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Solar System

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   Major features of the Solar System (not to scale): The Sun, the eight
   planets, the asteroid belt containing the dwarf planet Ceres, outermost
   there is the dwarf planet Pluto (the dwarf planet Eris not shown), and
   a comet.
   Enlarge
   Major features of the Solar System (not to scale): The Sun, the eight
   planets, the asteroid belt containing the dwarf planet Ceres, outermost
   there is the dwarf planet Pluto (the dwarf planet Eris not shown), and
   a comet.

   The Solar System or solar system comprises the Sun and the retinue of
   celestial objects gravitationally bound to it: the eight planets, their
   162 known moons, three currently identified dwarf planets and their
   four known moons, and thousands of small bodies. This last category
   includes asteroids, meteoroids, comets, and interplanetary dust.

   The principal component of the Solar System is the Sun or Sol (
   astronomical symbol ☉ ); a main sequence G2 star that contains 99.86%
   of the system's known mass and dominates it gravitationally. The Sun's
   large mass gives it an interior density high enough to sustain nuclear
   fusion, releasing enormous amounts of energy, most of which is radiated
   into space in the form of electromagnetic radiation including visible
   light. Jupiter and Saturn are the Sun's two largest orbiting bodies and
   account for more than 90% of the system's remaining mass. (The
   currently hypothetical Oort cloud would also hold a substantial
   percentage were its existence confirmed).

   In broad terms, the charted regions of the Solar System consist of the
   Sun, four rocky bodies close to it called the terrestrial planets, an
   inner belt of rocky asteroids, four gas giant planets and an outer belt
   of small icy bodies known as the Kuiper belt. In order of their
   distances from the Sun, the planets are Mercury ( ☿ ), Venus ( ♀}} ),
   Earth ( ⊕ ), Mars ( ♂ ), Jupiter ( ♃ ), Saturn ( ♄ ), Uranus ( ♅ ), and
   Neptune ( ♆ ). All planets but two are in turn orbited by natural
   satellites (usually termed "moons" after Earth's Moon) and every planet
   past the asteroid belt is encircled by planetary rings of dust and
   other particles. The planets other than Earth are named after gods and
   goddesses from Greco-Roman mythology.

   From 1930 to 2006, Pluto ( ♇ ), the largest known Kuiper belt object,
   was considered the Solar System's ninth planet. However, in 2006 the
   International Astronomical Union (IAU) created an official definition
   of the term "planet". Under this definition, Pluto is reclassified as a
   dwarf planet, and there are eight planets in the Solar System. In
   addition to Pluto, the IAU currently recognizes two other dwarf
   planets: Ceres ( Old symbol of Ceres ), the largest object in the
   asteroid belt, and Eris, which lies beyond the Kuiper belt in a region
   called the scattered disc. Of the known dwarf planets, only Ceres has
   no moons.

   For many years, the Solar System was the only known example of planets
   in orbit around a star. The discovery in recent years of many
   extrasolar planets has led to the term "solar system" being applied
   generically to all the newly discovered systems. Technically, however,
   it should strictly refer to Earth's system only, as the word " solar"
   is derived from the Sun's Latin name, Sol. Other such systems are
   usually referred to by the names of their parent star: "the Alpha
   Centauri system" or "the 51 Pegasi system".

Layout and structure

   The ecliptic viewed in sunlight from behind the Moon in this Clementine
   image. From left to right: Mercury, Mars, Saturn
   Enlarge
   The ecliptic viewed in sunlight from behind the Moon in this Clementine
   image. From left to right: Mercury, Mars, Saturn

   Most objects in orbit around the Sun lie within the ecliptic, a shallow
   plane which is roughly parallel to the Sun's equator. The planets are
   very close to the ecliptic while comets and kuiper belt objects are
   often at significant angles to it.

   All of the planets (and most other objects) also orbit with the Sun's
   rotation; in a counter-clockwise direction as viewed from a point above
   the Sun's north pole. There is a direct relationship between how far
   away a planet is from the Sun and how quickly it orbits. Mercury, the
   closest to the sun, travels the fastest, while Neptune, being much
   farther from the Sun, travels more slowly. Objects orbit in an ellipse
   around the Sun, so an orbiting object's distance from the Sun varies in
   the course of its year. Its closest approach to the Sun is known as its
   perihelion while its farthest point from the Sun is called its
   aphelion. Although the orbits of the planets are nearly circular (with
   perihelions roughly equal to their aphelions), many comets, asteroids
   and objects of the Kuiper belt follow highly elliptical orbits with
   large differences between perihelion and aphelion. The paths of objects
   around the Sun travel according to a law of planetary motion discovered
   by German astronomer Johannes Kepler in the early 1600's. The sun is
   slightly off to the side of the centre of each ellipse at a point
   called a focus. The focus is actually a point just outside the centre
   of the Sun called the barycenter of the solar system.

   Astronomers most often measure distances within the solar system in
   astronomical units or AU. One AU is the average distance between the
   Earth and the Sun or roughly 149 598 000 km (93,000,000 mi). Pluto is
   roughly 39 AU from the Sun while Jupiter lies at roughly 5.2 AU.

