Earth

Earth is the third planet from the Sun and the only known world to harbor life. With a diameter of 12,742 km and a mean distance of about 150 million km from the Sun (1 Astronomical Unit), Earth occupies the 'habitable zone' where liquid water can exist on the surface. About 71% of Earth's surface is covered by water, earning it the nickname 'the Blue Planet.' It is the densest planet in the solar system and the largest of the four terrestrial planets. Earth's dynamic geology features active plate tectonics that continuously reshape the surface, creating mountains, ocean basins, and driving volcanic and seismic activity. The planet has a powerful magnetic field generated by its liquid iron outer core, which creates the magnetosphere -- a protective shield that deflects harmful solar wind and cosmic radiation. This magnetic field, along with the ozone layer in the stratosphere, is essential for sustaining life on the surface. Earth has one natural satellite, the Moon, which is unusually large relative to its host planet. The Moon stabilizes Earth's axial tilt at about 23.4 degrees, which is responsible for the seasons and contributes to the long-term stability of the climate. Earth formed about 4.54 billion years ago, and the earliest evidence of life dates back to at least 3.5 billion years ago, making our planet the only confirmed biosphere in the universe.