Small ‘beads’ central to planet formation

Research points to the role of chondrules in Solar System development

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Planetesimals form via collisions amongst the debris orbiting a star. Image Credit: ISAS/JAXA

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Small, glassy beads called chondrules played a central role in the formation of planets, new research has suggested.

Asteroid-sized planetesimals, which are the building blocks of planets formed from dust and rock, sweep up the chondrules over time, causing them to grow in size.

“The big question is, ‘How did the planets come to be?’” says Mordecai-Mark Mac Low, a curator in the American Museum of Natural History’s Department of Astrophysics.

“When the Solar System first started forming, the largest solids were sub-micron dust.

The challenge is to figure out how all of that dust was gathered up into planet-building objects that then formed the diversity of planets and other smaller bodies that we see today.”

Planets begin forming in protoplanetary discs around young stars.

As this happens, according to current theory, dust particles contained within gas surrounding the star come together to form small clumps, which then become pebbles and, eventually, boulders.

However, as the boulders increase in size, so too does the drag caused by the surrounding gas.

In 2007, Mac Low and a team of collaborators, including Anders Johansen of Lund University in Sweden, first suggested a mechanism called the streaming instability, which predicts a wholly different scenario.

It says that boulders contained within the same orbit would begin to sweep the surrounding gas with them, reducing both the amount of drag and the rate at which they move closer to the star.

Then, as more boulders enter that orbit, they collapse together and form planetesimals big enough to overcome the gas drag completely.

“Access to some of the world’s fastest supercomputers allowed us to perform simulations of planetesimal formation in exquisite detail,” says Johansen.

We were able to measure the size distribution of the newly born planetesimals and compare this to the sizes of the asteroids in the Solar System.”

Current theories suggest that asteroids reflect the original size distribution of planetesimals because they are essentially planetesimals that were never swallowed up by a newly-forming planet.

But the team found that the size distribution predicted by their simulations did not match the distribution found in asteroids. Moreover, the simulations did not produce big enough planetesimals.

However, when the team ran through the simulation taking into account the presence of chondrules, they were then able to reproduce the size distribution found in asteroids.

“The interesting thing about chondrules is that they’re just the right size to get slowed down by the gas around planetesimals, which causes them to fall down and accumulate like sand piling up in a sandstorm,” Mac Low says.

The simulation also showed that the larger a planetesimal gets, the more easily it attracts and captures chondrules.

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These larger planetesimals eventually become unstable and can collide with others to form large planets the size of Mars or Earth.