Earth (or the 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.
Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four solar terrestrial planets in size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation. It also is the only terrestrial planet with active plate tectonics.
Home to millions of species including humans, Earth is currently the only astronomical body where life is known to exist. The planet formed 4.54 billion years ago, and life appeared on its surface within a billion years.
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. The planet is expected to continue supporting life for at least another 500 million years.
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 with salt water oceans, the remainder consisting of continents and islands which together have many lakes and other sources of water contributing to the hydrosphere.
Liquid water, necessary for all known life, is not known to exist in equilibrium on any other planet's surface. 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.
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.[note 8] 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.
Both the mineral resources of the planet, as well as the products of the biosphere, contribute resources that are used to support a global human population. These inhabitants are grouped into about 200 independent sovereign states, which interact through diplomacy, travel, trade, and military action. Human cultures have developed many views of the planet, including personification as a deity, a belief in a flat Earth or in the Earth as the center of the universe, and a modern perspective of the world as an integrated environment that requires stewardship.
Scientists have been able to reconstruct detailed information about the planet's past. The earliest dated Solar System material was formed 4.5672 ± 0.0006 billion years ago, and by 4.54 billion years ago (within an uncertainty of 1%) the Earth and the other planets in the Solar System had formed out of the solar nebula - a disk-shaped mass of dust and gas left over from the formation of the Sun.
This assembly of the Earth through accretion was thus largely completed within 10Š20 million years. 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 shortly thereafter, 4.53 billion years ago.
The current consensus model for the formation of the Moon is the giant impact hypothesis, in which the Moon was created when a Mars-sized object (sometimes called Theia) with about 10% of the Earth's mass impacted the Earth in a glancing blow. In this model, some of this object's mass would have merged with the Earth and a portion would have been ejected into space, but enough material would have been sent into orbit to coalesce into the Moon.
Outgassing and volcanic activity produced the primordial atmosphere of the Earth. Condensing water vapor, augmented by ice and liquid water delivered by asteroids and the larger proto-planets, comets, and trans-Neptunian objects produced the oceans. The newly formed Sun was only 70% of its present luminosity, yet evidence shows that the early oceans remained liquid - a contradiction dubbed the faint young Sun paradox.
A combination of greenhouse gases and higher levels of solar activity served to raise the Earth's surface temperature, preventing the oceans from freezing over. By 3.5 billion years ago, the Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.
Two major models have been proposed for the rate of continental growth: steady growth to the present-day and rapid growth early in Earth history. Current research shows that the second option is most likely, with rapid initial growth of continental crust followed by a long-term steady continental area.
On time scales lasting hundreds of millions of years, the surface continually reshaped as continents formed and broke up. The continents migrated across the surface, occasionally combining to form a supercontinent. Roughly 750 million years ago (Ma), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600Š540 Ma, then finally Pangaea, which broke apart 180 Ma.
At present, Earth provides the only example of an environment that has given rise to the evolution of life. 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 formed 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.
Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 Ma, 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.
Life may have survived 'Snowball Earth' in ocean pockets BBC - December 14, 2010
Life may have survived a cataclysmic global freeze some 700 million years ago in pockets of open ocean. Researchers claim to have found evidence in Australia that turbulent seas still raged during the period, where microorganisms may have clung on for life. Conditions on what is dubbed "Snowball Earth" were so harsh that most life is thought to have perished.
Following the Cambrian explosion, about 535 Ma, there have been five major mass extinctions. The most recent such event was 65 Ma, when an asteroid impact triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared some small animals such as mammals, which then resembled shrews.
Over the past 65 million years, mammalian life has diversified, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright. This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which allowed the evolution of the human race. 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 Ma and then intensified during the Pleistocene about 3 Ma. High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating every 40Š100,000 years. The last continental glaciation ended 10,000 years ago.
The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will grow by 10% over the next 1.1 Gyr (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 loss of the planet's oceans.
The Earth's increasing surface temperature will accelerate the inorganic CO2 cycle, reducing its concentration to levels lethally low for plants (10 ppm for C4 photosynthesis) in approximately 500 million to 900 million years. The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years. After another billion years all surface water will have disappeared and the mean global temperature will reach 70 °C (158 ”F).
The Earth is expected to be effectively habitable for about another 500 million years from that point, although this may be extended up to 2.3 billion years if the nitrogen is removed from the atmosphere. Even if the Sun were eternal and stable, the continued internal cooling of the Earth would result in a loss of much of its CO2 due to reduced volcanism, and 35% of the water in the oceans would descend to the mantle due to reduced steam venting from mid-ocean ridges.
The Sun, as part of its evolution, will become a red giant in about 5 Gyr. Models predict that the Sun will expand out to about 250 times its present radius, roughly 1 AU (150,000,000 km). Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will move to an orbit 1.7 AU (250,000,000 km) from the Sun when the star reaches it maximum radius.
