Earth is unique among all known planets, ones native to our Solar System or otherwise. It is the only planet that has liquid water on the surface and is also the only world containing life. Earth is the third planet out from the Sun and is the densest and largest of the four rocky terrestrial planets.
Formed at the same time as the rest of the Solar System the Earth would accrue enough mass to maintain an atmosphere, composed chiefly of nitrogen and carbon dioxide. It is believed that the Earth collided with another planetesimal early in its history, the remains of this collision would form the Moon. Earth's moon is its only natural satellite with its gravitational effects having great effects on the planet. The Earth was also just the right distance from the Sun for liquid water to form on the surface, filling the oceans and eventually covering 71% of the planet's surface.
Half a billion years after the formation of the planet the first self-replicating molecules were formed, and by a process of natural selection would go on to evolve into all life as we know it. The impact of life has been significant on the Earth, the oxygen in the atmosphere and the ozone layer are attributed to early plant life. The ozone layer protects the surface from many harmful rays from the Sun and has allowed colonisation of the land by multicellular organisms. Life would go on to evolve intelligence and humans. We are still the only known life in the universe, and most likely the only intelligent forms of life within millions of light years.
The Earth is almost a perfect sphere, with a slight bulge at the equator due to its rotation. The planet is mostly composed of iron, oxygen, silicon, magnesium, sulfur, nickel, calcium and aluminum. The planet is formed from a mostly iron core, a molten mantle and a thin rocky crust, much like the other rocky planets. The surface of Earth is split into several continental plates which all move around on the surface. This geological activity gives rise to earthquakes, mountains, tsunamis and volcanoes.
Similar to Mars, the Earth has polar caps with ice at the Northern and Southern tips. The Earth has a strong magnetic field, which deflects much of the solar wind and protects the Earth from the harmful radiation. The Earth is tilted on its axis, meaning that some areas are closer to the Sun that others during orbit. This tilt gives rise the seasons.
The human population of Earth is expanding at an ever increasing rate and many concerns are being raised about the treatment of the planet and how long it will be until the finite resources are consumed. A lot of human activity is now going into finding alternative power sources and environmentally friendly methods of industry. The evolution of humans has been the first time in over four billion years that one species has had such a negative impact on the well-being of the rest of the planet.
