Big Bang

  The Big Bang is one theory that is called the "beginning of Universe". The Big Bang theory developed from observations of the structure of the Universe and from theoretical considerations. In 1912 Vesto Slipher measured the first Doppler shift of a "spiral nebula" (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were "island universes" outside our Milky Way.

  Ten years later, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann equations from Albert Einstein's equations of general relativity, showing that the Universe might be expanding in contrast to the static Universe model advocated by Einstein at that time.  In 1924, Edwin Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Independently deriving Friedmann's equations in 1927, Georges Lemaître, a Belgian physicist and Roman Catholic priest, proposed that the inferred recession of the nebulae was due to the expansion of the Universe.

    In 1931 Lemaître went further and suggested that the evident expansion in forward time required that the Universe contracted backwards in time, and would continue to do so until it could contract no further, bringing all the mass of the Universe into a single point, a "primeval atom" where and when the fabric of time and space comes into existence.

   Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder, using the 100-inch (2,500 mm) Hooker telescope at Mount Wilson Observatory. This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recession velocity—now known as Hubble's law. Lemaître had already shown that this was expected, given the Cosmological Principle. During the 1930s other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the Milne model, the oscillatory Universe (originally suggested by Friedmann, but advocated by Albert Einstein and Richard Tolman) and Fritz Zwicky's tired light hypothesis.

   Two distinct possibilities emerged. One was Fred Hoyle's steady state model, whereby new matter would be created as the Universe seemed to expand. In this model, the Universe is roughly the same at any point in time. The other was Lemaître's Big Bang theory, advocated and developed by George Gamow, who introduced big bang nucleosynthesis (BBN) and whose associates, Ralph Alpher and Robert Herman, predicted the cosmic microwave background radiation (CMB).

   The discovery and confirmation of the cosmic microwave background radiation in 1964 secured the Big Bang as the best theory of the origin and evolution of the cosmos. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding the physics of the Universe at earlier and earlier times, and reconciling observations with the basic theory.

   Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity is debated—certainly not earlier than the Planck epoch. The early hot, dense phase is itself referred to as "the Big Bang", and is considered the "birth" of our Universe. Based on measurements of the expansion using Type Ia supernovae, measurements of temperature fluctuations in the cosmic microwave background, and measurements of the correlation function of galaxies, the Universe has a calculated age of 13.73 ± 0.12 billion years. The agreement of these three independent measurements strongly supports the ΛCDM model that describes in detail the contents of the Universe.

   The earliest phases of the Big Bang are subject to much speculation. In the most common models, the Universe was filled homogeneously and isotropically with an incredibly high energy density, huge temperatures and pressures, and was very rapidly expanding and cooling. Approximately 10−37 seconds into the expansion, a phase transition caused a cosmic inflation, during which the Universe grew exponentially. After inflation stopped, the Universe consisted of a quark–gluon plasma, as well as all other elementary particles. Temperatures were so high that the random motions of particles were at relativistic speeds, and particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point an unknown reaction called baryogenesis violated the conservation of baryon number, leading to a very small excess of quarks and leptons over antiquarks and antileptons—of the order of one part in 30 million. This resulted in the predominance of matter over antimatter in the present Universe.

  The Universe continued to grow in size and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form. After about 10−11 seconds, the picture becomes less speculative, since particle energies drop to values that can be attained in particle physics experiments. At about 10−6 seconds, quarks and gluons combined to form baryons such as protons and neutrons. The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. 

  The temperature was now no longer high enough to create new proton–antiproton pairs (similarly for neutrons–antineutrons), so a mass annihilation immediately followed, leaving just one in 1010 of the original protons and neutrons, and none of their antiparticles. A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the Universe was dominated by photons (with a minor contribution from neutrinos).

  A few minutes into the expansion, when the temperature was about a billion (one thousand million; 109; SI prefix giga-) kelvins and the density was about that of air, neutrons combined with protons to form the Universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis. Most protons remained uncombined as hydrogen nuclei. As the Universe cooled, the rest mass energy density of matter came to gravitationally dominate that of the photon radiation. After about 379,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is known as the cosmic microwave background radiation.

  Over a long period of time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the Universe. The three possible types of matter are known as cold dark matter, hot dark matter and baryonic matter. The best measurements available (from WMAP) show that the dominant form of matter in the Universe is cold dark matter. The other two types of matter make up less than 18% of the matter in the Universe.