The planet was therefore initially expected to escape envelopment by the expanded Sun's sparse outer atmosphere, though most, if not all, remaining life would have been destroyed by the Sun's increased luminosity (peaking at about 5000 times its present level). However, a 2008 simulation indicates that Earth's orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be vaporized.
The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties, but unlike the other terrestrial planets, it has a distinct outer and inner core. The outer layer of the Earth is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle.
The crust is separated from the mantle by the Mohorovicic discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30Š50 km on the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are comprised.
Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 kilometers below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core. The inner core may rotate at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1-0.5° per year.
Geologist's discoveries resolve debate about oxygen in Earth's mantle PhysOrg - December 15, 2010
Analysis of erupted rock from Agrigan volcano in the western Pacific near Guam found it to be highly oxidized as a result of its exposure to oxygen when it formed in the Earth's mantle. When, over millions of years, seafloor rocks are transported back into the Earth's mantle at subduction zones - sites on the seafloor where tectonic plates have collided, forcing one plate beneath the other - they deliver more oxygen into the mantle.
Earth's Core, Magnetic Field Changing Fast, Study Says National Geographic - July 1, 2008
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.
Researchers confirm discovery of Earth's inner, innermost core Science Daily - March 11, 2008
Geologists at the University of Illinois have confirmed the discovery of Earth's inner, innermost core, and have created a three-dimensional model that describes the seismic anisotropy and texturing of iron crystals within the inner core.
Earth's Cries Recorded in Space Space.com - July 1, 2008
Earth emits an ear-piercing series of chirps and whistles that could be
heard by any aliens who might be listening, astronomers have discovered.
Earth's Hum Sounds More Mysterious Than Ever Live Science - April 21, 2008
Earth gives off a relentless hum of countless notes completely imperceptible to the human ear, like a giant, exceptionally quiet symphony, but the origin of this sound remains a mystery. Now unexpected powerful tunes have been discovered in this hum. These new findings could shed light on the source of this enigma. The planet emanates a constant rumble far below the limits of human hearing, even when the ground isn't shaking from an earthquake. (It does not cause the ringing in the ear linked with tinnitus.) This sound, first discovered a decade ago, is one that only scientific instruments - seismometers - can detect. Researchers call it Earth's hum.
Earth's hum linked to coastal waves New Scientist - February 15, 2007
The Earth's hum comes from the bottom of the sea and not from turbulence in the atmosphere, says a US researcher, backing a novel theory put forward in 2004. The hum is a low rumble continually present in the ground even when there are no earthquakes happening, but is detectable only by very sensitive seismometers. Its frequency is near 10 millihertz, below the range of human hearing. The Japanese researchers who first described the hum in 1998 suggested it might be caused by turbulent air in the atmosphere pounding on land.
Study debunks theory on end of 'Snowball Earth' ice age PhysOrg - May 25, 2011
A team of scientists led by researchers from Caltech report in this week's issue of the journal Nature that the rocks on which much of a theory on how the "Snowball Earth" ice age ended was based were formed millions of years after the ice age ended, and were formed at temperatures so high there could have been no living creatures associated with them.
Ice Once Covered the Equator Live Science - March 5, 2010
"Snowball Earth" Confirmed: Ice Covered Equator National Geographic - March 5, 2010
But volcanoes would've made Earth more mud ball than snowball, scientists say.
Earth's now steamy Equator was covered with ice 716 million years ago.
Violent Planet: The Forces that Shape Earth Live Science - March 5, 2010
Ancient Rocks Show How Young Earth Avoided Becoming Giant Snowball Science Daily - February 6, 2007
A greenhouse gas that has become the bane of modern society may have saved Earth from completely freezing over early in the planet's history, according to the first detailed laboratory analysis of the world's oldest sedimentary rocks.
Snowball Earth Wikipedia
Humble moss helped to cool Earth and spurred on life BBC - February 2, 2012
Primitive moss-like plants could have triggered the cooling of the Earth some 470 million years ago, say researchers. A study published in Nature Geoscience may help explain why temperatures gradually began to fall, culminating in a series of "mini ice ages". Until now it had been thought that the process of global cooling began 100 million years later, when larger plants and trees emerged. The simple plants' interactions with rocks are believed to be the cause. The humble moss has created the climate which we enjoy today.
Infant, Magma-Ball Earth Glimpsed Via Newfound Rocks National Geographic - August 12, 2010
Canada's Baffin Island, home to perhaps the world's oldest rocks.
Arctic rocks may contain oldest remnants of Earth BBC - August 11, 2010
Scientists have found Arctic rocks that may preserve the earliest remnants of Earth. Over billions of years, much of the material that made up the early Earth was modified by processes such as melting and mixing. But the Arctic rocks seem to contain chemical signatures that date from just after the Earth's violent origin.
