Imagine, for a moment, that you're trying to reconstruct the pivotal Battle of Gettysburg during the U.S. Civil War. You want to know not only the two armies' initial positions and manpower, but also the height and weight of each soldier and how much ammunition he carried into battle. Yet all you have to work with is an aerial snapshot of where all the survivors were standing at the moment the fighting stopped and an assortment of musket balls collected from those bloody Pennsylvania fields.
It's a stretch, I know. But now you've got an idea of what planetary dynamicists are up against as they try to reconstruct how the solar system came together 4½ billion years ago.
In most models, it's a three-step process: (1) dust settles into a flattened disk and collects into countless planetesimals a mile or so (1 to 10 km) across; (2) the planetesimals collide and form Moon- to Mars-size planetary embryos; and (3) the embryos smash into one another until basically all that remains are a handful of rocky inner planets and a second handful of rocky "super-Earth" cores that eventually become the giant outer planets. In other words, the solar system's building blocks grew smoothly and systematically from small bodies to large ones.
But recent studies have (forgive me) punched huge holes in this traditional thinking. For one thing, there's no obvious way for Mother Nature to have assembled objects larger than about 1 meter across within the disk — the forces at play would have caused such bodies to smash themselves to bits or to spiral rapidly into the Sun. Fortunately, work in 2007 and 2008 by theorists Anders Johansen and Jeffrey Cuzzi (and their respective teams) suggests that really big planetesimals could have formed directly from pebble-size particles, bypassing the need for intermediate sizes and neatly sidestepping the "meter-size barrier."
Now a quartet of well-known dynamicists led by Alessandro Morbidelli (Côte d'Azur Observatory, France) has concluded that the Johansen and Cuzzi teams probably got it right. In particular, they find, the asteroid belt wouldn't have the assortment of big and small objects we see today (known as its size-frequency distribution, or SFD) if it initially teemed with smallish planetesimals. Instead, they argue, most of the original inhabitants would have been closer to the size of Ceres, the modern-day belt's undisputed king. "Asteroids Were Born Big" is the title of their analysis, which will appear in a forthcoming issue of Icarus.
Morbidelli & Co. tackled the question by developing new software to model the outcomes of colliding planetesimals (do they stick together, or shatter and scatter?) and then running computer models ad nauseum using different starting conditions. One important outcome is that planetesimals had to be at least 100 km across to end up with the the modern-day belt's SFD, which has a distinct "bump" at diameters near 100 km.
"Our model is fun, but we have much yet to do," notes coauthor Bill Bottke (Southwest Research Institute). "I suspect we are missing something big with the inner solar system's formation." Still, he thinks the asteroids-born-big concept will likely prove correct — and consequently our notions of how Earth and its siblings came together will change a lot.