What is a planetesimal? How did planetesimals give rise to the terrestrial planets?

The story of our solar system begins over 4.6 billion years ago with a giant molecular cloud composed of dust and gas. Under the inevitable pull of gravity, this cloud began to collapse, forming a spinning disk known as the solar nebula. At the center, the pressures and temperatures rose significantly, eventually leading to the fusion of hydrogen atoms into helium - this was the birth of our sun, the central star of our solar system.

Surrounding the newborn sun, the remnants of the solar nebula began cooling and condensing into solid particles. These particles collided and stuck together, forming larger bodies in a process that set the stage for planet formation.

As the nascent solar system swirled with countless particles, the stage was set for the accretion process to take over. The fundamental force of accretion is gravity. Small dust and ice particles, drawn together by their mutual gravitation, began to collide and coalesce, forming ever-larger clumps known as planetesimals. These planetesimals served as the construction sites for the planets, growing larger as they attracted more and more material.

Imagine a snowball rolling downhill, collecting more snow as it goes - this is similar to the accretion process in the early solar system. Over time, countless impacts and mergers of these icy and rocky bodies would lead to the creation of significant celestial objects: the protoplanets.

From planetesimals to protoplanets, the transition marks a critical phase in planetary formation. Protoplanets are larger bodies, substantial enough to be considered immature planets, that emerge from the coalescing material of planetesimals. They represent a midway point between small asteroid-sized bodies and the fully formed planets we observe today.

The journey from a planetesimal to a protoplanet is fascinating. Initially, numerous planetesimals accrete more matter, and as they grow, they begin to exert a stronger gravitational pull, which in turn allows them to draw in even more debris from their surroundings. This self-perpetuating cycle escalates until a protoplanet emerges, dominating its local region of space.

Planetary differentiation is like the maturing process of a young planet. When protoplanets reach a certain size and mass, their interiors heat up due to radioactive decay and the intense pressure from the overlying layers. This heat causes the interior to partially melt, allowing materials to move and separate based on density.

Heavier elements, such as iron, sink to the center to form the core, while lighter elements, such as silicon and oxygen, float to form the crust and mantle. This is how terrestrial planets, like Earth, ended up with a layered structure. Planetary differentiation is a definitive step towards a planet's maturity, as it leads to the diverse geological features and magnetic fields we observe today.