Today, there are eight planets orbiting the sun. The planets and the sun make up our solar system.

The planets formed a very long time ago in a cloud of gas and dust. The cloud is called a solar nebula. The cloud was spinning. A clump formed in the center where it was very hot. The clump formed the sun, the star at the center of our solar system. It happened 4,600 million years ago. The rest of our solar system formed later, a little less than 4,600 million years ago.

Spinning Space Clumps Made Planets

After the sun formed, there was some gas and dust left over. It came together in smaller clumps. The cloud was still spinning, and the clumps of matter crashed into each other. They stuck together. Some of the clumps grew into planets, including Earth. Earth formed about 4,500 million years ago.

Earth is one of the four inner planets in our solar system. Mercury, Venus, and Mars are the other inner planets. They are closest to the sun. These planets are small and rocky. They could handle the strong heat from the sun.

When it first formed, Earth did not look the same as it does now. At first, it was very hot and mostly melted rock. Over hundreds of millions of years, the planet began to cool and oceans of water formed. The planet separated into layers. The outer layer is light and solid. It is called the crust. The inner layers are heavier and only partly solid.

The Earth Forms

Earth formed in three different stages. The first stage was when clumps of gas, dust, rock, and ice come together and formed a planet. That is called accretion. Then shortly after Earth formed, a tiny planet crashed into it. This might be how the moon formed. In the last stage, many asteroids hit Earth. They held a lot of water. Some of that water stayed on Earth.

Later, a blanket of gas formed around the planet. This is called an atmosphere. The early atmosphere was made of simple gases. But there was no oxygen yet.

Volcanoes, Oceans and Life

As the planet changed, volcanoes erupted. These volcanoes gave off other gases like water and carbon dioxide. Slowly, the oceans began to take shape. Then, early life evolved in those oceans.

Around three billion years ago, bacteria formed. It made oxygen during a process called photosynthesis. Oxygen began to build up in the atmosphere. Over a few hundred million years, the atmosphere changed.

Today, about 20 percent of our atmosphere is oxygen. Life on Earth evolved to live within this atmosphere.