Millimetre-sized stones formed our planet

Asteroids are formed from tiny particles captured by gravity. Our own planet has its origins in the same process.

April 22, 2015

Fragments of asteroids regularly land on Earth as meteorites. When examined carefully, they are found to comprise 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 led by researchers from the Lund University in Sweden 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 Anders Johansen, an astronomy researcher at Lund University. As a result, they fall down and accumulate like sand piling up in a sandstorm.

Working with colleagues from the USA, Denmark and from the Max Planck Institute for Solar System Research (MPS) in Germany, Anders Johansen has developed a computer simulation for what the process may have looked like. “We 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”, says Pedro Lacerda from the MPS, who contributed to the work.

Millimeter-sized stones formed our planet

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”, explained Anders Johansen.

Researchers 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. The new study now shows that protoplanets may have formed very quickly from asteroids, by capturing chondrules in the same way as the asteroids did.

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.

In addition, the simple model is also capable of explaining the sizes of objects found in the Kuiper belt, a region far beyond the orbit of Neptine. ”The largest Kuiper belt objects such as Pluto and Eris are in excited orbits because they formed nearer the Sun where they were able to grow very large by pebble accretion and were later scattered to the Kuiper belt”, says Lacerda. ”The objects that were born at Kuiper belt distances remained relatively small - around 200 km in radius - because icy pebble accretion does not work that far from the Sun."

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