Earth

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, the Blue Planet, or by its Latin name, Terra.

## Background

Earth formed 4.54 billion years ago, and life appeared on its surface within one billion years. The planet is home to millions of species, including humans. Earth's biosphere has significantly altered the atmosphere and other abiotic conditions on the planet, enabling the proliferation of aerobic organisms as well as the formation of the ozone layer which, together with Earth's magnetic field, blocks harmful solar radiation, permitting life on land. The physical properties of the Earth, as well as its geological history and orbit, have allowed life to persist during this period. Estimates on how much longer the planet will to be able to continue to support life range from a mere 500 million years, to as long as 2.3 billion years.

## Makeup

Earth's outer surface is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. About 71% of the surface is covered by salt water oceans, with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with solid ice (Antarctic ice sheet) or sea ice (Arctic ice cap). The planet's interior remains active, with a thick layer of relatively solid mantle, a liquid outer core that generates a magnetic field, and a solid iron inner core.

## Sibling Incompatibility

Venus, the Earth's neighboring planet, is classified as its "sister" because of its similar size and shape. They are exact opposites though. Venus is a planet of volcanic activity and the surface are filled with lava. Earth, on the other hand, is 70.6% water and can support life. Venus and Earth are not the best set of siblings as you can see.

## Atmospheric Makeup

The Earth's atmosphere is mostly made of nitrogen. In fact, more than three fourths of the atmosphere is nitrogen. Oxygen makes up just more than one-fourth of the atmosphere. The rest is made up of less than one percent argon and carbon dioxide.

The atmosphere is split into two main layers: the troposphere and the stratosphere. The **troposphere** is the closest layer of the atmosphere to us. All of the weather on Earth occurs in this layer. The **stratosphere** is the lowest layer of the mesosphere, and is where the temperature of the atmosphere begins to increase due to the absorption of ultraviolet radiation from the Sun. This layer is also rich with ozone, for it contains a portion of the ozone layer.

Other layers include the **mesosphere**, the region in the atmosphere at which the temperatures drop to below -100 degrees Fahrenheit due to radiation being reflected away from this layer into the stratosphere. This layer is where the majority of the ozone layer is found. The ozone layer is the layer created by the build up of ozone, which is a gaseous substance that absorbs most of the ultraviolet radiation from the Sun.

## Interactions

Earth interacts with other objects in space, especially the Sun and the Moon. At present, Earth orbits the Sun once every 366.26 times it rotates about its own axis, which is equal to 365.26 solar days, or one sidereal year. The Earth's axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days). Earth's only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt, and gradually slows the planet's rotation. Between approximately 3.8 billion and 4.1 billion years ago, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment.