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In the protoplanetary disk, pebbles turn from rocks to asteroids to planetesimals. How do you get pebbles?

Jose Gonzalez, Bethlehem, Pennsylvania
RELATED TOPICS: DUST
A disk of planet-forming dust swirls around a young star.
In some ways, this is the hardest part of the process. Large objects like asteroids can smash into each other and end up partially embedded, or orbiting as a close pair. Planets have enough gravity to just haul in other bodies passing at similar speeds. But sticking what is essentially grit together is hard — as you might predict from emptying out a sack of gravel.

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