Information AboutSolar System |
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The solar system comprises the Earth 's Sun and the retinue of Celestial Objects Gravitationally bound to it. Traditionally, this is said to consist of the Sun, nine Planet s and their 158 currently known Moon s; however, a large number of other objects, including Asteroid s, Meteoroid s, Planetoid s, Comets , and Interplanetary Dust Orbit the Sun as well. Astronomers are debating the classification of a Tenth Planet . Although the term "solar system" is frequently applied to other name, '' Sol '', and thus the term sometimes appears as Solar System. When talking about another Stellar System or planetary system, including the Star s and bodies associated with them through gravity, it is usual to shorten it to drop the term "stellar" and form names such as "the Alpha Centauri system" or "the 51 Pegasi system". Structure and layout of the solar system The Sun ( Astronomical Symbol ☉) is a Main Sequence G2 Star that contains 99.86% of the system's known Mass . Its two largest orbiting bodies, Jupiter and Saturn , account for more than 90% of the remainder; the Oort Cloud might hold a substantial percentage, but as yet its existence is unconfirmed. In broad terms, the charted regions of the solar system consist of the Sun and its s, Planetoids , Meteoroids , or Planetesimal s. Pluto , the ninth planet, is also considered a member of the outer belt, and its status is currently uncertain, particularly since the discovery of (see Below) . Most objects in orbit round the Sun all lie within the same shallow plane, called the Ecliptic Plane , and orbit in the same direction. Many are in turn orbited by Moon s, and the largest are encircled by Planetary Ring s of dust and other particles. The major planets are, in order, Mercury (), Venus (♀), Earth (), Mars (♂), Jupiter (), Saturn (), Uranus (/), Neptune (), and Pluto (). Eight of the nine planets are named after or derived from gods and goddesses from Greco - Roman Mythology . Distances within the solar system are measured most often in Astronomical Unit s, or AU. One AU is the mean distance between the Earth and the Sun, or 149 598 000 kilometres. Other units in common use include the Gigametre (Gm, one million kilometres) and the Terametre (Tm, one billion/milliard kilometres). Pluto is roughly 38 AU (5.9 Tm ) from the Sun, while Jupiter lies at roughly 5.2 AU (778 Gm). gives a good idea of the layout of the solar system. Clockwise from top left: the inner solar system, the outer planets and the Kuiper Belt , the orbit of Sedna and the Oort Cloud .]] Despite the fact that many diagrams represent the solar system as having each orbit the same distance apart, in actuality the orbits are largely arranged geometrically, that is, each is roughly double the distance from the Sun as the one before it. Venus ’s distance from the Sun is roughly double that of Mercury , Earth ’s distance is roughly double that of Venus, Mars ’s double that of Earth etc. This relationship is expressed in the Titius-Bode Law , a mathematical formula for predicting the Semi-major Axes of planets in AU. In its simplest form, it is written: : where ''k''=0, 1, 2, 4, 8, 16, 32, 64, 128. By this formulation, one would expect Mercury's orbit (''k=0'') to be 0.4 AU, and Mars 's orbit (''k=4'') to be at 1.6 AU. In fact their orbits are 0.38 and 1.52 AU. Ceres , the largest asteroid, lies at ''k=8''. This law is only a rough guide, and doesn't fit all of the planets; Neptune is far closer than predicted, though Pluto lies at Neptune's predicted orbit. As of now, there is no scientific explanation for why this law applies, and many claim it is merely a coincidence, falling into the region of Uncomfortable Science . Age and origin of the solar system See Also: Age of the Earth History of Earth 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, vulcanism and plate tectonics. To estimate the age of the solar system scientists must use Meteorite s, 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 theory 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 has been refined over many years and now has a great deal of evidence supporting it. To briefly summarize, 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 it began to spin faster to conserve Angular Momentum , 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 center. Grains of dust (silicates and metals) and ice (hydrogen compounds) condensed from the gas, and began to accrete into larger and larger clumps, forming Planetesimals . Inside the Frost Line , planetesimals were composed of rock and metal, because those are the only grains that can condense at those temperatures, and remained relatively small because they were only 0.6% the mass of the disk. The larger icy planetesimals beyond the Frost Line became massive enough to capture and hold onto helium and then hydrogen gases, which caused them to rapidly grow into Jovian protoplanets. 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. Regions of the solar system ''', the largest structure in the solar system, results from the influence of the According to their location, the , including Terrestrial Planets and the Main Belt Of Asteroids ; Zone II, including the Giant Planet s, their satellites and the Centaurs , and Zone III, or the Outer Solar System , comprising the area of the Trans-Neptunian Objects including the Kuiper Belt , the Oort Cloud , and the vast region in between. Interplanetary medium The environment in which the solar system resides is called the Interplanetary Medium . The Sun radiates a continuous stream of charged particles, a Plasma known as Solar Wind , which forms a very tenuous "atmosphere" (the Heliosphere ), permeating the Interplanetary Medium in all directions for at least ten Billion (10) miles (16 Tm or 16 km) into space. Small quantities of dust are also present in the Interplanetary Medium and are responsible for the phenomenon of Zodiacal Light . Some of the dust is likely Interstellar Dust from outside the solar system. 