The formation of the Solar System began approximately 4.6 billion years ago with the collapse of a dense pocket within a giant molecular cloud. This collapse initiated the accretion phase, the fundamental process by which all larger objects in our solar system were constructed. Accretion describes the growth of a body by the gradual accumulation of surrounding material through gravitational attraction and collision. This process transformed a diffuse cloud of gas and dust into the diverse collection of planets, moons, and smaller bodies observed today.
The Protoplanetary Disk: Initial Conditions
The initial gravitational collapse caused the vast majority of the material to fall inward, forming the proto-Sun at the center. Conservation of angular momentum forced the remaining material to flatten into a vast, spinning structure known as the protoplanetary disk, or solar nebula. This disk was primarily composed of light gases, mostly hydrogen and helium, accounting for over 99% of its mass. Distributed within this gas was a small fraction of heavier elements in the form of microscopic dust grains, including silicates, iron, and various ices.
A strong temperature gradient existed across this disk, dictated by the heat radiating from the proto-Sun. The inner regions were extremely hot, allowing only materials with high melting points, such as rock and metal, to remain solid. Farther out, the temperature was low enough for volatile compounds like water, methane, and ammonia to condense into ice. This boundary, where water ice could first exist as a solid, is known as the “ice line” or “snow line,” and it fundamentally determined the composition of the forming planets.
Dust to Planetesimals: Coagulation and Growth
The planet-building process started with the gentle collision of the smallest dust grains throughout the disk. This initial stage, called coagulation, relied on non-gravitational forces to make the particles stick together. Electrostatic attraction and weak van der Waals forces were the primary mechanisms binding these dust particles into larger, fluffy aggregates. This growth was initially slow, transforming micron-sized dust into centimeter-sized pebbles.
As these particles grew, they faced the “meter-size barrier,” where aerodynamic drag from the surrounding gas would cause them to rapidly spiral into the Sun. To overcome this, a much faster process was required to jump from pebbles to kilometer-sized bodies, which were large enough for gravity to become effective. The “streaming instability” is the leading theory for this rapid growth, where collective drag forces cause dust to concentrate into extremely dense clumps. These high-density filaments could then collapse under their own gravity to form kilometer-sized objects called planetesimals, the first true building blocks of planets.
Runaway Accretion and Planetary Embryos
Once planetesimals reached a few kilometers in size, the growth mechanism shifted dramatically to the dominance of gravity. This next phase is known as “runaway accretion,” a period of explosive growth that was relatively short, lasting only about 100,000 to 300,000 years. During this time, the largest planetesimals grew exponentially faster than their smaller neighbors.
The massive objects possessed a greater gravitational reach, increasing their “gravitational cross-section” and allowing them to sweep up material at an accelerating rate. This created a positive feedback loop: the more mass a body gained, the stronger its gravity became, leading to faster accumulation. The result was the formation of numerous Moon- to Mars-sized objects known as planetary embryos or protoplanets.
In the inner Solar System, these embryos were composed of rock and metal and would become the cores of the terrestrial planets. Beyond the ice line, the embryos were substantially more massive because they accreted both rock and abundant water ice. This larger mass allowed them to reach a threshold of about 5 to 10 Earth masses, sufficient to gravitationally capture vast envelopes of hydrogen and helium gas from the nebula. This rapid core formation was a race against time, as the gas component of the disk was destined to dissipate quickly, paving the way for the formation of the gas and ice giants.
Final Assembly and Clearing the Nebula
Following runaway accretion, the process transitioned into a slower, more self-regulated phase known as “oligarchic growth.” The remaining planetary embryos had cleared out much of the local material in their orbits, forcing them to grow at a comparable rate rather than one body dominating the rest. This phase involved gravitational stirring and occasional, violent collisions between the remaining large planetary embryos.
The final construction of the terrestrial planets took tens to hundreds of millions of years and was characterized by catastrophic giant impacts. These collisions shaped the final size, spin, and orbital characteristics of the inner planets, including the event that formed Earth’s Moon. Material from various parts of the inner disk was mixed during these final mergers.
The end of the accretion phase was signaled by the Sun’s own evolution. During this early period, the young Sun was in its “T-Tauri phase,” characterized by intense stellar activity. This stage produced powerful stellar winds and ultraviolet radiation that acted as a cosmic broom, blowing the remaining gas and dust out of the inner solar system. By clearing the residual material, this event starved the growing planets of their primary fuel source, halting major accretion and establishing the final mass and structure of the planetary system.