The most prominent scientific theory about the origin of Earth involves a spinning cloud of dust called a solar nebula. This nebula is a product of the Big Bang. Philosophers, religious scholars and scientists have lots of ideas about where the universe came from, but the most widely held scientific theory is the Big Bang Theory. According to this theory, the universe originated in an enormous explosion.
Before the Big Bang, all of the matter and energy now in the universe was contained in a singularity. A singularity is a point with an extremely high temperature and infinite density. It's also what's found at the center of a black hole. This singularity floated in a complete vacuum until it exploded, flinging gas and energy in all directions. Imagine a bomb going off inside an egg — matter moved in all directions at high speeds.
As the gas from the explosion cooled, various physical forces caused particles to stick together. As they continued to cool, they slowed down and became more organized, eventually growing into stars. This process took about a billion years.
About 5 billion years ago, some of this gas and matter became our sun. At first, it was a hot, spinning cloud of gas that also included heavier elements. As the cloud spun, it collected into a disc — that solar nebula we mentioned a few paragraphs ago. Our planet and others probably formed inside this disc. The center of the cloud continued to condense, eventually igniting and becoming a sun.
There's no concrete evidence for exactly how Earth formed within this nebula. Scientists have two main theories. Both involve accretion, or the sticking together of molecules and particles. They have the same basic idea — once the sun ignited, it blew all of the extra particles away, leaving the solar system as we know it. Our moon formed in the solar nebula as well.
At first, Earth was very hot and volcanic. A solid crust formed as the planet cooled, and impacts from asteroids and other debris caused lots of craters. As the planet continued to cool, water filled the basins that had formed in the surface, creating oceans.
Through earthquakes, volcanic eruptions and other factors, Earth's surface eventually reached the shape that we know today. Its mass provides the gravity that holds everything together, and its surface provides a place for us to live. But the whole process would not have started without the sun.
This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.