  Independent lines of evidence from Type Ia supernovae and the CMB imply the Universe today is dominated by a mysterious form of energy known as dark energy, which apparently permeates all of space. The observations suggest 72% of the total energy density of today's Universe is in this form. When the Universe was very young, it was likely infused with dark energy, but with less space and everything closer together, gravity had the upper hand, and it was slowly braking the expansion. But eventually, after numerous billion years of expansion, the growing abundance of dark energy caused the expansion of the Universe to slowly begin to accelerate. Dark energy in its simplest formulation takes the form of the cosmological constant term in Einstein's field equations of general relativity, but its composition and mechanism are unknown and, more generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both observationally and theoretically.

  All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by the ΛCDM model of cosmology, which uses the independent frameworks of quantum mechanics and Einstein's General Relativity. As noted above, there is no well-supported model describing the action prior to 10−15 seconds or so. Apparently a new unified theory of quantum gravitation is needed to break this barrier. Understanding this earliest of eras in the history of the Universe is currently one of the greatest unsolved problems in physics.

  Using the Big Bang model it is possible to calculate the concentration of helium-4, helium-3, deuterium and lithium-7 in the Universe as ratios to the amount of ordinary hydrogen, H. All the abundances depend on a single parameter, the ratio of photons to baryons, which itself can be calculated independently from the detailed structure of CMB fluctuations. The ratios predicted (by mass, not by number) are about 0.25 for 4He/H, about 10−3 for 2H/H, about 10−4 for 3He/H and about 10−9 for 7Li/H.

  The measured abundances all agree at least roughly with those predicted from a single value of the baryon-to-photon ratio. The agreement is excellent for deuterium, close but formally discrepant for 4He, and a factor of two off for 7Li; in the latter two cases there are substantial systematic uncertainties. Nonetheless, the general consistency with abundances predicted by BBN is strong evidence for the Big Bang, as the theory is the only known explanation for the relative abundances of light elements, and it is virtually impossible to "tune" the Big Bang to produce much more or less than 20–30% helium. Indeed there is no obvious reason outside of the Big Bang that, for example, the young Universe (i.e., before star formation, as determined by studying matter supposedly free of stellar nucleosynthesis products) should have more helium than deuterium or more deuterium than 3He, and in constant ratios, too.

  During the first few days of the Universe, the Universe was in full thermal equilibrium, with photons being continually emitted and absorbed, giving the radiation a blackbody spectrum. As the Universe expanded, it cooled to a temperature at which photons could no longer be created or destroyed. The temperature was still high enough for electrons and nuclei to remain unbound, however, and photons were constantly "reflected" from these free electrons through a process called Thomson scattering. Because of this repeated scattering, the early Universe was opaque to light.

  When the temperature fell to a few thousand Kelvin, electrons and nuclei began to combine to form atoms, a process known as recombination. Since photons scatter infrequently from neutral atoms, radiation decoupled from matter when nearly all the electrons had recombined, at the epoch of last scattering, 379,000 years after the Big Bang.

   Observations of the morphology and distribution of galaxies and quasars provide strong evidence for the Big Bang. A combination of observations and theory suggest that the first quasars and galaxies formed about a billion years after the Big Bang, and since then larger structures have been forming, such as galaxy clusters and superclusters. Populations of stars have been aging and evolving, so that distant galaxies (which are observed as they were in the early Universe) appear very different from nearby galaxies (observed in a more recent state). Moreover, galaxies that formed relatively recently appear markedly different from galaxies formed at similar distances but shortly after the Big Bang. These observations are strong arguments against the steady-state model.  

   Before observations of dark energy, cosmologists considered two scenarios for the future of the Universe. If the mass density of the Universe were greater than the critical density, then the Universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state that was similar to that in which it started—a Big Crunch. Alternatively, if the density in the Universe were equal to or below the critical density, the expansion would slow down, but never stop. Star formation would cease as all the interstellar gas in each galaxy is consumed; stars would burn out leaving white dwarfs, neutron stars, and black holes. Very gradually, collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the Universe would asymptotically approach absolute zero—a Big Freeze. Moreover, if the proton were unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate by emitting Hawking radiation. The entropy of the Universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death.

  Modern observations of accelerated expansion imply that more and more of the currently visible Universe will pass beyond our event horizon and out of contact with us. The eventual result is not known. The ΛCDM model of the Universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, would remain together, and they too would be subject to heat death, as the Universe expands and cools. Other explanations of dark energy—so-called phantom energy theories—suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei and matter itself will be torn apart by the ever-increasing expansion in a so-called Big Rip.

