Earth is the third planet from the Sun, the densest planet in the Solar System, and the largest of the four terrestrials. It is — as far as we yet know — the only world with a biosphere, surface liquid water, and tectonic recycling of its crust.

Earth is 71% covered by salt-water oceans, with a total water inventory of roughly 1.386 × 10⁹ km³. Its atmosphere is dominated by nitrogen and molecular oxygen — the latter produced almost entirely by photosynthetic life over the past ~2.4 billion years. The planet’s magnetic dipole, generated by convection in its molten iron-nickel outer core, and the stratospheric ozone layer together block the bulk of the solar wind and ultraviolet radiation that would otherwise sterilize the surface.

Earth formed around 4.54 billion years ago. Its single large natural satellite, the Moon, formed about 4.5 billion years ago from debris ejected by a Mars-sized impactor (Theia). More than 13,000 operational satellites, 8 billion humans, and every known form of life share this single world as of 2026.