Earth formed around 4.54 billion years ago and is the third planet from the Sun. It is the fifth largest planet in the solar system and the largest of the terrestrial planets. It is also the densest of all the planets.
Earth orbits around the Sun once every 365.25 days and rotates itself with respect to the Sun once every 24 hours, causing the day and night cycle. The 24-hour cycle is the fastest among the terrestrial planets, but much slower than the gas planets. When the Earth rotates, it does so at a slight tilt, creating a different amount of sunlight in the northern hemisphere than in the southern hemisphere.
Earth has a core, a mantle and a crust. The core, or the center, has two layers: the inner layer consisting of solid nickel and iron and the outer layer consisting of molten nickel and iron. Solid silicate rock makes up the mantle, and silicate rock of varying densities makes up the crust.
Water makes up 70 percent of the Earth's surface and contained Earth's first life. The composition of Earth's atmosphere features 78 percent nitrogen, 21 percent oxygen and small amounts of other gases such as argon and carbon dioxide.