The Formation of the Solar System

About $4.6$ billion years ago, our Solar System formed from a huge, cold cloud of gas and dust in space. Gravity pulled this cloud together and set the process in motion.

From Cloud to Star and Disc

• The cloud contained many elements (mostly hydrogen, plus helium and tiny amounts of heavier elements).
• Gravity made the cloud collapse. As it shrank, it spun faster and flattened into a thin, rotating “accretion disc” (like pizza dough spreading out when spun).
• The centre became very hot and dense: a protostar. When it got hot enough, the Sun switched on as a stable star. The Sun contains about $\approx 99.8\%$ of the Solar System’s mass, so its gravity controls the orbits.

Building Planets: Accretion

In the disc, tiny dust grains stuck together. Small clumps grew into pebbles, then into kilometre-sized planetesimals. Collisions and gravity (“space glue”) built protoplanets and, eventually, the planets.

Why Inner Rocky Planets and Outer Gas Giants?

• Temperature gradient: Near the hot, young Sun, only materials with high melting points (rock and metal) could stay solid. Light gases (hydrogen, helium) and ices could not condense there.
• Farther out, it was cooler (beyond the “snow line”), so water, methane and ammonia could freeze into ices. Icy solids helped cores grow quickly and become massive. These large cores captured thick atmospheres of hydrogen and helium, forming the giant planets.
• Result: four small, rocky inner planets; four large, gaseous outer planets.

Leftovers and Moons

• Not all material became planets. Rocky leftovers orbit between Mars and Jupiter as the asteroid belt. Icy leftovers in distant regions formed comets.
• Some moons formed in mini-discs around growing planets, some were captured objects, and Earth’s Moon likely formed after a giant impact.

Orbits and Gravity

Because the material was in a rotating disc, most planets orbit the Sun in the same direction and in nearly the same plane. The Sun’s gravity keeps them in orbit. Farther from the Sun, gravity is weaker and orbital speeds are lower.

Common Misconceptions

• Planets were not pieces torn from the Sun; they formed from the surrounding disc.
• Formation was a slow collapse under gravity, not a single explosion.
• Rotation did not stop collapse; it spread material into a disc while gravity pulled inward.