Planetary accretion

Planetary accretion is the growth of planets as smaller solid bodies collide and merge under gravity. It builds planetary embryos and, eventually, planets within a protoplanetary disk.

Protoplanetary disk: A rotating disk of gas and dust surrounding a young star, from which planets can form. Accretion takes place within this reservoir of solid material and gas.
Protoplanetary disk turbulence: Irregular gas motion in a protoplanetary disk, driven by instabilities and other sources of stirring. Turbulence affects how dust concentrates, settles, and collides before large bodies form.
Terrestrial planet formation: The formation of rocky planets through the growth and collision of solid bodies in the inner part of a planetary system. Earth and the other terrestrial planets emerged through accretion of rocky material.
Disk instability model: A model in which parts of a massive, rapidly cooling protoplanetary disk collapse directly into giant planets. It contrasts with building a giant planet through solid accretion followed by gas capture.
Dust coagulation: The sticking and growth of microscopic solid particles through collisions in a gas. It begins the growth sequence that can ultimately produce planetary building blocks.
Pebble accretion: A growth mechanism in which planetary embryos capture centimeter- to meter-sized solids slowed by gas drag. It can grow embryos by capturing drifting pebbles rather than relying only on solid-body mergers.
Giant planet formation: The formation of massive planets, commonly involving a solid core that accretes surrounding gas. Accretion of solids can build the cores that trigger rapid gas capture.
Core accretion model: A model of giant planet formation in which a solid core grows and then attracts a substantial gaseous envelope. It describes how solid accretion can initiate the formation of a gas giant.
Planetesimal: A small solid body formed in a protoplanetary disk that can grow through gravitational interactions and collisions. Planetesimals are the kilometer-scale bodies whose mergers drive later accretion.
Gas drag: The force exerted by surrounding gas on a moving solid body, which alters its speed and orbit. Drag changes the motions and collision rates of small accreting solids.

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