A solar nebula is a spinning disc of matter around a new star.

Image courtesy NASA

Philosophers, religious scholars and scientists have lots of ideas on the creation of the universe and the Earth. Currently, the most prevalent scientific theory, known as the Big Bang Theory, is that 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 five 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 called a solar nebula. 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 the Earth formed within this nebula. Scientists have two main theories. Both involve accretion, or the sticking together of molecules and particles:

1. Similar elements stick together. 2. Heavier particles sink toward the center of the mass. 3. Outgassing creates the planet's atmosphere.

  • Homogenous accretion: Similar elements stick together, creating a solid mass. The heat generated in this process melts the particles. The heavier elements sink to the center because of gravity, creating the Earth's sold core. Outgassing from this solid body creates the atmosphere.
  • Heterogeneous accretion: First, particles of metal stick together, creating the Earth's core. Lighter elements stick to this core as it continues to cool. The gravity of this mass attracts most of the atoms that make up the atmosphere.

1. Heavier elements collect in the nebula. 2. Lighter particles adhere to this mass. 3. The atmosphere comes from outgassing and the collection of gaseous particles.

Both of those theories use the same basic idea -- about 4.6 billion years ago, the Earth formed as particles collected within a giant disc of gas orbiting a star. Once the sun ignited, it blew all of the extra particles away, leaving the solar system as we know it. The exact process probably included both homogenous and heterogeneous accretion.

At first, the 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, the Earth's surface eventually reached the shape that we know today.