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© Rex FeaturesBuilding blocks of the solar system
Perhaps we should thank rapid-assembly asteroids for spawning the planets. New simulations suggest that dense swarms of boulders collapsed under their own gravity to make the building blocks of our solar system.

The planets are thought to have formed from a disc of dust and gas around the infant sun. The initial process is well known: dust grains clumped together, forming objects in the millimetre-to-metre range. However, it is not known how the growth process continued. The gas in the disc should have put a drag on the new boulders, causing them to spiral into the sun before they could grow further.

Evidence is now mounting that the next step was a sudden leap forward, skipping intermediate sizes to make asteroids hundreds of kilometres across - massive enough to resist gas drag.

This basic idea is decades old, but it attracted renewed attention in 2007 and 2008 following simulations by a team led by Anders Johansen of the Max Planck Institute for Astronomy in Heidelberg, Germany, and by another team led by Jeffrey Cuzzi of NASA's Ames Research Center in Moffett Field, California. These showed that turbulence in the nebula could have concentrated objects less than a metre across in dense enough swarms to collapse under their mutual gravity and form large asteroids tens to hundreds of kilometres across.

"If either one of these models turns out to be right... this will be a big step forward," says John Chambers of the Carnegie Institution in Washington DC.

Now a new study has found evidence that such a process did occur in our solar system. It is based on the size of objects in the asteroid belt. Estimates from telescopic surveys suggest there are millions of the smallest asteroids, which are less than a kilometre across, with the numbers of larger ones dropping off sharply. Yet this size distribution and number would once have been different: asteroids can grow by sweeping up smaller objects, and shatter if they collide with an object of similar size.

Alessandro Morbidelli of the Côte D'Azur Observatory in Nice, France, led a team that simulated the evolution of the asteroid belt, modelling a variety of starting populations (Icarus, DOI: link).

When the team started with small asteroids a few hundred metres to a few kilometres across - a scenario that might have occurred in the absence of a "sudden leap" - they ended up with far more small asteroids than are seen today. Another scenario started solely with 100-kilometre objects, but ended up with too few asteroids at the high end of the size range. But they did find a good fit with today when they started with a mixture of sizes between 100 and 1000 kilometres across, suggesting that large asteroids did form spontaneously during the solar system's development.

"It's a nice story and they have a lot of evidence supporting their point of view," says Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. But he cautions that it may have been difficult to complete planet formation in a reasonable time if there were no small asteroids at the outset. Small asteroids boost the rate of collisions needed for growth through their gravitational interactions with larger ones, he says.