Milky Way Galaxy

  The Milky Way,  the Galaxy, is the galaxy in which the Solar System is located. It is a barred spiral galaxy that is part of the Local Group of galaxies. It is one of billions of galaxies in the observable universe. Its name is a translation of the Latin Via Lactea, in turn translated from the Greek Γαλαξίας (Galaxias), referring to the pale band of light formed by stars in the galactic plane.

All the stars that the eye can distinguish in the night sky are part of the Milky Way Galaxy, but aside from these relatively nearby stars, the galaxy appears as a hazy band of white light arching around the entire celestial sphere. The light originates from stars and other material that lie within the galactic plane. Dark regions within the band, such as the Great Rift and the Coalsack, correspond to areas where light from distant stars is blocked by dark nebulae. The Milky Way has a relatively low surface brightness due to the interstellar medium that fills the galactic disk, which prevents us from seeing the bright galactic center. It is thus difficult to see from any urban or suburban location suffering from light pollution.

   The center of the galaxy lies in the direction of Sagittarius, and it is here that Milky Way looks brightest. From Sagittarius, the Milky Way appears to pass westward through the constellations of Scorpius, Ara, Norma, Triangulum Australe, Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis Major, Monoceros, Orion and Gemini, Taurus, Auriga, Perseus, Andromeda, Cassiopeia, Cepheus and Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Ophiuchus, Scutum, and back to Sagittarius. The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that the Solar System lies close to the galactic plane.

    The galactic plane is inclined by about 60 degrees to the ecliptic (the plane of the Earth's orbit). Relative to the celestial equator, it passes as far north as the constellation of Cassiopeia and as far south as the constellation of Crux, indicating the high inclination of Earth's equatorial plane and the plane of the ecliptic relative to the galactic plane. In the Galactic Coordinate System (in which the equator corresponds with the galactic plane), the north galactic pole is situated at right ascension 12h 49m, declination +27.4° (B1950) near beta Comae Berenices, and the south galactic pole is near alpha Sculptoris.

   The stellar disk of the Milky Way Galaxy is approximately 100,000 light-years (9×1017 km) (6×1017 mi) in diameter, and is considered to be, on average, about 1,000 ly (9×1015 km) thick.[1] It is estimated to contain at least 200 billion stars and possibly up to 400 billion stars, the exact figure depending on the number of very low-mass stars, which is highly uncertain. This can be compared to the one trillion (1012) stars of the neighbouring Andromeda Galaxy. The stellar disc does not have a sharp edge, a radius beyond which there are no stars. Rather, the number of stars drops smoothly with distance from the centre of the Galaxy. Beyond a radius of roughly 40,000 light-years (4×1017 km) the number of stars drops much faster with radius, for reasons that are not understood.

   Extending beyond the stellar disk is a much thicker disk of gas. Recent observations indicate that the gaseous disk of the Milky Way has a thickness of around 12,000 ly (1×1017 km)—twice the previously accepted value. As a guide to the relative physical scale of the Milky Way, if it were reduced to 10m in diameter, the Solar System, including the hypothesized Oort cloud, would be no more than 0.1mm in width.

  The Galactic Halo extends outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose perigalacticon is at ~180,000 ly (2×1018 km). At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds, and the objects would likely be ejected from the vicinity of the Milky Way.

  A star in the Galactic halo, HE 1523-0901, was estimated to be about 13.2 billion years old, nearly as old as the Universe. As the oldest known object in the Milky Way at that time, it placed a lower limit on the age of the Milky Way. The age of stars in the Galactic thin disk can be estimated in the same way as HE 1523-0901. Measurements of thin disk stars yield an estimate that the thin disk formed between 8.8 ± 1.7 billion years ago. This suggest that there was a hiatus of almost 5 billion years between the formation of the Galactic halo and the thin disk.

   The galaxy consists of a bar-shaped core region surrounded by a disk of gas, dust and stars forming four distinct arm structures spiralling outward in a logarithmic spiral shape (see Spiral arms). The mass distribution within the galaxy closely resembles the Sbc Hubble classification, which is a spiral galaxy with relatively loosely-wound arms. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1990s. Their suspicions were confirmed by the Spitzer Space Telescope observations in 2005 which showed the galaxy's central bar to be larger than previously suspected.

