Solid particles, called “planetesimals” condensed out of the cloud. They're thought to have stuck together and created the early Earth. Bombarding planetesimals heated Earth to a molten state. The unequal heating of Earth is caused by differences in the angle of the Sun's rays that strike Earth, the amount of atmosphere that the Sun's rays must pass through before striking Earth's surface, and how much of the solar energy that reaches Earth is reflected rather than absorbed.