The current understanding of how planetary systems form is rooted in the Nebular Hypothesis. This model posits that the Sun and all orbiting bodies originated from the gravitational collapse of a single, immense cloud of interstellar gas and dust. The process describes a sequence of physical transformations that turns a diffuse, rotating cloud into the highly ordered structure of a solar system over tens of millions of years. This theory explains the composition and orbital characteristics observed in our solar system and countless others throughout the galaxy.
Initial Collapse and Formation of the Protoplanetary Disk
The first stage begins with a giant molecular cloud, a cold and vast reservoir of hydrogen, helium, and trace amounts of heavier elements. This cloud is inherently unstable and requires a trigger to initiate the collapse, such as a shockwave from a nearby supernova explosion or the gravitational influence of a passing star. Once triggered, the cloud begins to contract under its own self-gravity, pulling matter toward a central point.
As the cloud shrinks, two physical principles dictate its transformation: conservation of angular momentum and the force of gravity. The cloud’s initial, slow rotation is dramatically amplified as its radius decreases, much like a spinning figure skater pulling their arms inward. This rapid rotation prevents material from collapsing entirely onto the center, instead forcing it outward into a flattened, spinning structure.
The result is a flattened, pancake-like structure known as the protoplanetary disk, or solar nebula, surrounding a dense central protostar. This disk is composed of approximately 99% gas and 1% dust, and its temperature decreases sharply with distance from the center. The central region experiences immense pressure and heating, eventually becoming hot enough to ignite nuclear fusion, thus forming the star.
Condensation and Planetesimal Creation
The temperature gradient within the newly formed protoplanetary disk is the primary factor that determines the composition of the future planets. Closer to the protostar, temperatures can exceed 1,500 Kelvin, meaning only materials with high melting points could remain solid. These refractory materials include metals like iron and nickel, and silicate rocks.
Moving outward, a boundary known as the frost line marks the distance where temperatures drop low enough for volatile compounds to condense into solid ice grains. Beyond this boundary, the much more abundant ices of water, methane, and ammonia augment the available solid material.
Within the disk, microscopic dust grains begin to collide and stick together via electrostatic forces, a process known as coagulation. These collisions gradually build up larger clumps, progressing to pebble-sized objects. Once these solid bodies reach a size of roughly a kilometer, their self-gravity becomes strong enough to influence their neighbors, marking the formation of planetesimals—the foundational building blocks of planets.
Growth of Protoplanets and Runaway Accretion
Once kilometer-sized planetesimals are formed, their growth accelerates dramatically, transitioning to gravitationally dominated accretion. The largest planetesimals have a greater gravitational reach, allowing them to sweep up surrounding material more efficiently. This phase is termed “runaway accretion” because a slightly larger body quickly grows much larger, creating planetary embryos, or protoplanets, in a relatively short period of time.
This rapid growth phase is followed by a slower, self-limiting stage called oligarchic growth, where the few remaining large protoplanets dominate the disk. The final composition of these protoplanets depended heavily on their location relative to the frost line. In the inner system, terrestrial protoplanets formed by accreting only rock and metal, resulting in smaller, dense worlds.
In the outer system, protoplanets formed massive cores of rock and ice. If these icy cores formed before the gas in the protoplanetary disk dissipated, their powerful gravity allowed them to enter a second, intense phase of runaway accretion by pulling in vast envelopes of hydrogen and helium gas directly from the nebula. This core accretion model explains the creation of the gas and ice giants, which possess massive atmospheres surrounding their solid cores.
Planetary Differentiation and Final Configuration
As the protoplanets matured, internal heating became intense due to the energy released by impacts, gravitational compression, and the decay of short-lived radioactive isotopes. This heat caused the bodies to partially or fully melt, initiating the process of planetary differentiation. Differentiation is the physical separation of materials based on density, leading to a layered internal structure.
Denser materials, primarily molten iron and nickel, sank toward the center to form a metallic core. Simultaneously, lighter silicate materials floated upward to form the mantle and a thin outer crust. This process, which happened early in a planet’s history, established the fundamental internal architecture of the terrestrial worlds.
The final configuration of the planetary system was achieved when the central star entered its T Tauri phase, characterized by powerful stellar winds. These winds effectively swept away the remaining primordial gas and dust from the protoplanetary disk. The planets were left in their stable, cleared orbits, with the leftover planetesimals becoming the asteroids, comets, and other minor bodies of the system.