   Estimates for the mass of the Milky Way vary, depending upon the method and data used. Recent estimates at the low end have placed the mass of the Milky Way at 5.8 × 1011 solar masses (M☉), somewhat smaller than the Andromeda Galaxy. Other measurements by the Very Long Baseline Array (VLBA) have found velocities as large as 254 km/s for stars at the edge of the Milky Way, higher than the previously accepted value of 220 km/s. As the orbital velocity depends on the mass enclosed, this implies that the Milky Way is more massive, roughly equaling the mass of Andromeda Galaxy at 7 × 1011 solar masses (M☉) within 50 kiloparsecs of its center. A recent measurement of the radial velocity of halo stars finds the mass enclosed within 80 kiloparsecs is 7 × 1011 solar masses. Most of the mass of the galaxy is dark matter, which forms a dark matter halo that is spread out relatively uniformly to a distance beyond one hundred kiloparsecs from the Galactic center. The overall mass of the entire galaxy is estimated at 600–1000 billion M☉.

   This mass in baryonic matter is estimated to include 200 to 400 billion stars. Its integrated absolute visual magnitude has been estimated to be −20.9.

 The galactic disc, which bulges outward at the galactic center, has a diameter of between 70,000 and 100,000 light-years. The distance from the Sun to the galactic center is now estimated at 26,000 ± 1,400 light-years, while older estimates could put the Sun as far as 35,000 light-years from the central bulge.

   The galaxy's bar is thought to be about 27,000 light-years long, running through its center at a 44 ± 10 degree angle to the line between the Sun and the center of the galaxy. It is composed primarily of red stars, believed to be ancient (see red dwarf, red giant). The bar is surrounded by a ring called the "5-kpc ring" that contains a large fraction of the molecular hydrogen present in the galaxy, as well as most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of our own galaxy.

   Each spiral arm describes a logarithmic spiral (as do the arms of all spiral galaxies) with a pitch of approximately 12 degrees. Until recently, there were believed to be four major spiral arms which all start near the galaxy's center. Cyan 3-kpc and Perseus Arm, purple Norma and Outer arm (Along with a newly discovered extension) , green Scutum-Crux Arm , pink Carina and Sagittarius Arm .There are at least two smaller arms or spurs, including orange Orion-Cygnus arm (which contains the Sun and Solar System).

   The Sun (and therefore the Earth and the Solar System) may be found close to the inner rim of the galaxy's Orion Arm, in the Local Fluff inside the Local Bubble, and in the Gould Belt, at a distance of 7.62±0.32 kpc (~25,000±1,000 ly) from the Galactic Center.The Sun is currently 5–30 parsecs from the central plane of the galactic disc. The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years. The Sun, and thus the Solar System, is found in the galactic habitable zone.

  There are about 208 stars brighter than absolute magnitude 8.5 within 15 parsecs of the Sun, giving a density of 0.0147 such stars per cubic parsec, or 0.000424 per cubic light-year (from List of nearest bright stars). On the other hand, there are 64 known stars (of any magnitude, not counting 4 brown dwarfs) within 5 parsecs of the Sun, giving a density of 0.122 stars per cubic parsec, or 0.00352 per cubic light-year (from List of nearest stars), illustrating the fact that most stars are less bright than absolute magnitude 8.5.

  The Apex of the Sun's Way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun's galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center. The Sun's orbit around the Galaxy is expected to be roughly elliptical with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. In addition, the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. These oscillations often coincide with mass extinction periods on Earth; presumably the higher density of stars close to the galactic plane leads to more impact events.

  It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a galactic year), so it is thought to have completed 20–25 orbits during the lifetime of the Sun and 1/1250 of a revolution since the origin of humans. The orbital speed of the Solar System about the center of the Galaxy is approximately 220 km/s. At this speed, it takes around 1,400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU (astronomical unit).

   The Milky Way and the Andromeda Galaxy are a binary system of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the Local Group, itself being part of the Virgo Supercluster.

