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New study shows that Earth was formed by millimetre-sized pebbles over a short period

Landscape with rock formations and water. Illustration.
Illustration: Don Dixon (copyright/used with permission).

A Swedish-Danish research team is now launching a new theory of the process that led to the formation of Earth. Through advanced analyses of meteorites, astronomers can determine that Tellus went from being a baby planet made of ice and carbon to reaching its current size thanks to millimetre-sized pebbles. The study also shows that the Earth was formed over a much shorter period than previously thought.

About 4 550 million years ago, our planetary system was formed out of a rotating disk of gas and dust on the edge of the Milky Way. Common astronomical theories have entailed that the Earth was formed by collisions between asteroids that clumped together and that it took between 30 to 100 million years for Tellus to form its current shape and size. However, in a new study presented in the scientific journal Science Advances, a team of researchers from Lund University, among others, are launching a new ground-breaking theory about this process. Through careful analysis of meteorites’ isotope compositions in a laboratory at the University of Copenhagen, researchers can now establish that our planet was instead formed by millimetre-sized pebbles that were sucked together to form a celestial object.

“Our study shows that the Earth began as a baby planet consisting of ice and carbon. The Earth's isotope composition shows that it attracted pebbles first from the inner part and then from the outer part of the solar system”, explains Anders Johansen, professor of astronomy at Lund University and the University of Copenhagen.

It was not just the process of how the Earth was formed that surprised the researchers. Their extensive meteorite analyses, which were then compared with the Earth's isotope composition, also showed that the planet’s formation was much faster than previously thought by the scientific community.

“Our model shows that the Earth was formed over five million years, at which point the pebbles ran out. Five million years may not sound like a lightning-fast process, but it is a much shorter time than the 30 to 100 million years that researchers had previously estimated”, says Johansen.

According to the study, the Earth started out as a cold protoplanet – a precursor to a planet – consisting of ice and carbon. It was only when it grew to about 1 per cent of its current mass that its atmosphere became warm enough for the ice to evaporate and organic molecules containing carbon to be destroyed. In connection with this, the Earth stopped increasing its water and carbon content and instead began to capture only the dry, rocky components of the pebbles that at the time were whirling around the solar system. According to the study, Jupiter and Saturn were also formed by absorbing pebbles.

“There are many indications that Mars was also about to grow really big, but then the pebbles had run out and our red neighbouring planet ended up no larger than 10 per cent of the Earth's mass”, explains Johansen.

This new discovery is important because it can demonstrate how much water and carbon – both of which are essential in terms of the origin of life – is present when a planet is born. According to the research team, there are many indications that Earth-like planets orbiting other stars in the Milky Way are formed with the same composition. A typical result of the new planet formation theory – the pebble accretion theory – could be that Earth-like planets orbiting other stars, so-called exoplanets, have a surface consisting of 70 per cent water and 30 per cent land, something that significantly increases the chances of one day finding life in the universe, according to Johansen.

“Next year, the new generation of space telescope, JWST, will be launched, which will contribute to the discovery of a number of new exoplanets. It will be very interesting to study these Earth-like planets more closely”, adds Johansen.

In addition to Lund University, the following higher education institutions participated in the project: Center for Star and Planet Formation, GLOBE Institute (University of Copenhagen), Niels Bohr Institute (University of Copenhagen), Space Research Institute (Austrian Academy of Sciences).

The study is published in the research journal Science Advances.

Study: "A pebble accretion model for the formation of the terrestrial planets in the solar system" – advances.sciencemag.org