Early Theories

The story of how the Earth came to be is one of the oldest questions humans have asked. Before modern science had firm answers, philosophers and scientists built theories from the best observations available to them. These early attempts, though later revised or replaced, laid the groundwork for everything we now understand about planetary formation.

The first major and widely popular argument came from the German philosopher Immanuel Kant. His idea was later revised in 1796 by the mathematician Laplace, and together their work became known as the Nebular Hypothesis.

The Nebular Hypothesis proposed that the planets formed out of a cloud of material that surrounded a youthful, slowly rotating sun. In essence, the sun and its planets were seen as coming from the same original cloud of gas and dust, with the planets condensing out of the material circling the young star.

Much later, in 1950, two scientists working independently — Otto Schmidt in Russia and Carl Weizsäcker in Germany — revised the Nebular Hypothesis. Though they differed in details, their core idea was similar. They suggested that the sun was surrounded by a solar nebula, a vast cloud made mostly of hydrogen and helium, along with fine particles that could be called dust.

Within this cloud, the dust and gas particles did not stay still. They moved, collided, and experienced friction with one another. Over time, these collisions and the resulting friction caused the material to flatten out into a disk-shaped cloud. It was from this rotating disk that the planets eventually took shape.

The key process here is accretion — the gradual sticking together of smaller particles into larger and larger bodies. Through accretion, the scattered dust and gas of the disk slowly clumped together to form the planets we know today. …