  Two smaller galaxies and a number of dwarf galaxies in the Local Group orbit the Milky Way. The largest of these is the Large Magellanic Cloud with a diameter of 20,000 light-years. It has a close companion, the Small Magellanic Cloud. The Magellanic Stream is a peculiar streamer of neutral hydrogen gas connecting these two small galaxies. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Milky Way. Some of the dwarf galaxies orbiting the Milky Way are Canis Major Dwarf (the closest), Sagittarius Dwarf Elliptical Galaxy, Ursa Minor Dwarf, Sculptor Dwarf, Sextans Dwarf, Fornax Dwarf, and Leo I Dwarf. The smallest Milky Way dwarf galaxies are only 500 light-years in diameter. These include Carina Dwarf, Draco Dwarf, and Leo II Dwarf. There may still be undetected dwarf galaxies, which are dynamically bound to the Milky Way, as well as some that have already been absorbed by the Milky Way, such as Omega Centauri. Observations through the zone of avoidance are frequently detecting new distant and nearby galaxies. Some galaxies consisting mostly of gas and dust may also have evaded detection so far.

  In the general sense, the absolute velocity of any object through space is not a meaningful question according to Einstein's special theory of relativity, which declares that there is no "preferred" inertial frame of reference in space with which to compare the object's motion. (Motion must always be specified with respect to another object.) This must be kept in mind when discussing the Galaxy's motion.

  Astronomers believe the Milky Way is moving at approximately 630 km per second relative to the local co-moving frame of reference that moves with the Hubble flow. If the Galaxy is moving at 600 km/s, Earth travels 51.84 million km per day, or more than 18.9 billion km per year, about 4.5 times its closest distance from Pluto. The Milky Way is thought to be moving in the direction of the Great Attractor. The Local Group (a cluster of gravitationally bound galaxies containing, among others, the Milky Way and the Andromeda galaxy) is part of a supercluster called the Local Supercluster, centered near the Virgo Cluster: although they are moving away from each other at 967 km/s as part of the Hubble flow, the velocity is less than would be expected given the 16.8 million pc distance due to the gravitational attraction between the Local Group and the Virgo Cluster.

  Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at around 552 km/s with respect to the photons of the CMB, toward 10.5 right ascension, -24° declination (J2000 epoch, near the center of Hydra). This motion is observed by satellites such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.

  Current measurements suggest the Andromeda Galaxy is approaching us at 100 to 140 kilometers per second. The Milky Way may collide with it in 3 to 4 billion years, depending on the importance of unknown lateral components to the galaxies' relative motion. If they collide, individual stars within the galaxies would not collide, but instead the two galaxies will merge to form a single elliptical galaxy over the course of about a billion years.

Earth II

  Earth is the third planet from the Sun, the fifth-largest and the densest 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 Terra.

  Home to millions of species including humans, Earth is the only place in the universe where life is known to exist. The planet formed 4.54 billion years ago, and life appeared on its surface within a billion years. Since then, 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. Without intervention, the planet could be expected to continue supporting life for between 0.5 to 2.3 billion  years, after which the rising luminosity and expansion of the Sun – as a result of the gradual but inexorable depletion of its hydrogen fuel – would eventually eliminate the planet's biosphere.

  Earth'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. 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 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 for every roughly 366.26 times it rotates about its axis. This is a sidereal year, which is equal to 365.26 solar days. The Earth's axis of rotation is tilted 23.4° away from the perpendicular to 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 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 is dated to 4.5672 ± 0.0006 billion years ago, and by 4.54 billion years ago (within an uncertainty of 1%)[17] 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 largely completed within 10–20 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 formed as a result of a Mars-sized object (sometimes called Theia) with about 10% of the Earth's mass impacting 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 form the Moon.

  Outgassing and volcanic activity produced the primordial atmosphere. 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, 600–540 Ma, then finally Pangaea, which broke apart 180 Ma.

  The shape of the Earth is very close to that of an oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a bulge around the equator. This bulge results from the rotation of the Earth, and causes the diameter at the equator to be 43 km larger than the pole to pole diameter. The average diameter of the reference spheroid is about 12,742 km, which is approximately 40,000 km/π, as the meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris, France.

  Local topography deviates from this idealized spheroid, though on a global scale, these deviations are very small: Earth has a tolerance of about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed in billiard balls. The largest local deviations in the rocky surface of the Earth are Mount Everest (8848 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Because of the equatorial bulge, the surface locations farthest from the center of the Earth are the summits of Mount Chimborazo in Ecuador and Huascarán in Peru.

  The mass of the Earth is approximately 5.98 × 1024 kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements.

   The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties. 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 Mohorovičić discontinuity, and the thickness of the crust varies: averaging 6 km under the oceans and 30–50 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.

  Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%). The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232. At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa.

 The mechanically rigid outer layer of the Earth, the lithosphere, is broken into pieces called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: Convergent boundaries, at which two plates come together, Divergent boundaries, at which two plates are pulled apart, and Transform boundaries, in which two plates slide past one another laterally. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries. The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates, and their motion is strongly coupled with convection patterns inside the Earth's mantle.

  The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the Solar System. The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of −10,911.4 m. The average depth of the oceans is 3,800 m, more than four times the average height of the continents.

 The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km. It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the troposphere varies with latitude, ranging between 8 km at the poles to 17 km at the equator, with some variation resulting from weather and seasonal factors.

  Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 billion years ago, forming the primarily nitrogen-oxygen atmosphere of today. This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life on Earth include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature. This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Carbon dioxide, water vapor, methane and ozone are the primary greenhouse gases in the Earth's atmosphere. Without this heat-retention effect, the average surface temperature would be −18 °C and life would likely not exist.

   The Earth's magnetic field is shaped roughly as a magnetic dipole, with the poles currently located proximate to the planet's geographic poles. According to dynamo theory, the field is generated within the molten outer core region where heat creates convection motions of conducting materials, generating electric currents. These in turn produce the Earth's magnetic field. The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This results in field reversals at irregular intervals averaging a few times every million years. The most recent reversal occurred approximately 700,000 years ago.

  The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth's atmosphere at the magnetic poles, it forms the aurora. The future of  planet Earth is not confirmed but will disapear maybe in 5 bilion years, because the Sun´s mass will increase and became a Red Giant. That is one of many possibilities.

Mercury

  Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about the axis for every two orbits. The perihelion of Mercury's orbit precesses around the Sun at an excess of 43 arcseconds per century; a phenomenon that was explained in the 20th century by Albert Einstein's General Theory of Relativity.

  The first of two spacecraft to visit the planet was Mariner 10, which mapped about 45% of the planet’s surface from 1974 to 1975. The second is the MESSENGER spacecraft, which mapped a further 30% during its flyby of January 14, 2008. MESSENGERs final flyby took place in September 2009 and it is scheduled to attain orbit around Mercury in 2011, where it will begin mapping the rest of the planet.

  Mercury is similar in appearance to the Moon: it is heavily cratered with regions of smooth plains, has no natural satellites and no substantial atmosphere. However, unlike the moon, it has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth. It is an exceptionally dense planet due to the large relative size of its core. Surface temperatures range from about 90 to 700 K (−183 °C to 427 °C, −297 °F to 801 °F), with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest. Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like the Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 km. Mercury is even smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material. Mercury's density is the second highest in the Solar System at 5.427 g/cm³, only slightly less than Earth’s density of 5.515 g/cm³. If the effect of gravitational compression were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3 g/cm³ versus Earth’s 4.4 g/cm³.

    Mercury’s density can be used to infer details of its inner structure. While the Earth’s high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not nearly as strongly compressed. Therefore, for it to have such a high density, its core must be large and rich in iron. Geologists estimate that Mercury’s core occupies about 42% of its volume; for Earth this proportion is 17%. Recent research strongly suggests Mercury has a molten core. Surrounding the core is a 500–700 km mantle consisting of silicates. Based on data from the Mariner 10 mission and Earth-based observation, Mercury’s crust is believed to be 100–300 km thick. One distinctive feature of Mercury’s surface is the presence of numerous narrow ridges, and these can extend up to several hundred kilometers. It is believed that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.

   Mercury's core has a higher iron content than that of any other major planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal-silicate ratio similar to common chondrite meteors, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass. However, early in the solar system’s history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass and several hundred kilometers across. The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component. A similar process has been proposed to explain the formation of Earth’s Moon (see giant impact theory).

   Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. The planet would initially have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between 2,500 and 3,500 K (Celsius equivalents about 273 degrees less), and possibly even as high as 10,000 K. Much of Mercury’s surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" which could have been carried away by the solar wind.

   A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material. Each hypothesis predicts a different surface composition, and two upcoming space missions, MESSENGER and BepiColombo, both aim to make observations to test them.