   Informally, the Solar System is sometimes divided into separate
   "zones"; the first zone, known as the inner Solar System, comprises the
   inner planets and the main asteroid belt. The outer solar system is
   sometimes defined as everything beyond the asteroids; however it is
   also the name often given to the region beyond Neptune with the gas
   giants as a separate "middle zone."
   The orbits of the bodies in the solar system to scale (clockwise from
   top left)
   Enlarge
   The orbits of the bodies in the solar system to scale (clockwise from
   top left)

   One common misconception is that the orbits of the major objects within
   the Solar System (planets, Pluto and asteroids) are equidistant. To
   cope with the vast distances involved, many representations of the
   Solar System simplify these orbits by showing them the same distance
   apart. However, in reality, with a few exceptions, the Solar System is
   arranged so that the farther a planet or belt is from the Sun, the
   larger the distance between it and the previous orbit. For example,
   Venus is approximately 0.33 AU farther out than Mercury while Jupiter
   is 1.9 AU from the farthest extent of the asteroid belt and Neptune's
   orbit is roughly 20 AU farther out than that of Uranus. Attempts have
   been made to determine a correlation between these distances (see
   Bode's Law) but to date there is no accepted theory that explains the
   orbital distances.

Planets, dwarf planets, and small solar system bodies

   Planets and Dwarf Planets of the solar system. While the size is to
   scale, the relative distances from the Sun are not.
   Enlarge
   Planets and Dwarf Planets of the solar system. While the size is to
   scale, the relative distances from the Sun are not.

   In a decision passed by the International Astronomical Union General
   Assembly on August 24, 2006, the objects in the Solar System other than
   the Sun and natural satellites were divided into three separate groups:
   planets, dwarf planets and small solar system bodies.

   Under this classification, a planet is any body in orbit around the Sun
   that a) has enough mass to form itself into a spherical shape and b)
   has cleared its immediate neighbourhood of all smaller objects. Eight
   objects in the Solar System currently meet this definition; they are
   Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

   Dwarf planet was a second and new classification. The key difference
   between planets and dwarf planets is that while both are required to
   orbit the Sun and be of large enough mass that their own gravity pulls
   them into a nearly round shape, dwarf planets are not required to clear
   their neighbourhood of other celestial bodies. Three objects in the
   solar system are currently included in this category; they are Pluto
   (formerly considered a planet), the asteroid Ceres, and the scattered
   disc object Eris. The IAU will begin evaluating other known objects to
   see if they fit within the definition of dwarf planets. The most likely
   candidates are some of the larger asteroids and several Trans-Neptunian
   Objects such as Sedna, Orcus, and Quaoar.

   The remainder of the objects in the Solar System were classified as
   small solar system bodies (SSSBs). As the IAU noted in its resolution:

          These currently include most of the Solar System asteroids, most
          Trans-Neptunian Objects (TNOs), comets, and other small bodies.

Formation

   Artist's conception of a protoplanetary disc
   Enlarge
   Artist's conception of a protoplanetary disc

   Using radiometric dating, scientists can estimate that the solar system
   is 4.6 billion years old. The oldest rocks on Earth are approximately
   3.9 billion years old. Rocks this old are rare, as the Earth's surface
   is constantly being reshaped by erosion, volcanism and plate tectonics.
   To estimate the age of the solar system scientists must use meteorites,
   which were formed during the early condensation of the solar nebula.
   The oldest meteorites (such as the Canyon Diablo meteorite) are found
   to have an age of 4.6 billion years, hence the solar system must be at
   least 4.6 billion years old.

   The current hypothesis of Solar System formation is the nebular
   hypothesis, first proposed in 1755 by Immanuel Kant and independently
   formulated by Pierre-Simon Laplace. The nebular theory holds that the
   Solar System was formed from the gravitational collapse of a gaseous
   cloud called the solar nebula. It had a diameter of 100  AU and was 2–3
   times the mass of the Sun. Over time, a disturbance (possibly a nearby
   supernova) squeezed the nebula, pushing matter inward until
   gravitational forces overcame the internal gas pressure and it began to
   collapse. As the nebula collapsed, conservation of angular momentum
   meant that it spun faster, and became warmer. As the competing forces
   associated with gravity, gas pressure, magnetic fields, and rotation
   acted on it, the contracting nebula began to flatten into a spinning
   protoplanetary disk with a gradually contracting protostar at the
   centre. Studies of young, pre-fusing solar mass stars, called T Tauri
   stars, show that these discs extend to several hundred AU and are
   rather cool, reaching only a thousand kelvins at their hottest.

   From this cloud and its gas and dust, the various planets formed. The
   currently accepted method by which the planets formed is known as
   accretion, in which the planets began as dust grains in orbit around
   the central protostar, which initially formed by direct contact into
   clumps between one and ten kilometres in diameter, which in turn
   collided to form larger bodies ( planetesimals), of roughly 5 km in
   size gradually increasing by further collisions by roughly 15 cm per
   year over the course of the next few million years.

