spaceSpace and Physics

Pebble Theory Of Planet Formation Could Explain Why Earth Is Bigger Than Mars


Jonathan O'Callaghan

Senior Staff Writer

3250 Pebble Theory Of Planet Formation Could Explain Why Earth Is Bigger Than Mars

Two months ago, Dr. Hal Levison from the Southwest Research Institute (SwRI) in Boulder, Colorado, co-authored a paper that described a theory of “pebble formation” for gas giants. He has now expanded that theory in a new study to include some of the terrestrial planets in the Solar System, a major breakthrough in planet formation.

“I think it’s the most important paper I’ve ever written,” Levison told IFLScience. It is published in the Proceedings of the National Academy of Sciences Early Edition and is available on Arxiv.


How planets form remains an unresolved area of astronomy. It’s generally agreed that, in our Solar System at least, dust and gas stuck together early on to grow ever larger objects, eventually forming minor and major planets such as Ceres and Earth. But despite being proposed decades ago, the theory remains in its infancy.

Particular problems include why Mars is so much smaller than Earth, and also how to overcome the “meter barrier” – how very early planetesimals (the building blocks of planets) grew beyond a meter (3.3 feet) in size without falling into the Sun. According to Levison, it’s all about the position of the growing planet, and the pebble theory is essential.

The pebble theory states that a growing object will continue to accumulate pebble-sized objects a few inches in diameter, rather than sticking to similarly sized objects, to eventually form a planet. As on object grows in size, on a timescale of thousands of years, it will attract more and more pebbles into its orbit via aerodynamic drag, which ultimately fuse with the body, as proposed by Michiel Lambrechts and Anders Johansen of Lund University in Sweden in 2012.

“The new work in this paper was to show that if you have the same growing planet, let’s say Ceres, and you put it at 1 AU [astronomical unit; 1 AU is the Earth-Sun distance], it can grow by this process,” Levison told IFLScience. “But at 2 AU it can’t because of the time scale in order to get the process to work,” referring to the fact that aerodynamic drag is too weak this far out.


Our planets formed about 4.6 billion years ago. NASA/JPL-Caltech. NASA/JPL-Caltech/T. Pyle (SSC).

According to this latest paper, which took months to perform a simulation of the early Solar System using high-powered computer clusters, 1.5 AU seems to be the limit within which objects form fully fledged rocky planets under what is called Viscously Stirred Pebble Accretion (VSPA). This explains why Mars – at an average distance of 1.52 AU – has just 10% the mass of Earth. The model can’t yet explain how Mercury was able to form so close to the Sun – but crucially no other model can definitively, either.

As mentioned earlier, the same model was used in August to explain how the gas giants formed. And Levison said it was a “relief” to discover the terrestrial planets probably followed the same process. “It really sort of ties a bow around the planet formation process,” he said. “It can reproduce the structure of the Solar System, at least for Venus, Earth, Mars, the asteroid belt, Jupiter, Saturn, Uranus, Neptune, and the Kuiper Belt. Nobody’s been able to do that before.”

Levison and his team plan to continue their work and hope that it will also be able to explain the formation of Mercury, and possibly other planetary systems too. For now, it’s rapidly emerging that the pebble theory is our best explanation yet for how planets formed.


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