  Craters on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins hundreds of kilometers across. They appear in all states of degradation, from relatively fresh rayed craters to highly degraded crater remnants. Mercurian craters differ subtly from lunar craters in that the area blanketed by their ejecta is much smaller, a consequence of Mercury's stronger surface gravity. The largest known crater is Caloris Basin, with a diameter of 1,550 km. The mean surface temperature of Mercury is 442.5 K, but it ranges from 100 K to 700 K due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. The subsolar point reaches about 700 K during perihelion then drops to 550 K at aphelion. On the dark side of the planet, temperatures average 110 K. The intensity of sunlight on Mercury’s surface ranges between 4.59 and 10.61 times the solar constant (1,370 W·m−2).

   Despite the generally extremely high temperature of its surface, observations strongly suggest that ice exists on Mercury. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102 K; far lower than the global average. Water ice strongly reflects radar, and observations by the 70 m Goldstone telescope and the VLA in the early 1990s revealed that there are patches of very high radar reflection near the poles. While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.

  The icy regions are believed to contain about 1014–1015 kg of ice, and may be covered by a layer of regolith that inhibits sublimation. By comparison, the Antarctic ice sheet on Earth has a mass of about 4 × 1018 kg, and Mars' south polar cap contains about 1016 kg of water. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet’s interior or deposition by impacts of comets.

 Mercury is too small for its gravity to retain any significant atmosphere over long periods of time; however, it does have a "tenuous surface-bounded exosphere" containing hydrogen, helium, oxygen, sodium, calcium, potassium and others. This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen and helium atoms probably come from the solar wind, diffusing into Mercury’s magnetosphere before later escaping back into space. Radioactive decay of elements within Mercury’s crust is another source of helium, as well as sodium and potassium. MESSENGER found high proportions of calcium, helium, hydroxide, magnesium, oxygen, potassium, silicon and sodium. Water vapor is present, released by a combination of processes such as: comets striking its surface, sputtering creating water out of hydrogen from the solar wind and oxygen from rock, and sublimation from reservoirs of water ice in the permanently shadowed polar craters. The detection of high amounts of water-related ions like O+, OH-, and H2O+ was a surprise. Because of the quantities of these ions that were detected in Mercury's space environment, scientists surmise that these molecules were blasted from the surface or exosphere by the solar wind.

  Sodium, potassium and calcium were discovered in the atmosphere during the 1980–1990s, and are believed to result primarily from the vaporization of surface rock struck by micrometeorite impacts. In 2008 magnesium was discovered by MESSENGER probe. Studies indicate that, at times, sodium emissions are localized at points that correspond to the planet's magnetic poles. This would indicate an interaction between the magnetosphere and the planet's surface.

 Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field. According to measurements taken by Mariner 10, it is about 1.1% as strong as the Earth’s. The magnetic field strength at the Mercurian equator is about 300 nT. Like that of Earth, Mercury's magnetic field is dipolar. Unlike Earth, however, Mercury's poles are nearly aligned with the planet's spin axis. Measurements from both the Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable.

  It is likely that this magnetic field is generated by way of a dynamo effect, in a manner similar to the magnetic field of Earth. This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong tidal effects caused by the planet's high orbital eccentricity would serve to keep the core in the liquid state necessary for this dynamo effect.

  Mercury’s magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. The planet's magnetosphere, though small enough to fit within the Earth, is strong enough to trap solar wind plasma. This contributes to the space weathering of the planet's surface. Observations taken by the Mariner 10 spacecraft detected this low energy plasma in the magnetosphere of the planet's nightside. Bursts of energetic particles were detected in the planet's magnetotail, which indicates a dynamic quality to the planet's magnetosphere. Mercury’s magnetic field can be extremely "leaky." The spacecraft encountered magnetic "tornadoes" – twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space – that were up to 800 km wide or a third of the radius of the planet. These 'tornadoes' form when magnetic fields carried by the solar wind connect to Mercury's magnetic field. As the solar wind blows past Mercury's field, these joined magnetic fields are carried with it and twist up into vortex-like structures. These twisted magnetic flux tubes, technically known as flux transfer events, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface.

  The process of linking interplanetary and planetary magnetic fields, called magnetic reconnection, is common throughout the cosmos.

  Mercury has the most eccentric orbit of all the planets; its eccentricity is 0.21 with its distance from the Sun ranging from 46 to 70 million kilometers. It takes 88 days to complete an orbit. A solar day on Mercury lasts about 176 Earth days, which is about twice as long as Mercury's orbital period, roughly 88 Earth Days. As a result, a Mercury year is about 0.5 Mercury days long, and one Mercury day lasts approximately two Mercury years.

Hubble Deep Field