Earth

The third planet from a star named the Sun. The only planet in our solar system whose surface has liquid water, covering over 70% of the surface and dividing it into four oceans and seven continents. There is a relatively thick atmosphere composed of about 2/3 Nitrogen, 1/3 Oxygen, plus trace other elements.

The interior of the Earth is, like the other terrestrial planets, divided into an outer silicaceous solid mantle and an inner nickel-iron core. Our planet is big enough to have the core differentiated into an liquid outer core, which gives rise to a weak magnetic field, and a solid inner core.

The Earth is unusual in its solar system in having a terrestrial planet-like Moon that is about 1/4 of the Earth's diameter, usually called simply "the Moon," though satellites orbiting other planets are also called "moons." By coincidence, the Moon is just far enough away to have, when seen from the Earth, the same apparent angular size as the Sun. The origin of the Moon is presently unknown, but one popular theory has it that it was formed from the collision of a Mars-sized planet into the early Earth, which explains for instance its lack of iron.

The Earth is the only known place in the universe that supports life, notably intelligent life including humans and to a lesser extent apes, dolphins, and maybe a few others. Scientists have not ruled the existence of life in other places in the universe, and some think it likely.