Planet Earth
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. Home to myriad species including humans, it is also referred to as "the Earth", "Planet Earth", "Gaia", "Terra", and "the World".
The Earth is the first planet known to have liquid water on the surface and is the only place in the universe known to harbor life. Earth has a magnetic field that, together with a primarily nitrogen-oxygen atmosphere, protects the surface from radiation that is harmful to life. The atmosphere also serves as a shield that causes smaller meteors to burn up before they strike the surface.
The Earth formed around 4.57 billion years ago and its only known natural satellite, the Moon, began orbiting it around 4.53 billion years ago. At present, the Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis (which is equal to 365.26 solar days). The Earth's axis of rotation is tilted 23.5° (away from the perpendicular to its orbital plane), producing seasonal variations on the planet's surface.
Atmospheric conditions on Earth have been significantly altered by the presence of life forms, which create an ecological balance that modifies the surface conditions. About 71% of the surface is covered with salt-water oceans. The remaining 29% consists of continents and islands. The planet's outer surface is divided into several rigid segments, or tectonic plates, that gradually migrate across the surface over periods of many millions of years. Earth'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.
Earth interacts with outer space to a significant degree. Its relatively large moon provides ocean tides, stabilizes the axial tilt and has gradually modified the length of the planet's rotation period. A cometary bombardment during the early history of the planet played a role in the formation of the oceans. Later, asteroid impacts caused significant changes to the surface environment. Long term periodic changes in the orbit of the planet are believed to have caused the ice ages that have covered significant portions of the surface in glacial sheets.
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 with about 10% of the Earth's mass, known as Theia, 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.
Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice delivered by comets, produced the oceans. The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago, and half a billion years later, the last common ancestor of all life existed.
The development of photosynthesis allowed the sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and resulted in a layer of ozone (a form of molecular oxygen [O3]) in the upper atmosphere. The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes. True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.
As the surface continually reshaped itself, over hundreds of millions of years, continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago (mya), the earliest known supercontinent Rodinia, began to break apart. The continents later recombined to form Pannotia, 600540 mya, then finally Pangaea, which broke apart 180 mya.
Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.
Following the Cambrian explosion, about 535 mya, there have been five mass extinctions. The last extinction event occurred 65 mya, when a meteorite collision probably triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared small animals such as mammals, which then resembled shrews. Over the past 65 mya, mammalian life has diversified, and several mya, an African ape-like animal gained the ability to stand upright. This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had, affecting both the nature and quantity of other life forms. The present pattern of ice ages began about 40 mya, then intensified during the Pleistocene about 3 mya. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40100,000 years. The last ice age ended 10,000 years ago.
Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant such as Jupiter. It is the largest of the four solar terrestrial planets, both in terms of size and total mass. Of these four planets, Earth also has the highest density, the highest surface gravity and the strongest magnetic field.
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Earth's biogeochemical cycles, once in concert, falling out of sync PhysOrg - August 4, 2009
Comet Dust Older Than the Sun Found in Earth's Atmosphere National Geographic - April 28, 2009
Earth's crust melts easier than previously thought PhysOrg - March 18, 2009
Quebec: Oldest Rocks on Earth Found Live Science - September 25, 2008
Earth's Cries Recorded in Space Space.com - July 1, 2008
Diamonds hint at 'earliest life' on Earth BBC - July 2, 2008
Earth's Core, Magnetic Field Changing Fast, Study Says National Geographic - July 1, 2008
History of Ancient Supercontinent's Breakup Detailed Live Science - April 29, 2008
In the News ...
How Earth Survived Its Birth: New Simulation Reveals Planet Migration Prevents Plunge Into Sun Science Daily - January 8, 2010
How Earth avoided falling into the sun MSNBC - January 7, 2010
Earth emits an ear-piercing series of chirps and whistles that could be
heard by any aliens who might be listening, astronomers have discovered.
Tiny slivers of diamond forged on an infant Earth may contain the earliest
traces of life, a study has shown. Analysis of the crystals showed they
Rapid changes in the churning movement of Earth's liquid outer core are
weakening the magnetic field in some regions of the planet's surface.
Electrified Deep Earth Changing Length of Day National Geographic - April 3, 2008
Top 10 Questions About Earth Live Science - March 14, 2008
Researchers confirm discovery of Earth's inner, innermost core Science Daily - March 11, 2008
Fossil Hunt Finds Warning for Warming Earths Live Science - March 7, 2008
New Discovery Of 'Old Growth' Crystals Provides New Record Of Planetary Evolution Science Daily - March 5, 2008
Earth's final sunset predicted MSNBC - February 26, 2008
New calculation predicts planet's destruction in 7.6 billion years
The Expanding Earth Debate - Part One Thunderbolts.com - February 21, 2008
The Expanding Earth Debate - Part Two Thunderbolts.com - February 22, 2008
The Expanding Earth Debate - Part Three Thunderbolts.com - February 26, 2008
NASA Releases New Views of Earth Live Science - October 10, 2007
Early Earth Was Purple, Study Suggests Live Science - April 11, 2007
Oldest Known Ocean Crust Found on Greenland National Geographic - March 26, 2007
Scientists have discovered a 3.8-billion-year-old rock formation in Greenland that they say is the earliest example of oceanic crust ever to be discovered.
Sea floor records ancient Earth BBC - March 26, 2007
A sliver of four-billion-year-old sea floor has offered a glimpse into the inner workings of an adolescent Earth. The baked and twisted rocks, now part of Greenland, show the earliest evidence of plate tectonics, colossal movements of the planet's outer shell. Until now, researchers were unable to say when the process, which explains how oceans and continents form, began. Plate tectonics is a geological theory used to explain the observed large-scale motions of the Earth's surface.
Scientists probe 'hole in Earth' in the mid-Atlantic BBC - March 1, 2007
The hole in the crust is midway between the Cape Verde Islands and the Caribbean, on the Mid-Atlantic Ridge.
Mission to Study Earth's Gaping 'Open Wound' Live Science - March 1, 2007
A team of scientists will embark on a voyage next week to study an 'open wound' on the
Atlantic seafloor where the Earthıs deep interior lies exposed without any crust covering.
Earth's hum linked to coastal waves New Scientist - February 15, 2007
New data shakes accepted models of collisions of the Earth's crust PhysOrg - February 7, 2007
Ancient Rocks Show How Young Earth Avoided Becoming Giant Snowball Science Daily - February 6, 2007
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