Today, our solar system is made up of four rocky planets and four gas giant planets. Billions of years ago, the dust and gas that would become our solar system existed only as an enormous cloud. At some point, the dust cloud was disturbed. This led to the formation of life as we know it.

Scientists think a distant star blew up causing waves of force to travel through space. It's called a supernova. It compressed, or squeezed, the dust cloud until it began to shrink. The dust cloud's own gravity, a force that pulls objects together, helped the cloud to shrink. Eventually, it formed a spinning disc of gas and dust called a solar nebula. The faster the cloud spun, the more dust and gas built up in the center. This caused the nebula to spin even faster.

Over time, the gravity at the center of the cloud got even stronger, pulling in more matter. This formed the star at the center of our solar system—the sun—roughly 4.6 billion years ago. One billion is equal to a thousand million. Our solar system formed 4,600 million years ago.

Clumps of Crashing Matter

Almost all the matter in the nebula went into forming the sun. What was left, began to clump together into smaller masses. The cloud was still spinning, and the clumps of matter crashed into each other. Some of those clumps grew into the planets and dwarf planets that make up our solar system today.

Earth is one of the four inner, terrestrial planets in our solar system. Just like the other inner planets—Mercury, Venus, and Mars—it is relatively small and rocky. Early in the history of the solar system, only rocky planets could be close to the sun because of the heat.

At its beginning, Earth did not look like it does now. At first, it was very hot. The planet was mostly melted rock. Over the course of a few hundred million years, the planet began to cool and oceans of liquid water formed. Heavy elements, like iron and nickel, began sinking toward the center of the planet. As this happened, Earth separated into layers. Lighter material formed the outer layer. Denser, heavier material sank to the center of Earth.

Earth Formed in Three Stages

After the supernova, scientists define the formation of Earth in three different stages. The first stage is known as accretion. This is when particles within the solar system crash into each other and stick together. They form larger and larger bodies.

During the next stage, a protoplanet crashed into the very young planet Earth. The crash happened more than 4.5 billion years ago. It may have formed the moon.

In the final stage, the planet was hit by many asteroids. Scientists think the asteroids that slammed into Earth, the moon, and other inner planets held a lot of water. The asteroids hit at a great speed, shattered and melted. Some of the water in the asteroids stayed on Earth.

A Perfect Atmosphere

The atmosphere is the blanket of gas formed around the planet. The early atmosphere was made of simple elements like hydrogen and helium. As the planet changed, a crust formed and volcanoes erupted. These volcanoes released water vapor, ammonia, and carbon dioxide. Slowly, the oceans began to take shape. Then, early life evolved in those oceans.

Around 2.7 billion years ago, bacteria started to make oxygen during photosynthesis. Oxygen began to build up in the atmosphere. Over a few hundred million years, the atmosphere changed. Today, our atmosphere is made up of 78 percent nitrogen and 21 percent oxygen. Life on Earth evolved to thrive within this atmosphere.