   The inner solar system was too warm for volatile molecules like water
   and methane to condense, and so the planetesimals which formed there
   were relatively small (comprising only 0.6% the mass of the disc) and
   composed largely of compounds with high melting points, such as
   silicates and metals. These rocky bodies eventually became the
   terrestrial planets. Farther out, the gravitational effects of Jupiter
   made it impossible for the protoplanetary objects present to come
   together, leaving behind the asteroid belt.

   Farther out still, beyond the frost line, where more volatile icy
   compounds could remain solid, Jupiter and Saturn were able to gather
   more material than the terrestrial planets, as those compounds were
   more common. They became the gas giants, while Uranus and Neptune
   captured much less material and are known as ice giants because their
   cores are believed to be made mostly of ices (hydrogen compounds).

   After 100 million years, the pressure and density of hydrogen in the
   centre of the collapsing nebula became great enough for the protosun to
   begin thermonuclear fusion, which increased until hydrostatic
   equilibrium was achieved.

   The young Sun's solar wind then cleared away all the gas and dust in
   the protoplanetary disk, blowing it into interstellar space, thus
   ending the growth of the planets.

Sun

   The Sun as seen from Earth.
   Enlarge
   The Sun as seen from Earth.

   The Sun is the Solar System's parent star, and far and away its chief
   component. It is classed as a moderately large yellow dwarf. However,
   this name is misleading, as on the scale of stars in our galaxy, the
   Sun is rather large and bright. Stars are classified based on their
   position on the Hertzsprung-Russell diagram, a graph which plots the
   brightness of stars against their surface temperatures. Generally
   speaking, the hotter a star is, the brighter it is. Stars which follow
   this pattern are said to be on the main sequence, and the Sun lies
   right in the middle of it. This has led many astronomy textbooks to
   label the Sun as "average;" however, stars brighter and hotter than it
   are rare, whereas stars dimmer and cooler than it are common. The vast
   majority of stars are dim red dwarfs, though they are under-represented
   in star catalogues as we can observe only those few that are very near
   the Sun in space.
   The Hertzsprung-Russell diagram. The main sequence is from bottom right
   to top left
   Enlarge
   The Hertzsprung-Russell diagram. The main sequence is from bottom right
   to top left

   The Sun's position on the main sequence means, according to current
   theories of stellar evolution, that it is in the "prime of life" for a
   star, in that it has not yet exhausted its store of hydrogen for
   nuclear fusion, and been forced, as older red giants must, to fuse more
   inefficient elements such as helium and carbon. The Sun is growing
   increasingly bright as it ages. Early in its history, it was roughly 75
   percent as bright as it is today. Calculations of the ratios of
   hydrogen and helium within the Sun suggest it is roughly halfway
   through its life cycle, and will eventually begin moving off the main
   sequence, becoming larger, brighter and redder, until, about five
   billion years from now, it too will become a red giant.

   The Sun is a population I star, meaning that it is fairly new in
   galactic terms, having been born in the later stages of the universe's
   evolution. As such, it contains more elements heavier than hydrogen and
   helium ("metals" in astronomical parlance) than older population II
   stars such as those found in globular clusters. Since elements heavier
   than hydrogen and helium were formed in the cores of ancient and
   exploding stars, the first generation of stars had to die before the
   universe could be enriched with them. For this reason, the very oldest
   stars contain very few metals, while stars born later have more. This
   high metallicity is thought to have been crucial in the Sun's
   developing a planetary system, because planets form from accretion of
   metals.
   The heliospheric current sheet
   Enlarge
   The heliospheric current sheet

   The Sun radiates a continuous stream of charged particles, a plasma
   known as solar wind, ejecting it outwards at speeds greater than 2
   million kilometres per hour, creating a very tenuous "atmosphere" (the
   heliosphere), that permeates the solar system for at least 100 AU. This
   environment is known as the interplanetary medium. Small quantities of
   cosmic dust (some of it arguably interstellar in origin) are also
   present in the interplanetary medium and are responsible for the
   phenomenon of zodiacal light. The influence of the Sun's rotating
   magnetic field on the interplanetary medium creates the largest
   structure in the solar system, the heliospheric current sheet.

   Earth's magnetic field protects its atmosphere from interacting with
   the solar wind. However, Venus and Mars do not have magnetic fields,
   and the solar wind causes their atmospheres to gradually bleed away
   into space.

Inner planets

   The inner planets. From left to right: Mercury, Venus, Earth, and Mars
   (sizes to scale)
   Enlarge
   The inner planets. From left to right: Mercury, Venus, Earth, and Mars
   (sizes to scale)

   The four inner or terrestrial planets are characterised by their dense,
   rocky composition, few or no moons, and lack of ring systems. They are
   composed largely of minerals with high melting points such as silicates
   to form the planets' solid crusts and semi-liquid mantles, and metals
   such as iron and nickel, which form their cores. Three of the four
   inner planets have substantial atmospheres. All have impact craters and
   possess tectonic surface features, such as rift valleys and volcanoes.
   The term inner planet should not be confused with inferior planet,
   which designates those planets which are closer to the Sun than the
   Earth is (i.e. Mercury and Venus).

