By Sasha Rousseau | Updated Mar 24, 2022
Image credit: LightFieldStudios/iStock/GettyImages
The condensation theory explains why the planets orbit the Sun in a flat, coplanar disk, why they share a common direction of motion, and why inner planets are rocky while outer planets are gas giants. Terrestrial worlds such as Earth differ fundamentally from Jovian giants like Jupiter.
Giant molecular clouds (GMCs) are vast interstellar clouds composed of roughly 90 % hydrogen, 9 % helium, and 1 % trace heavier elements. As a GMC collapses, a rotation axis develops. Over time, the rotating clump contracts, heats, and densifies, eventually encompassing most of the GMC’s mass. The cloud’s angular momentum forces the material to condense toward the axis, while centrifugal forces flatten the structure into a disk. This disk—known as the solar nebula—provides the geometric framework for the planetary system: all planets orbit in the same, relatively flat plane, and in the same direction as the original rotation.
At the heart of the solar nebula lies the densest, hottest region that will become the proto‑Sun. As the nebula spins, dust grains—mixtures of ice, silicates, carbon, and iron—collide and stick together, forming planetesimals that are a few hundred kilometers in diameter. These planetesimals gravitationally attract one another, merging into protoplanets that continue to orbit the proto‑Sun in the same sense as the initial GMC rotation.
Protoplanets draw in hydrogen and helium from the surrounding nebula. Their ability to accrete gas depends on distance from the hot center: the farther a protoplanet is, the cooler its environment, allowing more solid material to condense and build a larger core. A larger core exerts stronger gravity, enabling the capture of more gas. Consequently, inner protoplanets remain small and rocky, while those farther out grow massive enough to become gas giants.
As the proto‑Sun ignites nuclear fusion, it emits a powerful solar wind that sweeps away the remaining gas from the nebula. This outflow terminates the accretion of gaseous material, effectively freezing the final masses of the planets. Protoplanets farther from the Sun, where material was more sparse, may finish with thin atmospheres or remain primarily icy cores. The solar wind clears the system roughly 100 million years after the Sun’s formation.
