The Earth is the third planet in the Solar System. It is the largest of the terrestrial planets and is the only known planet which has life as we know it. The surface of the planet is approximately two-thirds covered in water. The atmosphere is roughly 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide, and other elements.

Earth is surrounded by a magnetic field powerful enough to prevent most of the Sun's radiation from reaching the Earth and harming the life on it. This field has been decaying at a known exponential rate, as decades of recordkeeping reveal.
