Scientists have long puzzled over how our solar system’s planets formed, because this isn’t a process they can watch unfold. Researchers have generally thought that Jupiter and Saturn grew out of collisions of smaller clumps of rock and ice, building up proto-planets called planetesimals. But the prevailing model produced far too many. Scientists’ new computer model found a way for these massive wonders to form from mere collections of pebbles.
When our solar system formed, it looked quite different. The four giant planets – Jupiter, Saturn, Uranus and Neptune – were spaced closer together and nearer to the Sun than they are today, until their interactions flung the latter two further out into space. So the giant planets likely formed closer in than they are now. Their rock and ice cores formed first, before pulling in layers of gas to balloon them out to their current giant sizes. But the problem has been in explaining how their cores built up. The leading theory is that small clumps ranging between a few feet wide and a few hundred miles wide attracted one another to build up the cores. But models have created far too many of these – hundreds of them, instead of just the four that we see in our solar system.
As a way to get around this problem, some scientists have proposed that maybe the smaller starting materials – called pebbles – didn’t all exist at the same time. In the evolution of planets, pebbles come before planetesimals. At first, dust clumps with dust, the small rocks clump with material, and finally boulders clump with material — until the solar system is filled with rocky objects between meters and hundreds of kilometers wide. In planetary science lingo, these are “pebbles.” Michiel Lambrechts and Anders Johansen’s “pebble accretion” theory posited that it was these pebbles that formed the basic ingredients for Jupiter and the others. Pebbles are not interacting just by gravity. Because they’re embedded in a gas disk, aerodynamic drag also affects how they clump. Lambrechts and Johansen’s theory incorporated this drag, but the problem was that, under that theory, the young planets grew way too fast. “I have to admit, I hated this idea when I first read it,” says lead author Hal Levison, “I was trying to prove it wrong.” But he and his colleagues built off of the pebble accretion theory for their study, published today in Nature.
Crunching the Numbers
Levison and his colleagues ran computer simulations to solve the equations that Lambrechts and Johansen’s model were based off of. In a simulation of our solar system under the basic pebble accretion model, the scientists ended up with about 300 Earth-mass planets – way off. But they wondered what would happen if instead of having pebbles coalesce from dust all at the same time, the pebbles formed at different rates. That would create fewer at one time, maybe encouraging them to clump into fewer planets. And that was just what they saw. Planet cores began forming, but gravitational interactions quickly segregated many baby planets, throwing the smaller planets out of the disk of gas. “The smaller guys, which become more eccentric and more inclined, get scattered out of the disk where the pebbles are. Therefore they’re starved and can’t grow,” says Levison. “Only the biggest planets remain in the same location as the pebbles,” which means they can continue to grow. And that arrangement produced an impressively accurate rendition of our outer solar system. Researchers found that their simulations produced one to four gas giants between 5 and 15 astronomical units from the sun, which is where those planets orbit today.
The finding is far from the last word on solar system formation. For one thing, it doesn’t take into account the inner planets. Simulating the formation of the inner planets is much harder because they took longer to form, so simulations need to span longer amounts of time. The inner solar system also crams more objects into a smaller space and those objects move around their orbits faster. Both of these reasons mean simulations require far more resolution elements than the outer planet models use. Levison’s team plans to next investigate how the inner solar system might have formed in a “pebble accretion” model. If this theory can reproduce the formation of the four inner planets as well, planetary scientists will be one step closer to understanding how planets formed – and it would be a triumph of the interplay of computer simulations and the pebble accretion model.
Top image by Lsmpascal