National Geographic : 2013 Jul
52 national geographic • JULY 2013 the past and future orbits of the planets. The first clue came from Pluto. The oddball of the solar system, it dips far above and below the pancake-like plane in which the eight planets travel; it swoops on an elongated orbit that takes it from 30 to 50 times Earth’s distance from the sun. But the most curious thing about Pluto is its bond with Neptune. It’s called a resonance: For every three times that Neptune orbits the sun, Pluto orbits twice, and in such a way that the bodies never approach each other. In 1993 Renu Malhotra figured out how that exact synchrony could have evolved. She pro- posed that when the solar system was young and full of asteroids and comets, Neptune was closer to the sun. If one of those bodies approached Neptune, the planet’s powerful gravity might either fling the object closer to the sun or out of the solar system entirely, in a cosmic version of crack the whip. Because action begets reac- tion, Neptune’s orbit would shift a tiny bit too. A human, even a Newton, could never calculate the effect of trillions of such interactions—but Malhotra’s computer model showed that on av- erage they would compel Neptune to migrate away from the sun. In her scenario, that led it to “capture” Pluto, which was already farther out, and sweep it into gravitational lockstep. Her colleagues were doubtful, but Malhotra was proved right within a decade. In the Kuiper belt, a dark region extending far beyond Neptune, telescopes unveiled bunches of Plutinos—icy dwarf worlds that have the same two-to-three resonance with Neptune. That could only have happened, says Malhotra, if Neptune had advanced toward the Kuiper belt like a gravita- tional snowplow, piling up dwarf planets into new orbits. “Once the Plutinos were discovered, it was a slam dunk,” she says. “Planet migration practically became a textbook idea.” The notion of migrating planets came along at a time when planetary scientists were puzzled by several other features of the solar system. By the early 2000s they had long since realized that the birth pangs of the solar system had been violent. The planets had not condensed gently from the solar nebula; instead they had grown to full size by absorbing planetesimals—rocky asteroids, icy comets, and larger objects—that smashed into them at high speed. According to one theory, the moon coalesced from the spray of molten rock that was blasted into orbit when a body the size of Mars collided with Earth. All this probably happened in the first 100 million years. The puzzle was that the extreme violence didn’t end then. Many hundreds of millions of years later, the moon suffered a series of major impacts that left it permanently scarred with huge craters. This so-called Late Heavy Bom- bardment would have pounded Earth even more viciously. Scientists had no good explanation for what sparked it, since by the time it happened, the planets had swept their orbits mostly clean of debris. Telescopes were unveiling a similar enigma in the Kuiper belt. Besides Plutinos, it was littered with bodies on wildly different orbits. Some of the bodies were grouped in a flat disk, some in a puffy doughnut-shaped cloud; others were on orbits even more crazily eccentric (the techni- cal term for elongated) than Pluto’s. “It looked like a train wreck,” says Harold Levison, Stern’s colleague at the Southwest Research Institute. The smooth outward migration of Neptune that Malhotra had used to explain the Plutinos would not have strewn debris so widely. Meanwhile, astronomers had started to discover planets around other stars—and to radically expand their notions of what’s possible in a planetary system. Hundreds of extrasolar If an asteroid or comet approached Neptune, the planet’s powerful gravity might fling it toward the sun or out of the solar system, in a cosmic ver- sion of crack the whip.