Silver tells a volatile story of Earth's origin: Water was present during its birth PhysOrg - May 13, 2010
Tiny variations in the isotopic composition of silver in meteorites and Earth rocks are helping scientists put together a timetable of how our planet was assembled beginning 4.568 billion years ago. The new study indicates that water and other key volatiles may have been present in at least some of Earth's original building blocks, rather than acquired later from comets, as some scientists have suggested.
Quantum mechanics reveals new details of deep earth PhysOrg - May 11, 2010
Scientists have used quantum mechanics to reveal that the most common mineral on Earth, silica, is relatively uncommon deep within the planet.
'Goldilocks Zone' may go colder than previously thought PhysOrg - April 20, 2010
The survival of life on Earth is possible only within a relatively narrow temperature range known as the "Goldilocks Zone," which ranges from around 0 to 100”C. In many ecosystems ....
Goldilocks Planet Wikipedia
Goldilocks Planet Google Videos
Chile Earthquake Altered Earth Axis, Shortened Day National Geographic - March 2, 2010
Chilean Quake May Have Shortened Earth Days NASA - March 2, 2010
The Earth has its own set of rules PhysOrg - March 3, 2010
Early in our history it didn't make any difference how we viewed our environment. We could change it, and if we didn't like what we did to it, we could move and natural processes would soon obliterate whatever we had done. Over the years, models of our relationship to the environment have been based on religious views, with the world provided for us to dominate and subdue as described in Genesis, and philosophical views, seeing wisdom and virtue in nature as described by Thoreau.
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
In short, temperature differences in the space around the sun, 4.6 billion years ago, caused Earth to migrate outward as much as gravity was trying to pull it inward, and so the fledgling world found equilibrium in what we now know to be a very habitable orbit.
Comet Dust Older Than the Sun Found in Earth's Atmosphere National Geographic - April 28, 2009
Meteorites on the ground or icy comets millions of miles away are usually the only sources of ancient matter from the early days of the solar system. But in a high-flying experiment, researchers have used a sort of chemical flypaper to scoop up comet dust from Earth's atmosphere - and it appears to be some of the oldest matter in our cosmic neighborhood.
Earth's crust melts easier than previously thought PhysOrg - March 18, 2009
In the study, researchers measured how well rocks conduct heat at different temperatures and found that as rocks get hotter in the Earth's crust, they become better insulators and poorer conductors. This finding provides insight into how magmas are formed and will lead to better models of continental collision and the formation of mountain belts.
Quebec: Oldest Rocks on Earth Found Live Science - September 25, 2008
Scientists have found the oldest known rocks on Earth. They are 4.28 billion years old, making them 250 million years more ancient than any previously discovered rocks. Earth formed about 4.6 billion years ago from a disk of gas and dust circling the sun. Remnants of crust from Earth's infancy are hard to come by because most of that material has been recycled into Earth's interior several times by the plate tectonics that continue to shape our planet's surface.
Diamonds hint at 'earliest life' on Earth BBC - July 2, 2008
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
History of Ancient Supercontinent's Breakup Detailed Live Science - April 29, 2008
The breakup of the supercontinent Gondwana eventually formed the continents in the Southern Hemisphere. Exactly how this happened has been debated by geologists for years. Most theories say Gondwana broke into many different pieces, but new research suggests the large land mass simply split in two.
New Discovery Of 'Old Growth' Crystals Provides New Record Of Planetary Evolution Science Daily - March 5, 2008
Three-billion-year-old zircon microcrystals found in northern Ontario are proving to be a new record of the processes that form continents and their natural resources, including gold and diamonds.
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
Early Earth Was Purple, Study Suggests Live Science - April 11, 2007
The earliest life on Earth might have been just as purple as it is green today, a scientist claims. Ancient microbes might have used a molecule other than chlorophyll to harness the Sun's rays, one that gave the organisms a violet hue. Chlorophyll, the main photosynthetic pigment of plants, absorbs mainly blue and red wavelengths from the Sun and reflects green ones, and it is this reflected light that gives plants their leafy color. This fact puzzles some biologists because the sun transmits most of its energy in the green part of the visible spectrum.
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. Usually the plates are pulled apart and to fill the gap the mantle underneath has to rise up. As it comes up it starts to melt. That forms the magma. That's the normal process. Here it has gone awry for some reason. The crust does not seem to be repairing itself.
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 EarthÕs deep interior lies exposed without any crust covering. There are two popular hypotheses about how these holes in the EarthÕs crust form.
EARTH'S MOON
EARTH'S MAGNETIC FIELD
EARTHQUAKES
VOLCANOES
PROPHECIES