering and will still be on the same orbits five billion years from now, when the aging sun is expected to balloon outward and engulf the inner planets.