Earth

Earth is the third closest planet to the Sun in the solar system. It is classified as a rocky "terrestrial planet" in the solar system, and is the largest in both size and mass. Meteorite dating using radioactive elements has estimated that it was formed about 4.6 billion years ago. Since the age of the solar system is also based on meteorite dating, it is safe to say that Earth was formed almost simultaneously with the birth of the solar system. It is believed to have been formed by the collision and merging of about 10 Mars-sized planets (protoplanets).

Its composition varies with depth from the surface. Near the surface, it is mainly composed of oxygen and silicon, with other metallic elements such as aluminum, iron, calcium, sodium, potassium, and magnesium. Most of these exist in the form of oxides. In contrast, the central part is mainly composed of iron and nickel. 70% of the surface is covered by liquid water (oceans), and from the surface to about 100 km above the surface there is an atmosphere composed mainly of nitrogen and oxygen. The composition of the atmosphere changes with altitude.