   The four inner planets are:

Mercury

   Mercury (0.4 AU), the closest planet to the Sun, is also the least
   massive of the planets, at only 0.055 Earth masses. Mercury has a very
   thin atmosphere consisting of atoms blasted off its surface by the
   solar wind. Because Mercury is so hot, these atoms quickly escape into
   space. Thus in contrast to the Earth and Venus whose atmospheres are
   stable, Mercury's atmosphere is constantly being replenished. It has no
   natural satellite, and it's only known geological features besides
   impact craters are "wrinkle ridges" probably produced by a period of
   contraction early in its history. Its relatively large iron core and
   thin mantle have not yet been adequately explained. Hypotheses include
   that its outer layers were stripped off by a giant impact, and that it
   was prevented from fully accreting by the Sun's gravity.

Venus

   Venus (0.7 AU) is of comparable mass to the Earth (0.815 Earth masses),
   and, like Earth, possesses a thick silicate mantle around an iron core,
   as well as a substantial atmosphere and evidence of internal geological
   activity, such as volcanoes. However, it is much drier than Earth and
   its atmosphere is 90 times as dense and is composed overwhelmingly
   (96.5%) of carbon dioxide. Venus has no natural satellite. It is the
   hottest planet, despite being farther from the sun than Mercury, with
   temperatures reaching more than 400 degrees Celsius. This is most
   likely because of the amount of greenhouse gases in the atmosphere.
   Although no definitive evidence of geological activity has yet been
   detected on Venus, its substantial atmosphere and lack of a magnetic
   field to protect it from depletion by the solar wind suggest that it
   must be regularly replenished by volcanic eruptions, perhaps as
   massive, global volcanic events which resurface the entire planet at a
   stroke, though other studies have shown that these events may have been
   continuous rather than instantaneous.

Earth

   The largest and densest of the inner planets, Earth (1 AU) is also the
   only one to demonstrate unequivocal evidence of current geological
   activity. Earth is the only planet known to have life. Its liquid
   hydrosphere, unique among the terrestrial planets, is probably the
   reason Earth is also the only planet where plate tectonics has been
   observed, because water acts as a lubricant for subduction. Its
   atmosphere is radically different from the other terrestrial planets,
   having been altered by the presence of life to contain 21 percent free
   oxygen. It has one satellite, the Moon; the only large satellite of a
   terrestrial planet in the Solar System. In fact, the Moon is in
   co-orbit around the Sun with the Earth; its annual orbit around the Sun
   is essentially circular. The Moon possesses many features in common
   with other terrestrial planets, though differs in that its core is much
   smaller.

Mars

   Mars (1.5 AU), at only 0.107 Earth masses, is less massive than either
   Earth or Venus. It possesses a tenuous atmosphere of carbon dioxide.
   Its surface, peppered with vast volcanoes and rift valleys such as
   Valles Marineris, shows that it was once geologically active and recent
   evidence suggests this may have been true until very recently. Mars
   possesses two tiny moons ( Deimos and Phobos) thought to be captured
   asteroids.

Asteroid belt

   Image of the main asteroid belt and the Trojan asteroids.
   Enlarge
   Image of the main asteroid belt and the Trojan asteroids.

   Asteroids are mostly small solar system bodies that are composed in
   significant part of rocky and metallic non-volatile minerals.

   The main asteroid belt occupies the orbit between Mars and Jupiter,
   between 2.3 and 3.3 AU from the Sun. It is thought to be the remnants
   from the Solar System's formation that failed to coalesce because of
   the gravitational interference of Jupiter. Asteroids range in size from
   hundreds of kilometers to as small as dust. All asteroids save the
   largest, Ceres, are classified as small solar system bodies; however, a
   number of other asteroids, such as Vesta and Hygeia, could potentially
   be reclassed as dwarf planets if it can be conclusively shown that they
   have achieved hydrostatic equilibrium. The asteroid belt contains tens
   of thousands - and potentially millions - of objects over one kilometre
   in diameter. However, despite their large numbers, the total mass of
   the main belt is unlikely to be more than a thousandth of that of the
   Earth. In contrast to its various depictions in science fiction, the
   main belt is very sparsely populated; spacecraft routinely pass through
   without incident. Asteroids with a diameter of less than 50 m are
   called meteoroids.

Ceres

   Ceres
   Enlarge
   Ceres

   Ceres (2.77 AU) is the largest astronomical body in the asteroid belt
   and the only known dwarf planet in this region. It has a diameter of
   slightly under 1000 km, large enough for its own gravity to pull it
   into a spherical shape. Ceres was considered a planet when it was
   discovered in the nineteenth century, but was reclassified as an
   asteroid as further observation revealed additional asteroids. It has
   since been again reclassified as a dwarf planet.

Asteroid groups

   Asteroids in the main belt are subdivided into asteroid groups and
   families based on their specific orbital characteristics. Asteroid
   moons are asteroids that orbit larger asteroids. They are not as
   clearly distinguished as planetary moons, sometimes being almost as
   large as their partners. The asteroid belt also contains main-belt
   comets which may have been the source of Earth's water.

   Trojan asteroids are located in either of Jupiter's L[4] or L[5]
   points, (gravitationally stable regions leading and trailing a planet
   in its orbit) though the term is also sometimes used for asteroids in
   any other planetary Lagrange point as well. Hilda asteroids are those
   Trojans whose orbits are in a 2:3 resonance with Jupiter; that is, they
   go around the Sun three times for every two Jupiter orbits.

