Formation of Planets

The birth of a planet is not a single event but a slow, step-by-step process. The textbook describes it as unfolding in three clear stages, starting from a swirling cloud of gas and ending with solid, rounded worlds. The driving force throughout is gravity, which pulls scattered matter together into ever-larger bodies.

Stage one: the rotating disc. A nebula is not a uniform cloud; it contains localised lumps of gas. Within each lump, gravity pulls material inward, forming a dense core at the centre of the gas cloud. Around this core, the remaining gas and dust spread out into a huge, rotating disc. So the first step is the creation of a central core surrounded by a flat, spinning disc of material.

Stage two: planetesimals. The gas cloud now begins to condense. The matter in the disc around the core does not stay as a smooth sheet — it clumps into small, rounded objects. These objects then grow through a process called cohesion, where material sticks together. The result is a large number of smaller bodies known as planetesimals. Once these exist, larger bodies begin to form through collision; when two planetesimals crash into each other, gravitational attraction makes the material stick together rather than bounce apart. So a planetesimal is simply a small, solid body that acts as a building block for something bigger. …