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 . The inner planets The four inner or Terrestrial Planet s are characterised by their dense, Rocky composition, lack of primary atmospheres, and few or no moons or ring systems. They formed in the hotter regions close to the Sun, where hydrogen compounds remained gaseous, leaving only those mineral dust grains with high melting points such as Silicates to form the planets' solid Crusts and semi-liquid Mantle s, and metallic dust grains such as Iron , which forms their Cores . All have Impact Crater s and many possess tectonic surface features, such as Rift Valley s and Volcanoes . The term ''inner planet'' should not be confused with ''' Inferior Planet ''', which designates those planets which lie inside the Earth's orbit (thus Mercury and Venus only). The four inner planets are:
The asteroid belt Asteroid s are objects smaller than planets that mostly occupy the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun, and are composed in significant part of non-volatile minerals. The main Asteroid Belt contains tens of thousands (possibly millions) of asteroids over 1 km across, though they can be as small as dust. Despite their large numbers, the total mass of the main asteroid belt is unlikely to be more than a thousandth that of the Earth . Asteroids with a diameter of less than 50 m are called Meteoroids . The largest asteroid, Ceres , has a diameter of roughly 1000 km; large enough to be spherical, which would make it a planet by some definitions of the word. The asteroids are thought to be the remnants of a small terrestrial planet that failed to coalesce due to the gravitational interference of Jupiter. They are subdivided into based on their specific orbital characteristics. Asteroid Moon s are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. Trojan Asteroid s are located in either of Jupiter's L4 Or L5 Points , though the term is also sometimes used for asteroids in any other planetary Lagrange point as well. The asteroid belt also contains Main-belt Comets {Link without Title} which may have been the source of Earth's water. The inner solar system is dusted with rogue asteroids, many of which cross the orbits of the inner planets. The outer planets The four outer planets, or Gas Giant s, (sometimes called Jovian Planet s) are so large they collectively make up 99 percent of the mass known to orbit the Sun. Their large sizes and distance from the Sun meant they could hold on to much of the hydrogen and helium too light for the smaller and hotter terrestrial planets to retain. 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 ).
Centaur s are icy comet-like bodies that have less-eccentric orbits so that they remain in the region between Jupiter and Neptune. The first centaur to be discovered, 2060 Chiron , has been called a comet since it has been shown to develop a tail, or Coma , just as comets do when they approach the sun. The trans-Neptunian region .]] The area beyond Neptune, often referred to as the Outer Solar System or simply the " Trans-Neptunian Region ", is still largely unexplored. The Kuiper belt This region's first formation, which actually begins inside the orbit of Neptune, 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 Comet s, such as Halley's Comet . Though there are estimated to be over 70,000 Kuiper Belt Object s with a diameter greater than 100 km, the total mass of the Kuiper belt is relatively low, perhaps equalling or just exceeding the mass of the Earth. Many Kuiper belt objects have orbits that take them outside the plane of the ecliptic. =Pluto
Kuiper belt objects with Pluto-like orbits are called Plutino s. Other Kuiper belt objects have Resonant Orbits and are grouped accordingly. The remaining Kuiper belt objects, in more "classical" orbits, are classified as Cubewano s. The Kuiper Belt has a very sharply defined edge. At around 49 AU, a sharp dropoff occurs in the number of objects observed. This dropoff is known as the "Kuiper Cliff", and as yet its cause is unknown. Some speculate that something must exist beyond the belt large enough to sweep up the remaining debris, perhaps as large as Earth or Mars. This view is still controversial, however. The scattered disc 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. =2003 UB313 One particular scattered disc object, originally found in 2003 but confirmed two years later by has not yet been resolved. Possibility of a new region Sedna , the newly discovered Pluto-like object with a gigantic, highly elliptical 10,500-year orbit that takes it from about 76 to 928 AU, has too distant a Perihelion to be a scattered member of the Kuiper Belt and could be the first in an entirely new population. is also believed to be a member of this population. Comets Comet s are composed largely of volatile ices and have highly eccentric orbits, generally having a Perihelion within the orbit of the inner planets and an Aphelion far beyond Pluto. Short-period comets exist with Apoapses closer than this, however, and old comets that have had most of their volatiles driven out by solar warming are often categorized as asteroids. Long period comets have orbits lasting thousands of years. Some comets with Hyperbolic orbits may originate outside the solar system. 