Around a newborn star, leftover gas and dust settle into a rotating disk. Micron-sized dust grains orbiting at slightly different speeds gently collide; static and weak molecular forces make some of them stick. Over hundreds of thousands of years those clumps grow from dust, to pebbles, to kilometre-scale planetesimals, and eventually to Moon- or Mars-sized protoplanets that go on sweeping up everything else in their orbital path.
The main unsolved puzzle in planet formation is the "metre-size barrier": metre-scale rocks should be blown inward by gas drag or shattered by collisions before gravity can take over — yet planets clearly exist. Streaming instabilities and pebble accretion are two leading ideas for how nature skips past that barrier.
A rotating disk of gas and dust orbits a young star; speed up simulated time to watch micron-sized grains collide, stick, and grow through planetesimals into glowing protoplanets.
Each grain follows a real Keplerian orbit — faster close to the star, slower farther out. Nearby grains merge probabilistically, conserving mass and volume, and are reclassified from dust to planetesimal to protoplanet as they grow past size thresholds.
Raise time speed to fast-forward accretion, tune stickiness to see how collision efficiency changes growth rate, and toggle the gas disk and snow line to compare rocky grains inside it against icy grains beyond it.
Beyond the snow line, water and other volatiles freeze onto grains, roughly doubling the solid material available — one reason gas giants like Jupiter and Saturn formed farther from the Sun than the rocky inner planets.