   The inner solar system is also dusted with rogue asteroids, many of
   which cross the orbits of the inner planets.

Outer planets

   From top to bottom: Neptune, Uranus, Saturn, and Jupiter (sizes not to
   scale).
   Enlarge
   From top to bottom: Neptune, Uranus, Saturn, and Jupiter (sizes not to
   scale).

   The four outer planets, or gas giants, (sometimes called Jovian
   planets) are so large they collectively make up 99 percent of the mass
   known to orbit the Sun. Jupiter and Saturn are true giants, at 318 and
   95 Earth masses, respectively, and composed largely of hydrogen and
   helium. Uranus and Neptune are both substantially smaller, being only
   14 and 17 Earth masses, respectively. Their atmospheres contain a
   smaller percentage of hydrogen and helium, and a higher percentage of
   “ices”, such as water, ammonia and methane. For this reason some
   astronomers suggested that they belong in their own category, “Uranian
   planets,” or “ice giants.” All four of the gas giants exhibit orbital
   debris rings, although only the ring system of Saturn is easily
   observable from Earth. The term outer planet should not be confused
   with superior planet, which designates those planets which lie outside
   Earth's orbit (thus consisting of the outer planets plus Mars).

Jupiter

   Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the
   other planets put together. Its composition of largely hydrogen and
   helium is not very different from that of the Sun, and the planet has
   been described as a "failed star". Jupiter's strong internal heat
   creates a number of semi-permanent features in its atmosphere, such as
   cloud bands and the Great Red Spot. The four largest of its 63
   satellites, Ganymede, Callisto, Io, and Europa (the Galilean
   satellites) share elements in common with the terrestrial planets, such
   as volcanism and internal heating. Ganymede, the largest satellite in
   the Solar System, has a diameter larger than Mercury.

Saturn

   Saturn (9.5 AU), famous for its extensive ring system, has many
   qualities in common with Jupiter, including its atmospheric
   composition, though it is far less massive, being only 95 Earth masses.
   Two of its 56 moons, Titan and Enceladus, show signs of geological
   activity, though they are largely made of ice. Titan, like Ganymede, is
   larger than Mercury; it is also the only satellite in the solar system
   with a substantial atmosphere, similar in composition to that of the
   atmosphere of the early Earth.

Uranus

   Uranus (19.6 AU) at 14 Earth masses, is the lightest of the outer
   planets. Uniquely among the planets, it orbits the Sun on its side; its
   axial tilt lies at over ninety degrees to the ecliptic. Its core is
   remarkably cold compared with the other gas giants, and radiates very
   little heat into space. Uranus has 27 satellites, the largest being
   Titania, Oberon, Umbriel, Ariel and Miranda.

Neptune

   Neptune (30 AU), though slightly smaller than Uranus, is denser and
   slightly more massive, at 17 Earth masses, and radiates more internal
   heat than Uranus, but not as much as Jupiter or Saturn. Its peculiar
   ring system is composed of a number of dense "arcs" of material
   separated by gaps. Neptune has 13 moons. The largest, Triton, is
   geologically active, with geysers of liquid nitrogen, and is the only
   large satellite to revolve around its host planet in a prograde
   (clockwise) motion. Neptune possesses a number of Trojan asteroids.

Kuiper belt

   Diagram showing the resonant and classical Kuiper belt
   Enlarge
   Diagram showing the resonant and classical Kuiper belt

   The area beyond Neptune, often referred to as the outer solar system or
   simply the " trans-Neptunian region", is still largely unexplored.

   This region's first formation is the Kuiper belt, a great ring of
   debris, similar to the asteroid belt but composed mainly of ice and far
   greater in extent, which lies between 30 and 50 AU from the Sun. This
   region is thought to be the place of origin for short-period comets,
   such as Halley's comet. Though it is composed mainly of small solar
   system bodies, many of the largest Kuiper belt objects could soon be
   reclassified as dwarf planets. There are estimated to be over 100,000
   Kuiper belt objects with a diameter greater than 50 km; however, the
   total mass of the Kuiper belt is relatively low, perhaps barely
   equalling the mass of the Earth. Many Kuiper belt objects have multiple
   satellites and most have orbits that take them outside the plane of the
   ecliptic.

   The Kuiper belt can be roughly divided into two regions: the "resonant"
   belt, consisting of objects whose orbits are in some way linked to that
   of Neptune (orbiting, for instance, three times for every two Neptune
   orbits, or twice for every one), which actually begins within the orbit
   of Neptune itself, and the "classical" belt, consisting of objects that
   don't have any resonance with Neptune, and which extends from roughly
   39.4 AU to 47.7 AU. Members of the classical Kuiper belt are classified
   as Cubewanos, after the first of their kind to be discovered, 1992 QB1.

