The formation of planets, known as planetary accretion, describes how immense celestial bodies arise from microscopic particles of dust and gas. This assembly is governed by the interplay of material forces and the influence of gravity. The modern understanding of this process is rooted in the Solar Nebular Hypothesis, which posits that planetary systems begin as a vast, rotating cloud of gas and dust. This framework provides a chronological sequence for how dispersed matter clumps together, grows, and culminates in the diverse planetary systems observed today.
The Protoplanetary Disk: Raw Materials
The journey begins immediately following the birth of a star, where the remnants of the collapsing molecular cloud form a flattened, rotating structure called a protoplanetary disk. This disk is composed overwhelmingly of gas, primarily hydrogen and helium, with only about one percent of its mass existing as solid materials. The solid component includes silicate dust grains and various ices of water, methane, and ammonia farther from the star. The disk shape is a natural consequence of the conservation of angular momentum during the initial gravitational collapse.
As the bulk of the material falls inward to form the central star, the remaining matter flattens into a disk. This rotational motion resists the star’s gravity, resulting in a thin, pancake-like structure where all the raw ingredients orbit in a common plane. The disk environment is dynamic, with temperatures and gas densities decreasing rapidly with distance from the young star. The properties of this disk, which lasts only a few million years, determine the composition and architecture of the planetary system.
From Dust Grains to Planetesimals
The initial stage of growth involves microscopic dust particles, which must stick together without being destroyed by collisions or lost to the central star. At this scale, gravity is negligible, and the particles are held together by non-gravitational forces, such as van der Waals forces and static electricity. These forces allow the dust grains to gently collide and coagulate into fluffy aggregates, eventually forming centimeter or meter-sized objects known as pebbles.
This growth phase faces the “meter-size barrier,” where objects between one centimeter and one meter experience significant aerodynamic drag from the surrounding gas. This drag causes them to lose angular momentum and rapidly spiral inward toward the central star. One proposed solution is the streaming instability, where the drag force concentrates pebbles into dense clumps that collapse under their own weak self-gravity. This gravitational collapse bypasses slow collisional growth and directly forms planetesimals, solid bodies roughly one kilometer in diameter, which are the building blocks of planets.
Gravitational Dominance and Runaway Growth
Once kilometer-sized planetesimals have formed, gravity becomes the dominant force driving accretion. A larger body’s gravitational pull influences the trajectories of nearby smaller planetesimals, an effect known as gravitational focusing. This phenomenon increases the collision cross-section of the growing body far beyond its physical size, leading to a rapid increase in mass.
This process initiates “runaway growth,” where the most massive planetesimals grow exponentially faster than their neighbors, creating a positive feedback loop. This phase continues until the largest bodies have consumed most of the nearby material, leading to a more ordered growth phase called oligarchic growth. The result of this rapid accumulation is the formation of planetary embryos or protoplanets, which range in size from that of Earth’s Moon to Mars.
Final Assembly: From Protoplanet to Planet
The final stage of planet formation involves violent, large-scale collisions between these planetary embryos over tens of millions of years. In the inner solar system, terrestrial planets formed as these Moon-to-Mars-sized bodies smashed into one another, contributing to the final mass and orbital characteristics of the emerging planet. The energy released by these massive impacts also played a significant role in melting the planet’s interior.
The continuous accumulation of material eventually leads to internal differentiation. During this process, heavier elements, such as iron and nickel, sink toward the center to form a dense core, while lighter silicate materials rise to form the mantle and crust. The final criterion for a planet is “orbital clearing,” where the body becomes gravitationally dominant in its orbital zone, either accreting or ejecting the remaining planetesimals and protoplanets.