test 1 Astronomy - May 2018 USA | Page 26

The movements of the giant planets disrupted the distant Kuiper Belt and its most prominent member, Pluto. There couldn’t have been enough material in the outer solar system to create Pluto in place. NASA/JHUAPL/S W RI The battered lunar highlands testify to an intense period of bombardment that lasted some 300 million years. The migration of the giant planets likely caused the barrage by jostling the distant Kuiper Belt. CONSOLIDATED LUNAR ATLAS/UA/LPL This cleared out the area that one day would be the asteroid belt, and left less material in the region for Mars to grow. Basically, Jupiter’s presence removed a lot of the matter that could have gone into planet-building. Orbiting the Sun faster than Jupiter itself, the remaining gas and dust inside Jupiter’s orbit exerted an outward force, and the two giant planets started to retreat. “It’s a tidal effect,” says Morbidelli. “It’s the same as the Earth which, rotating faster than the Moon, pushes the Moon out. So the still massive inner disk wins — Jupiter moves out and drags Saturn with it, thanks to the fact that Saturn is locked in resonance.” Fortunately for us, Jupiter did “tack,” migrating out again, and the remaining planetesimals formed a ring around the Sun centered in the region between where Earth and Venus are now. Mars, on the outer edge of that ring, was never able to grow large. As the two giant worlds moved ever outward, they scattered material back into the future asteroid belt. The Grand Tack explains why Mars is relatively small, the lack of super-Earths, and the dearth of mass in the asteroid belt. It also can help solve the problem of Earth’s water. Many planetary scientists think asteroids delivered the water. With Jupiter and Saturn scattering objects from the current asteroid belt region, it’s plau- sible that lots of water-rich objects would smack into Earth. One version of the Grand Tack model, proposed by Konstantin Batygin at Caltech and Greg Laughlin at the University of California, Santa Cruz, posits the existence of super-Earths in the early solar system. But Jupiter and Saturn caused such a dis- ruption in the inner system that the super- Earths spiraled into the Sun. As Jupiter and Saturn tacked back to the outer part of the solar system, they cap- tured Uranus and Neptune in resonance, keeping other large bodies from wandering into the inner solar system. “Without Jupiter, Uranus and Neptune would be inside Mercury’s orbit,” says Morbidelli. “Maybe there wouldn’t have been enough mass left to create terrestrial planets.” 26 A ST R O N O M Y • MAY 2018 Nice ice, baby Getting the solar system to its current con- figuration requires a bit more rearrange- ment, however. In a series of papers pub- lished in Nature in 2005, Kleomenis Tsiganis, then at the Observatoire de la Côte d’Azur; Rodney Gomes of Brazil’s National Observatory in Rio de Janeiro; Hal Levison of the Southwest Research Institute; and Morbidelli proposed a theory called the Nice model (named for the French city in which it was formulated). This scenario takes place after the dust and gas in the planet-forming disk had dissipat- ed at the conclusion of the Grand Tack. In the Nice model, Uranus and Neptune formed closer in than they are now and migrated outward. Some later versions of the theory have Uranus and Neptune switching places, or even flinging a third ice giant into deep space. The clues are in models of the early solar nebula. Estimates of its density don’t leave enough matter in the outer reaches of the solar system to form planets, at least not fast enough to yield ice giants. “It’s obvious you can’t form Neptune at 30 AU,” says Steven Desch, a theoretical astrophysi- cist at Arizona State University. Desch was among the proponents of the idea that Uranus and Neptune might have swapped positions. Building Neptune requires a lot of hydrogen, some two Earth masses at least. “There’s no model where you can grow Neptune to where it can accrete enough gas,” he says. “It’s got to be 10 Earth masses to hold on to it. If you form it closer in — like 15 AU — then maybe it can grow big enough to hold on to the hydrogen gas.” Unlike the Grand Tack model, the Nice model plays out over millions of years. This is how Uranus and Neptune could exchange enough angular momentum with the Kuiper Belt to switch places. Although each interaction with a Kuiper Belt object (KBO) wouldn’t do much, over time, many KBOs would be scattered toward the Sun and the ice giants would 20° be nudged to their current positions. Morbidelli doesn’t think the planet-switching is likely, however. He refers to it as a “dead hypothesis,” and 80° Telltale orbits Classical KBOs Resonant KBOs Scattered disk Detached 60° 40° The orbits of many Kuiper Belt objects (KBOs) tip significantly to that of the major planets. Astronomers suspect that the migrations of Uranus and Neptune likely scattered these KBOs into their highly inclined orbits. ASTRONOMY: ROEN KELLY 20° Sun 0 50 Astronomical units 100 150