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The Earth is very old -- 4.5 billion years or more according to recent estimates. Most of the evidence for an ancient Earth is contained in the rocks that form the Earth's crust. The rock layers themselves -- like pages in a long and complicated history -- record the surface-shaping events of the past, and buried within them are traces of life --the plants and animals that evolved from organic structures that existed perhaps 3 billion years ago. Also contained in rocks once molten are radioactive elements whose isotopes provide Earth with an atomic clock. Within these rocks, "parent" isotopes decay at a predictable rate to form "daughter" isotopes. By determining the relative amounts of parent and daughter isotopes, the age of these rocks can be calculated.
Earth is the third planet from the Sun , and the densest and fifth-largest of the eight planets in the Solar System . It is also the largest of the Solar System's four terrestrial planets . It is sometimes referred to as the world or the Blue Planet. [ 22 ] Earth formed approximately 4.54 billion years ago, and life appeared on its surface within one billion years. [ 23 ] Earth's biosphere then significantly altered the atmospheric and other basic physical conditions, which enabled the proliferation of organisms as well as the formation of the ozone layer , which together with Earth's magnetic field blocked harmful solar radiation , and permitted formerly ocean-confined life to move safely to land. [ 24 ] The physical properties of the Earth , as well as its geological history and orbit, have allowed life to persist.