By Jove, they've done it! Planetary scientists have overcome a key problem in explaining the emergence of the solar system's behemoths—Jupiter and Saturn. Previous models predicted too many gas giants. But a new study shows how just a few such monsters should emerge from a swirling protoplanetary disk of gas and dust.
“We can now start with a pretty simple disk, pretty simple physics, and reproduce the outer solar system—and that’s never been done before,” says Hal Levison, a planetary scientist at Southwest Research Institute in Boulder, Colorado, who led the study.
The 4.56-billion-year-old solar system began in a hurry. Within a few million years, the sun had already eaten up most of its disk of gas and dust. So Jupiter and Saturn—which are shrouded in immense envelopes of gas—had to form quickly, before that disk disappeared. Many theorists believe that the gas giants began with rocky cores with masses equal to about 10 Earths, giving them enough gravity to gobble up their gas shells. But modeling the formation of those cores from bits of dust and getting the right number of cores in the right orbits has long challenged planetary scientists. Now, Levison and colleagues have developed a new model that predicts the formation of just a few gas giants the size of Jupiter and Saturn, as they report today in Nature.
The model builds on a theory, called pebble accretion, that explains the formation of cores. Small dust grains can grow as they collide and stick together with static electricity. But beyond a certain size—about a meter—growth stops as collisions rupture the dust ball rather than add to it. This “meter-scale problem” was surmounted about a decade ago, when theorists realized that pebbles less than a meter in size are constantly moving in the wind of the spinning gas disk. When they encounter other pebbles, they clump together and take advantage of a wake in the wind of gas, like flocking birds. These clumps quickly reach a size where they are gravitationally bound to one another. “That solves the meter barrier—you go directly from pebbles to 100-kilometer-sized things, almost overnight,” Levison says. These “embryos” then coalesce and add more material until they reach the size of a core.
But in simple models, pebble accretion had its own problem: It was too efficient. In many simulations, dozens or even hundreds of Earth-sized bodies tended to form. So Levison says he initially set out to “kill” the pebble accretion theory. But he ended up advancing it further, instead. His team found that by tuning the model so that the pebble formation process takes a bit longer, there is more time for the large planetary embryos to interact with each other gravitationally. All but the biggest get kicked outside the plane of the solar system, allowing the few that remain to mop up remaining pebbles and become the cores of gas giants.
The researchers’ modeling runs—which each take weeks on a cluster of five computers—typically predict one to four gas giants like Jupiter and Saturn in orbits between 5 and 15 astronomical units (AU) from the sun. (An AU is equal to the distance between the sun and Earth, with Jupiter at 5.2 AU and Saturn at 9.6 AU from the sun.) As a bonus, says Levison, the model creates a few ice giants—planets like Uranus and Neptune—in the right range of orbits. It also predicts that no large planets would have formed in the Kuiper belt, the region of small icy worlds in which Pluto resides.
“They found a solution to the problem of too many planets growing,” says Anders Johansen, an astrophysicist at the University of Lund in Sweden who helped develop the theory of pebble accretion. “It’s about giving gravity more time, so large embryos can bully the small ones.”
Other planetary formation theories still cling to life, however. Alan Boss, a theorist at the Carnegie Institution for Science in Washington, D.C., has pushed an idea called gravitational instability, in which an especially cool and massive protoplanetary disk can develop ripples that can coalesce into gas giants, with or without cores. Boss notes that some exoplanets far more massive than Jupiter sit in far-flung orbits tens or hundreds of AU from their stars. At those distances, it may be difficult for cores to form, because the population of pebbles is thought to die off with distance. The gravitational instability might explain those planets better than core accretion, Boss says. “I think there’s room for both mechanisms,” he says.
Still, Andrew Youdin, an astronomer at the University of Arizona, Tucson, thinks the giants with distant orbits may be the anomalies. He predicts that with further observations, more “normal” gas giants will be found, such as one described last week that has twice the mass of Jupiter and orbits its star at 13 AU. That would firm up core accretion as the theory of choice. “People would say this is the prevailing hypothesis,” he says.
Johansen is also heartened by a recent discovery from ALMA, a submillimeter array of telescopes in Chile, of young planets sweeping clear paths through millimeter-sized dust grains in a protoplanetary disk no more than a million years old. The observation shows that pebbles remain plentiful on the timescales that Levison proposes, Johansen says. “It seems that protoplanetary disks are really good pebble factories,” he says.
Levison says he is next turning to the modeling of the inner solar system, where the rocky planets are thought to have formed much more slowly. That, combined with the planets' shorter orbital periods, makes the modeling effort much tougher.