Giant Gas Planets Started From Tiny Pebbles

Gas-giant planets such as Jupiter and Saturn form quickly by scooping up pebble-size building blocks and pushing smaller potential planets out of the way, new research suggests.

Stars are born from slowly rotating clouds of gas that collapse inward; protostars spin faster and faster at the heart of a disk made of gas and dust. Over time, the dust grains clump together to form pebbles, asteroid-size planetesimals and eventually whole planets, clearing out paths through the dust and debris. But the details of that planet-forming process are still up for debate.

A new simulation answers important questions about how the fast-acting pebble-accretion model of gas-giant formation works, letting just a few planets reign supreme and banishing smaller planets and asteroids to the outskirts, as in our solar system. [Video: Best-Ever View of Exoplanet Birth]

“This is the first model that we know about that you start out with a pretty simple structure for the solar nebula from which planets form, and end up with the giant-planet system that we see,” study lead author Harold Levison, an astronomer at the Southwest Research Institute (SwRI) in Colorado, told

Pebble power

Earlier theories of planet formation relied on a slow buildup from dust to pebble-size debris to mountain- or asteroid-size planetesimals, until these burgeoning cores grew large enough to grab hydrogen and helium gas nearby and form gas giants like those orbiting the sun. But observational evidence suggested that the planets formed quickly, before the gas surrounding the newborn sun dissipated — and the chances of the giant planetesimals crashing into one another and combining were too low to form massive planets quickly enough.

In 2012, researchers Michiel Lambrechts and Anders Johansen from Lund University in Sweden proposed a model whereby tiny pebbles, formed from dust grains that fused together in collisions, hold the key to making gas giants quickly — up to 1000 times faster than the earlier model. “They showed that the leftover pebbles from this formation process, which previously were thought to be unimportant, could actually be a huge solution to the planet-forming problem,” Levison said.

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