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Earth is the third planet from the Sun and is the largest of the terrestrial planets in the Solar System, in both diameter and mass. It is also referred to as "the Earth", "Planet Earth", "Gaia", "Terra", and "the World".
Scientists have been able to reconstruct detailed information about the planet's past. Earth and the other planets in the Solar System
formed 4.57 billion years ago out of the solar nebula, a disk-shaped mass of dust and gas left over from the formation of the Sun.
Initially molten, the outer layer of the planet Earth cooled to form a solid crust when water began accumulating in the atmosphere. The Moon formed
soon afterwards, possibly as the result of a Mars-sized object (sometimes called Theia with about 10% of the Earth's mass impacting the Earth in a glancing blow. Some of this object's mass
merged with the Earth and a portion was ejected into space, but enough material survived to form an orbiting moon.
A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides the (currently understood) requisite conditions of liquid water, an environment
where complex organic molecules can assemble, and sufficient energy to sustain metabolism. The distance of the Earth from the Sun, as well as
its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the conditions necessary to originate and
sustain life on this planet.
Earth has approximately 6,600,000,000 human inhabitants. Projections indicate that the world's human population will reach seven
billion in 2013 and 9.2 billion in 2050. Most of the growth is expected to take place in developing nations. Human population density varies widely around the world, but a majority live in Asia. By 2020, 60% of
the world's population is expected to be living in urban, rather than rural, areas
The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium ash at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will increase by 10 percent over the next 1.1 billion years, and by 40% over the next 3.5 Gyr.
Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the possible loss of the planet's oceans.
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