Pluto and Charon

   Pluto, and its three known moons
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   Pluto, and its three known moons

   Pluto (39 AU average), is the largest known object in the Kuiper belt
   and was previously accepted as the smallest planet in the Solar System.
   In 2006, it was reclassified as a dwarf planet by the Astronomers
   Congress organized by the International Astronomers Union (IAU). Pluto
   has a relatively eccentric orbit inclined 17 degrees to the ecliptic
   plane and ranging from 29.7 AU from the Sun at perihelion (within the
   orbit of Neptune) to 49.5 AU at aphelion. Prior to the 2006
   redefinitions, Charon was considered a moon of Pluto, but in light of
   the redefinition it is unclear whether Charon will continue to be
   classified as a moon of Pluto or as a dwarf planet itself. Charon does
   not exactly orbit Pluto in a traditional sense; Charon is about
   one-tenth the mass of Pluto and the centre of gravity of the pair is
   not within Pluto. Both bodies orbit a barycenter of gravity above the
   surface of Pluto (in empty space), making Pluto-Charon a binary system.
   Two much smaller moons, Nix and Hydra, orbit Pluto and Charon. Those
   Kuiper belt objects which, like Pluto, possess a 3:2 orbital resonance
   with Neptune (ie, they orbit twice for every three Neptunian orbits)
   are called Plutinos.

Scattered disc

   Black: scattered disc; blue: classical Kuiper belt; green: resonant
   KBOs inc. Pluto.
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   Black: scattered disc; blue: classical Kuiper belt; green: resonant
   KBOs inc. Pluto.

   Overlapping the Kuiper belt but extending much further outwards is the
   scattered disc. Scattered disc objects are believed to have been
   originally native to the Kuiper belt, but were ejected into erratic
   orbits in the outer fringes by the gravitational influence of Neptune's
   outward migration (see Formation and evolution of the Solar System).
   Most scattered disc objects have perihelia within the Kuiper belt but
   aphelia as far as 150 AU from the Sun. Their orbits are also highly
   inclined to the ecliptic plane, and are often almost perpendicular to
   it. Some astronomers, such as Kuiper belt co-discoverer David Jewitt,
   consider the scattered disc to be merely another region of the Kuiper
   belt, and describe scattered disc objects as "scattered Kuiper belt
   objects."

   Centaurs, which roughly extend from 9 to 30 AU, are icy comet-like
   bodies that orbit in the region between Jupiter and Neptune. The
   largest known Centaur, 10199 Chariklo, has a diameter of between 200
   and 250 km. The first centaur to be discovered, 2060 Chiron, has been
   called a comet since it has been shown to develop a coma just as comets
   do when they approach the sun. Some asronomers class Centaurs, along
   with scattered disc objects, as Scattered Kuiper belt objects, as they
   consider them Kuiper belt objects scattered inward, rather than
   outward..

Eris

   Eris (68 AU average) is the largest known scattered disc object and was
   the cause of the most recent debate about what constitutes a planet
   since it is at least 5% larger than Pluto with an estimated diameter of
   2400 km (1500 mi). It is now the largest of the known dwarf planets. It
   has one moon, Dysnomia.

   The object has many similarities with Pluto: its orbit is highly
   eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from
   the Sun) and an aphelion of 97.6 AU, and is steeply inclined to the
   ecliptic plane, at 44 degrees, more so than any known object in the
   solar system except the newly-discovered object 2004 XR[190] (also
   known as "Buffy") and is believed to consist largely of rock and ice.

Comets

   Comet Hale-Bopp
   Enlarge
   Comet Hale-Bopp

   Comets are small solar system bodies (usually only a few kilometres
   across) composed largely of volatile ices, which possess highly
   eccentric orbits, generally having a perihelion within the orbit of the
   inner planets and an aphelion far beyond Pluto. When a comet approaches
   the Sun, its icy surface begins to sublimate, or boil away, creating a
   coma; a long tail of gas and dust which is often visible with the naked
   eye.

   There are two basic types of comet: short-period comets, with orbits
   less than 200 years, and long-period comets, with orbits lasting
   thousands of years. Short-period comets, such as Halley's Comet, are
   believed to originate in the Kuiper belt, while long period comets,
   such as Hale-Bopp (pictured), are believed to originate in the Oort
   Cloud. Some comets with hyperbolic orbits may originate outside the
   solar system. Old comets that have had most of their volatiles driven
   out by solar warming are often categorized as asteroids.

Farthest regions

   The point at which the solar system ends and interstellar space begins
   is not precisely defined, since its outer boundaries are delineated by
   two separate forces: the solar wind and the Sun's gravity. The solar
   wind extends to a point roughly 130 AU from the Sun, whereupon it
   surrenders to the surrounding environment of the interstellar medium.
   The Sun's gravity, however, holds sway to almost halfway to the next
   star system. The vast majority of the solar system, therefore, is
   completely unknown; however, recent observations of both the solar
   system and other star systems have led to an increased understanding of
   what is or may be lying at its outer edge.