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 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 . But even this point is not classified as outside the solar system, for the Sun's gravity will still hold sway even up to the Oort Cloud , which is a great mass of icy objects, currently hypothetical, 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 , or almost halfway to the next star system. The vast majority of the solar system, therefore, is completely unknown. The solar system in galactic context The solar system is located in the Local Fluff in the Orion Arm of the Milky Way Galaxy , a Barred Spiral Galaxy with a diameter estimated at about 100,000 Light Year s containing approximately 200 billion stars, of which the Sun is a rather large and bright one. The vast majority of stars are Red Dwarfs ; the Sun is placed near the middle of the Hertzsprung-Russell Diagram , but stars larger and hotter than it are rare, whereas stars dimmer and cooler than it are very common, although we can observe only those few other red dwarfs that are very near the Sun in space. Estimates place the solar system at between 25,000 and 28,000 light years from the Galactic Center . Its speed is about 220 Kilometres Per Second , and it completes one revolution every 226 Million Years . 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. The solar system appears to have a very unusual 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. For many years, the solar system had the only Planetary System known, and so theories of planetary formation only had to explain one system to be plausible. The discovery in recent years of many Extrasolar Planet s has uncovered systems very different compared to Earth's solar system, and theories have had to be revised accordingly. 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. ''See main article: Extrasolar Planet '' Exploration of the solar system 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 catagorically different from the "wandering stars" (planets) that moved through the sky. The conceptual advances of the 17th century, led by Nicholas 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 goverened by the same laws that governed the Earth, and therefore could be similar to it. 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. ''See main article: Geocentric Model '' Since the start of the '' passed Mercury in 1973 . 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 Pluto '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. Pluto remains the only planet not having been visited by a man-made spacecraft, though that will change with the successful launch of the '' New Horizons '' spacecraft on 19 January 2006 . This Unmanned Mission is scheduled to fly by Pluto in July 2015 and then make an extensive study of as many Kuiper Belt objects as it can. Through these unmanned missions, humans have been able to get close-up photographs of most of the planets and, in the case of landers, perform tests of their soils and Atmospheres . ''See main article: Space Exploration '' Attributes of major planets All attributes below are measured relative to the Earth: Of the other objects, Ganymede has the largest mass, 2% that of the Earth. For a more comprehensive table, see Table Of Planets In The Solar System . Attributes of the largest minor planets The largest Minor Planets (also known as planetoids) are smoothly rounded, like major planets, because their gravity overcomes material strength that keeps smaller bodies in non-spherical shapes. Before the discovery of 2060 Chiron and the Trans-Neptunian Object s, the term "minor planet" was a synonym for Asteroid , but many people now prefer to restrict the use of "asteroid" to refer to rocky bodies of the inner solar system. Most trans-Neptunian objects are icy, like Comets , although those we can detect at that distance are much larger than comets. Several asteroids, in the strict sense, are large enough to be spherical. The largest known trans-Neptunian objects are much larger than the large asteroids. (Natural satellites of major planets also range smoothly from small non-spherical objects to large spherical ones, and the largest are larger than 1 Ceres , the largest asteroid). All attributes below are measured relative to the Earth: Other facts The total Surface Area of the solar system's objects that have solid surfaces and a diameter greater than 1 km is approximately 1.7 Km 2 —about 11 times the area of the Earth's land masses. It has been suggested that the Sun may be part of a Binary Star system, with a distant companion named Nemesis . Nemesis was proposed to explain some timing regularities of the great extinctions of life on Earth. The hypothesis says that Nemesis creates periodical perturbations in the Oort Cloud of comets surrounding the solar system, causing a "comet shower". Some of them hit Earth, causing destruction of life. This hypothesis is no longer taken seriously by most scientists, mostly because Infrared surveys failed to spot any such object, which should have been very conspicuous at those wavelengths. The concept of the tenth planet has frequently been explored in Science Fiction works and Conspiracy Theories (see also Planet X , and Hypothetical Planet ). See also
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