A new study challenges the conventional view that asteroids are the building blocks of planets that never formed. The basis of this new study relies on chondrules—spheres of rock within meteorites that have melted and resolidified—which scientists consider as time capsules of the early solar system.

From the time the asteroid belt was discovered, two explanations seemed obvious: These were either the remnants of a planet that somehow got broken apart, or they were the bits that might have become planets but never got the chance to form.

Chondrules are spherical grains roughly 0.001-1.5 centimeters across that are formed from once molten rock and are dotted through non-metallic meteorites. Some chondrites, the name given to these meteorites, are 60-80% chondrule by volume, others much less.

The formation process for chondrules is poorly understood, but many scientists believe they hold clues to the early solar system. It is hoped they may settle the debate between the dominant view that asteroids are planets that never formed and the less popular impact model for asteroid formation.

"Understanding the origin of chondrules is like looking through the keyhole of a door; while we can't see all that is happening behind the door, it gives us a clear view of one part of the room and a glimpse into the very beginnings of our solar system," says Purdue University's Professor Jay Melosh.

Melosh has now published a paper in Nature arguing that chondrules reveal evidence of having been formed when objects collided at a velocity of least 2.5 kilometers per second. He proposes that in the first 5 million years of planetary formation when protoplanets were smashing into each other, rocky material would have melted and been ejected to form chondrules that eventually coalesced to form the rocky asteroids of today.

"Chondrules are identical in size, shape and texture to spherules on Earth and spherules found in the lunar soil," Melosh says. Since these spherules are agreed to have been made when asteroids hit the surface, Melosh thinks the same applies to those in meteorites. "The only difference among chondrules, impact spherules and lunar soil particles is in their chemical composition, which fits because they are made of different starting materials from impacts on different bodies."

Melosh's work revives the idea of chondrules being the byproduct of impacts, but explains the process in a new way. Early solar system collisions are thought to have occurred at relatively low speeds, since the colliding objects were in similar orbits – like two cars travelling in the same direction that nudge against each other gently, rather than experiencing a head-on strike. Such slow impacts were not thought to melt rock.

However, Melosh argues that if the bodies were large enough, even with a slow impact debris at the surface would have melted in the compression. This might be small compared to the volume broken off, but it would be enough to produce the chondrules we see today.

In another example of chondrules' potential as time capsules, a recent study used them to demonstrate for the first time that the formation of the solar system was shaped by a powerful magnetic field.