Accretion is the fundamental process in astrophysics describing the growth of a massive object by attracting and accumulating surrounding matter. In planet formation, accretion is a multi-stage, bottom-up mechanism where micron-sized particles coalesce over millions of years to form massive planetary bodies. This process built every planet in the solar system, from terrestrial worlds to gas-dominated giants. The process transitions from non-gravitational sticking to the overwhelming dominance of gravitational attraction.
The Environment of Planet Formation
The process begins within a protoplanetary disk, a flattened, rotating cloud of gas and dust surrounding a newly formed star. The disk is primarily hydrogen and helium gas, containing about one percent solid material in the form of microscopic dust grains. Physical conditions vary dramatically with distance from the star, known as a radial gradient. High temperatures in the inner region vaporize volatile compounds, leaving only refractory materials like silicates and iron.
Moving outward past the “ice line,” temperatures drop low enough for water, methane, and ammonia to condense into ice mantles on the dust grains. This increases the available solid material, influencing the composition and mass of the outer planets. Dust particles settle toward the disk’s mid-plane due to the star’s gravity, forming a thin, dense layer where the initial phase of planet building occurs.
From Dust Grains to Rocky Planetesimals
The first step of accretion is coagulation, where gravity plays no role in the sticking process. Microscopic dust grains, typically less than a micrometer in size, gently collide and adhere due to short-range forces, such as van der Waals forces and electrostatic charges. These slow, non-destructive collisions allow the particles to grow into fluffy, fractal aggregates. As growth continues, these porous clumps compact and transition into centimeter-sized pebbles.
This growth faces the “meter-size barrier,” where aerodynamic drag causes objects between a meter and a kilometer in size to rapidly spiral into the star. A proposed solution is the streaming instability, a hydrodynamic mechanism that concentrates pebbles into dense, gravitationally bound clumps. Once these clumps collapse, they bypass the dangerous size range and directly form kilometer-sized planetesimals. These planetesimals are the stable, self-gravitating building blocks of all planets, representing the largest bodies formed by non-gravitational means.
Gravitational Assembly of Planetary Cores
Once planetesimals reach kilometer-scale sizes, their own gravity becomes the dominant force, initiating the true phase of gravitational accretion. This growth proceeds through two distinct stages, beginning with runaway growth. During this phase, the largest planetesimals have a greater gravitational reach, allowing them to sweep up surrounding material much faster than smaller neighbors. This positive feedback loop means the most massive objects grow exponentially, quickly reaching the size of planetary embryos, roughly the mass of the Moon or Mars.
Runaway growth transitions into oligarchic growth when planetary embryos become massive enough to dynamically influence a large region of the disk. These dominant bodies, or “oligarchs,” accrete or scatter all remaining planetesimals in their local orbital zone, clearing their feeding lanes. This slows the growth rate because the oligarchs must wait for new material to diffuse into these cleared zones. This extended oligarchic phase forms the solid cores of terrestrial planets and the larger rocky or icy cores required for outer solar system gas giants.
The Specialized Process of Gas Accretion
The formation of giant planets follows the core accretion model, a two-stage process beginning with the gravitational assembly of a large solid core. A massive core of rock and ice must form in the outer disk, reaching an estimated 10 to 20 Earth masses. This must be completed quickly, before the surrounding gaseous disk dissipates. The core’s immense gravitational field initially draws in an envelope of hydrogen and helium gas from the nebula.
Once the core surpasses a critical mass threshold, the planet’s gravitational pull becomes so powerful that the gas envelope can no longer maintain hydrostatic equilibrium. This triggers the second stage: rapid, runaway gas accretion. The planet dramatically increases its mass by capturing vast amounts of nebula gas. This explosive growth allows a gas giant like Jupiter to become hundreds of times the mass of Earth, dominated by light elements from the primordial disk.