Heliopause

   The Voyagers entering the heliosheath
   Enlarge
   The Voyagers entering the heliosheath

   The heliosphere expands outward in a great bubble to about 95 AU, or
   three times the orbit of Pluto. The edge of this bubble is known as the
   termination shock; the point at which the solar wind collides with the
   opposing winds of the interstellar medium. Here the wind slows,
   condenses and becomes more turbulent, forming a great oval structure
   known as the heliosheath that looks and behaves very much like a
   comet's tail; extending outward for a further 40 AU at its
   stellar-windward side, but tailing many times that distance in the
   opposite direction. The outer boundary of the sheath, the heliopause,
   is the point at which the solar wind finally terminates, and one enters
   the environment of interstellar space. Beyond the heliopause, at around
   230 AU, lies the bow shock, a plasma "wake" left by the Sun as it
   travels through the Milky Way.

Sedna

   An artist's conception of Sedna
   Enlarge
   An artist's conception of Sedna

   Sedna is a large, reddish Pluto-like object with a gigantic, highly
   elliptical orbit that takes it from about 76 AU at perihelion to 928 AU
   at aphelion and takes 12,050 years to complete. Mike Brown, who
   discovered the object in 2003, asserts that it cannot be part of the
   scattered disc or the Kuiper Belt as it has too distant a perihelion to
   have been affected by Neptune's migration. He and other astronomers
   consider it to be the first in an entirely new population, one which
   also may include the object 2000 CR[105], which has a perihelion of
   45 AU, an aphelion of 415 AU, and an orbital period of 3420 years.
   Sedna is very likely a dwarf planet, though its shape has yet to be
   determined with certainty.

Oort cloud

   Artist's rendering of the Kuiper Belt and hypothetical Oort cloud.
   Enlarge
   Artist's rendering of the Kuiper Belt and hypothetical Oort cloud.

   The Oort cloud, currently only hypothetical, is a great mass of up to a
   trillion icy objects that is believed to be the source for all
   long-period comets and to surround the solar system like a shell from
   50,000 to 100,000 AU beyond the Sun. It is believed to be composed of
   comets which where ejected from the inward Solar System by
   gravitational interactions with the outer planets. Because the Sun's
   gravitational hold on them is so weak, Oort cloud objects move only
   very slowly, though they can be perturbed by such rare events as
   collisions, or the gravitational effects of a passing star or the
   galactic tides.

Galactic context

   Artist's conception of the Local Bubble
   Enlarge
   Artist's conception of the Local Bubble

   The solar system is located in the Milky Way galaxy, a barred spiral
   galaxy with a diameter estimated at about 100,000 light years
   containing approximately 200 billion stars. Our Sun resides in one of
   the Milky Way's outer spiral arms, known as the Orion Arm or Local
   Spur. The immediate galactic neighbourhood of the solar system is known
   as the Local Fluff, an area of dense cloud in an otherwise sparse
   region known as the Local Bubble, an hourglass-shaped cavity in the
   interstellar medium roughly 300 light-years across. The bubble is
   suffused with high-temperature plasma that suggests it is the product
   of several recent supernovae.

   Estimates place the solar system at between 25,000 and 28,000 light
   years from the galactic centre. Its speed is about 220 kilometres per
   second, and it completes one revolution every 226 million years. The
   apex of solar motion--that is, the direction in which the Sun is
   heading--is near the current location of the bright star Vega. At the
   galactic location of the solar system, the escape velocity with regard
   to the gravity of the Milky Way is about 1000 km/s.
   Presumed location of the solar system within our galaxy
   Enlarge
   Presumed location of the solar system within our galaxy

   The solar system appears to have a very remarkable orbit. It is both
   extremely close to being circular, and at nearly the exact distance at
   which the orbital speed matches the speed of the compression waves that
   form the spiral arms. The solar system appears to have remained between
   spiral arms for most of the existence of life on Earth. The radiation
   from supernovae in spiral arms could theoretically sterilize planetary
   surfaces, preventing the formation of large animal life on land. By
   remaining out of the spiral arms, Earth may be unusually free to form
   large animal life on its surface. The solar system also lies well
   outside the star-crowded environs of the galactic centre. The opposing
   gravitational tugs from so many close stars within the galactic centre
   would have prevented planets from forming.

   Recent studies of Extrasolar systems neighboring Earth's have shown
   that our system's configuration might not be common, as the vast
   majority so far discovered have been found to be markedly different.
   For instance, many extrasolar planetary systems contain a " hot
   Jupiter"; a planet of comparable size to Jupiter that nonetheless
   orbits very close to its star, at, for instance, 0.05 AU. It has been
   hypothesised that while the giant planets in these systems formed in
   the same place as the gas giants in Earth's solar system did, some sort
   of migration took place which resulted in the giant planet spiralling
   in towards the parent star. Any terrestrial planets which had
   previously existed would presumably either be destroyed or ejected from
   the system. On the other hand, the apparent prevalence of hot Jupiters
   could result from a sampling error, as planets of similar size at
   greater distances from their stars are more difficult to detect.

Discovery and exploration

   For many thousands of years, people, with a few notable exceptions, did
   not believe the solar system existed. The Earth was believed not only
   to be stationary at the centre of the universe, but to be categorically
   different from the divine or ethereal objects that moved through the
   sky. The conceptual advances of the 17th century, led by Nicolaus
   Copernicus, Galileo Galilei, Johannes Kepler, and Isaac Newton, led
   gradually to the acceptance of the idea not only that Earth moved round
   the Sun, but that the planets were governed by the same laws that
   governed the Earth, and therefore could be similar to it.

