The Earth is the third planet from he Sun and the largest of the terresrial planets in our solar system in diameter, mass and density. It is also referred to as the World and Terra. It is home to millions of species, including humans, and is estimated to have formed some 4.5 billions years ago. Its atmosphere and other environmental factors enable the proliferation of the breathing organisms as well as the formation of the ozone layer, which blocks harmful radiation, permitting life on land. There are five principal parts of the earth system. Each part has its own collection of materials and processes. However, the parts do not operate on their own. They all interact in many ways to contribute to and sustain the whole. The atmosphere extends up from the earth surface for several hundred kilometers. The lowest part is home to clouds and weather. The biosphere is all living things, from single-celled bacteria to plants and animals. The geosphere includes all minerals, rocks, sediments and soils. The hydrosphere includes the ocean, rivers, lakes, streams, groundwater and even puddles. The cryosphere is the frozen part of the earth system and includes snow, glacier and sea ice.