KnowraPlanetesimalPlanetesimalA small solid body formed in a protoplanetary disk that can grow by accretion and merge with others to build planets.BriefConnectDust coagulation: The sticking and growth of dust grains through collisions in a gas-rich disk. It describes the earliest proposed steps toward planetesimal formation.Protoplanetary disk: A rotating disk of gas and dust surrounding a young star, where planets can form. Planetesimals originate within this planet-forming environment.Asteroid: A small rocky or metallic body orbiting the Sun, mostly found in the inner Solar System. Many asteroids are surviving or altered remnants of early planetesimals.Hierarchical accretion: A growth model in which progressively larger bodies form through repeated collisions and mergers of smaller ones. It contrasts gradual collisional assembly with direct collapse of concentrated particles.Streaming instability: A process in which gas drag concentrates drifting solid particles, potentially triggering gravitational collapse. It can gather disk solids densely enough to form planetesimals directly.Interstellar dust: Microscopic solid particles in space, composed of materials such as silicates, carbon, and ice. Dust grains inherited by disks supply the initial solid material.Comet: An icy Solar System body that releases gas and dust when heated near the Sun. Comets preserve volatile-rich material from the outer planetesimal-forming regions.Planetesimal hypothesis: A historical model in which planets grow through the accumulation of small solid bodies. It frames planetesimals as the central intermediate stage in planet formation.Gravitational collapse: The contraction of matter under its own gravity when self-gravity overcomes internal support. Dense particle clumps can collapse into planetesimals.Snow line: The disk radius beyond which water vapor can condense as ice under prevailing conditions. The availability of ice changes solid abundance and planetesimal growth across the disk.Show all 21Linked from 22 pagesPlanetBroader topic: Planetesimals are the building blocks that collide and grow into planets.Impact eventRelated: Planetesimal collisions helped assemble larger planetary bodies.Comet nucleusNarrower topic: Comet nuclei are thought to descend from icy planetesimals formed in the early Solar System.Solar System formationBroader topic: Planetesimals were intermediate building blocks between dust grains and planets.Debris diskNarrower topic: Planetesimals are the parent bodies whose collisions replenish the disk’s observable dust.Oort cloudBroader topic: Objects scattered outward during planet formation began as planetesimals.ArrokothNarrower topic: Arrokoth is interpreted as a preserved planetesimal from the Solar System’s formation.Planet formationBroader topic: Planetesimals are the intermediate building blocks between dust and planetary embryos.Show all 22