How did the titans of our solar system form? That’s a question researchers at the Southwest Research Institute (SwRI) and Queen’s University in Canada tackled in a new study published in the Aug. 20 issue of Nature.

The most common theory of gas giant formation is known as the core accretion model. Small rocks merge and become big rocks. Big rocks merge and become mountains and so on. Then, interstellar gas and dust attaches to the developing planet. There’s just one issue. Planets like Jupiter and Saturn had to evolve quickly to grab the significant amounts of lighter gases that make up their atmosphere.

Jupiter and Saturn would need to form somewhere between 1 to 10 million years. Core accretion can’t explain how gas giant formation in a time period that short.

accretion planets

Dr. Hal Levison, an Institute scientist in the SwRI Planetary Science Directorate and lead author of the study, touched on this glaring issue.

“The timescale problem has been sticking in our throats for some time,” said Levison. “It wasn’t clear how objects like Jupiter and Saturn could exist at all.”

The answer lies in tiny rocks

Levison and his team found the answer in planetary pebbles. These pebbles are icy objects about a foot in diameter. Similar models show how the planetary pebbles are concentrated by aerodynamic drag and then come together to form objects 100 to 1,000 kilometers in size. These larger objects can then grab any leftover pebbles. Cores up to ten times that of the Earth can be created in just a few thousand years. But, that’s way too fast. Full simulations have shown our solar system would be stuck with hundreds of Earth-mass objects.

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So we need a core ten times the size of Earth that forms quickly, but not in a few thousand years quick.

Dr. Katherine Kretke, a SwRI research scientist and co-author on the study, explains how this would work. “The growing cores need some time to fling their competitors away from the pebbles, effectively starving them. This is why only a couple of gas giants formed.”

Why are researchers confident with this explanation? Levison says it’s the first model that reproduces the structure of our outer solar system. The model explains everything we see in our outer solar system. Saturn, Jupiter, Uranus, Neptune and even the Kuiper Belt are all shown using this model.

“After many years of performing computer simulations of the standard model without success, it is a relief to find a new model that is so successful,” says Dr. Martin Duncan, a professor at Queen’s University.

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