5 June 2014
Earth, also known as “the Earth” and “the World” and sometimes referred to as the “Blue Planet”, the “Blue Marble”, Terra or “Gaia”, is the third-closest planet to the Sun, the densest planet in the Solar System, the largest of the Solar System’s four terrestrial planets and the only celestial body known to accommodate life. It is home to millions of species, including a global population of humans, that are supported and nourished by its biosphere and minerals. The human population is grouped into around two-hundred independent sovereign states that interact, among other means, through diplomacy, conflict, travel, trade and media.
According to evidence from sources such as radiometric dating, Earth was formed around four and a half billion years ago. Within its first billion years, life appeared in its oceans and began to affect its atmosphere and surface, promoting the proliferation of aerobic as well as anaerobic organisms and causing the formation of the atmosphere’s ozone layer. This layer and Earth’s magnetic field block the most life-threatening parts of the Sun’s radiation, so life was able to flourish on land as well as in water. Since then, Earth’s position in the Solar System, its physical properties and its geological history have allowed life to persist.