A computer simulation studying how asteroids formed from the tiny millimetre-sized chondrules suggests that the Earth was also possibly formed from these round stones.

Chondrules are believed to be the original building blocks of the solar system.

The study shows that asteroids were formed by capturing chondrules using the gravitational force.

"The chondrules are of exactly the right size to be slowed down by the gas that orbited the young sun, and they could then be captured by the asteroids' gravity," says Dr Anders Johansen, an astronomy researcher at Lund University.

Working with colleagues from the USA, Denmark and Germany, Johansen developed a computer simulation for the asteroid forming process.

Formed in a cosmic ocean of chondrules, the asteroids started out much smaller and grew quickly to present sizes of diameters going up to 1000km.

The largest asteroids in the simulation continued to grow till they acquired the mass of Mars.

Rocky planets like Earth begin life as dusty material circling around young stars. The material clumps together to form asteroids that break up or merge to form planets over a period of 100 million years.

It has been believed that the Earth too was formed through collisions between protoplanets the size of Mars, over a period of a 100 million years.

But how these protoplanets in turn formed was a puzzle.

The latest study suggests protoplanets formed very quickly from asteroids, by capturing chondrules in the same way as the asteroids did, says Martin Bizzarro, an expert on chondrules from Copenhagen University and co-author of the paper.

The theory is supported by studies of Martian meteorites that show the planet was formed over a period of only one to three million years, the same time span as obtained in the computer simulation.