Fragments of asteroids regularly land on Earth as meteorites. If you examine such a find, you can see that it comprises millimetre-sized round stones, known as chondrules. These small particles are believed to be the original building blocks of the solar system. However, the research community has not previously been able to explain how the chondrules formed asteroids. A new study shows that asteroids were formed by capturing chondrules with the help of 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.

"This causes them to fall down and accumulate like sand piling up in a sandstorm," adds co-author Mordecai-Mark Mac Low from the American Museum of Natural History.

Working with colleagues from the USA, Denmark and Germany, Anders Johansen has developed a computer simulation for what the process may have looked like. They assumed that the asteroids were formed in a kind of cosmic ocean of chondrules and that the asteroids started out much smaller than they are today.

According to the computer simulations, the asteroids grew quickly to a diameter of up to 1,000 km, the same size as those found today in the asteroid belt between Mars and Jupiter. The largest asteroids continued to grow to the same mass as the planet Mars, which has ten per cent of the mass of Earth.

"We suddenly realised that this rapid process could say something about the formation of the Earth as well," explains Anders Johansen.

The research community had previously believed that the Earth was formed through collisions between protoplanets, of the size of Mars, over a period of 100 million years. However, the researchers have not yet understood how the protoplanets themselves were formed.

"Our study shows that protoplanets may have 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 also co-author of the paper.

The researchers' theory is supported by studies of meteorites from Mars. These studies have previously shown that Mars was formed over a period of only 1-3 million years, which is within the same time span as the researchers have obtained in the computer simulation.

"Traces of this process remain in asteroids that still contain intact chondrules. The terrestrial planets, however, have all melted after their birth and therefore do not show any direct traces of their original building blocks," concludes Anders Johansen.

The study has been published in the journal Science Advances.