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The nebular hypothesis holds that the solar system formed from a rotating cloud of gas and dust — a solar nebula — that collapsed under gravity roughly 4.6 billion years ago. As the cloud contracted it spun faster (conservation of angular momentum), flattening into a protoplanetary disk. Solid particles in the disk collided and grew by accretion from dust to planetesimals to protoplanets. The frost line separates inner rocky planets from outer icy and gas giants, because only refractory materials could condense in the hot inner disk.
Trace the sequence from collapsing nebula to differentiated planet. Explore the frost-line concept: which materials condense at which temperatures, and how this determines the composition gradient from inner to outer solar system.
You already know the broad structure of the solar system — a central star, four rocky inner planets, an asteroid belt, and four outer gas or ice giants. The nebular hypothesis is our best explanation for how this arrangement arose from a much simpler starting point: a slowly rotating cloud of interstellar gas and dust roughly 4.6 billion years ago.
The story begins with gravitational collapse. A slight overdensity in the cloud — perhaps triggered by a nearby supernova shock wave — caused material to fall inward under its own gravity. As it contracted, two things happened simultaneously: it spun faster (just as a figure skater pulls in their arms to spin faster, conservation of angular momentum spun up the collapsing cloud), and it flattened into a disk. The young Sun ignited at the center, and a protoplanetary disk of gas and dust spread out around it.
Within the disk, solid material began to clump together through collisions. Dust grains stuck electrostatically to form pebbles, pebbles collided to form boulders, and boulders accumulated into kilometer-scale planetesimals — the building blocks of planets. From there, larger bodies grew faster because they had stronger gravity, sweeping up material more efficiently in a process called runaway accretion. Protoplanets hundreds of kilometers across eventually formed, and their final mergers — including enormous giant impacts — shaped the planets we see today.
The frost line, located roughly where the asteroid belt is now, was the critical compositional dividing line. Inside it, only silicates and metals could condense from the nebular gas; volatiles like water and ammonia remained gaseous and were eventually blown away by solar radiation and the solar wind. Outside the frost line, these same compounds froze solid, dramatically increasing the density of solid material available for accretion. This surplus allowed the outer solar system to build massive rocky cores quickly — massive enough to gravitationally capture the abundant hydrogen and helium gas before the disk dispersed. The inner planets, starved of building material, grew slowly and small.
One important correction to a common simplification: the frost line was not a fixed fence. As the young Sun grew hotter in its first few million years, the frost line migrated outward; as the disk cooled and the Sun settled, it moved inward. Some bodies may have formed on one side of the frost line and migrated to the other. The asteroid belt preserves remnants from both sides of this boundary — carbonaceous asteroids rich in water-bearing minerals coexist with dry, rocky S-type asteroids — hinting at the complex history the simple frost-line picture only partially captures.