Telescopic observations

   The first exploration of the solar system was conducted by telescope,
   with astronomers learning that the Moon and other planets possessed
   such Earthlike features as craters, ice caps, and seasons.

   Galileo Galilei was the first to discover physical details about the
   individual bodies of the Solar System. He discovered that the Moon was
   cratered, that the Sun was marked with sunspots, and that Jupiter had
   four satellites in orbit around it. Christiaan Huygens followed on from
   Galileo's discoveries by discovering Saturn's moon Titan and the shape
   of the rings of Saturn. Giovanni Domenico Cassini later discovered four
   more moons of Saturn, the Cassini division in Saturn's rings, and the
   Great Red Spot of Jupiter.

   In 1682, Edmund Halley realised that repeated sightings of a comet were
   in fact recording the same object, returning regularly once every 75-6
   years. This proved once and for all that comets were not atmospheric
   phenomena, as had been previously thought, and was the first evidence
   that anything other than the planets orbited the Sun.

   In 1781, William Herschel was looking for binary stars in the
   constellation of Taurus when he observed what he thought was a new
   comet. In fact, its orbit revealed that it was a new planet, Uranus,
   the first ever discovered.

   In 1801, Giuseppe Piazzi discoverd Ceres, a small world between Mars
   and Jupiter that was initially considered a new planet. However,
   subsequent discoveries of thousands of other small worlds in the same
   region led to their eventual separate reclassification: asteroids.

   In 1846, discrepancies in the orbit of Uranus led many to suspect a
   large planet must be tugging at it from farther out. Urbain Le
   Verrier's calculations eventually led to the discovery of Neptune.

   Further discrepancies in the orbits of the planets led Percival Lowell
   to conclude yet another planet, " Planet X" must still be out there.
   After his death, his Lowell Observatory conducted a search, which
   ultimately led to Clyde Tombaugh's discovery of Pluto in 1930. Pluto
   was, however, found to be too small to have disrupted the orbits of the
   outer planets, and its discovery was therefore coincidental. Like
   Ceres, it was initially considered to be a planet, but after the
   discovery of many other similarly sized objects in its vicinity it was
   eventually reclassified as a Kuiper belt object.

   In 1992, astronomers David Jewitt of the University of Hawaii and Jane
   Luu of the Massachusetts Institute of Technology discovered 1992 QB1,
   the first object found beyond Neptune in 62 years. This object proved
   to be the first of a new population, which came to be known as the
   Kuiper Belt; an icy analogue to the asteroid belt of which such objects
   as Pluto and Charon were deemed a part. Many of the largest of these
   objects, such as Chaos, Quaoar, Varuna and Ixion, where discovered by
   astronomer Mike Brown.

   In 2005, Mike Brown announced the discovery of Eris, a Scattered disc
   object larger than Pluto and the largest object discovered in the solar
   system since Neptune.

Observations by spacecraft

   The Pale Blue Dot photo, a photo of Earth as a tiny dot (taken 4
   billion miles from Earth by Voyager 1 at the edge of the solar system)
   Enlarge
   The Pale Blue Dot photo, a photo of Earth as a tiny dot (taken 4
   billion miles from Earth by Voyager 1 at the edge of the solar system)

   Since the start of the space age, a great deal of exploration has been
   performed by unmanned space missions that have been organized and
   executed by various space agencies. The first probe to land on another
   solar system body was the Soviet Union's Luna 2 probe, which impacted
   on the Moon in 1959. Since then, increasingly distant planets have been
   reached, with probes landing on Venus in 1965, Mars in 1976, the
   asteroid 433 Eros in 2001, and Saturn's moon Titan in 2005. Spacecraft
   have also made close approaches to other planets: Mariner 10 passed
   Mercury in 1973.
   The planned Phoenix Mars lander
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   The planned Phoenix Mars lander

   The first probe to explore the outer planets was Pioneer 10, which flew
   by Jupiter in 1973. Pioneer 11 was the first to visit Saturn, in 1979.
   The Voyager probes performed a grand tour of the outer planets
   following their launch in 1977, with both probes passing Jupiter in
   1979 and Saturn in 1980 – 1981. Voyager 2 then went on to make close
   approaches to Uranus in 1986 and Neptune in 1989. The Voyager probes
   are now far beyond Neptune's orbit, and astronomers anticipate that
   they will encounter the heliopause which defines the outer edge of the
   solar system in the next few years.

   All planets in the solar system have now been visited to varying
   degrees by spacecraft launched from Earth, the last being Neptune in
   1989. Through these unmanned missions, humans have been able to get
   close-up photographs of all of the planets and, in the case of landers,
   perform tests of the soils and atmospheres of some.

   No Kuiper belt object has been visited by a man-made spacecraft.
   Launched in 19 January 2006, the New Horizons is currently enroute to
   becoming the first man-made spacecraft to explore this area. This
   unmanned mission is scheduled to fly by Pluto in July 2015. Should it
   prove feasible, the mission will then be extended to observe a number
   of other Kuiper belt objects.

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