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This article is about the Solar System. For other planetary systems or star systems, see extrasolar planet.
Planets and dwarf planets of the Solar System; while the sizes are to scale, the relative distances from the Sun are not.
The Solar System or solar system[a] consists of the Sun and the other celestial objects gravitationally bound to it: the eight planets, their 166 known moons,Scott S. Sheppard. The Jupiter Satellite Page. University of Hawaii. Retrieved on 2006-07-23. three dwarf planets (Ceres, Pluto, and Eris and their four known moons), and billions of small bodies. This last category includes asteroids, Kuiper belt objects, comets, meteoroids, and interplanetary dust.
In broad terms, the charted regions of the Solar System consist of the Sun, four terrestrial inner planets, an asteroid belt composed of small rocky bodies, four gas giant outer planets, and a second belt, the Kuiper belt, composed of icy objects. Beyond the Kuiper belt is the scattered disc, the heliopause, and ultimately the hypothetical Oort cloud.
In order of their distances from the Sun, the terrestrial planets are:
The outer gas giants (or jovians) are:
The three dwarf planets are
Six of the eight planets and two of the dwarf planets are in turn orbited by natural satellites, usually termed "moons" after Earth\'s Moon, and each of the outer planets is encircled by planetary rings of dust and other particles. All the planets except Earth are named after deities from Greco-Roman mythology.
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The zones of the Solar system: the inner solar system, the asteroid belt, the giant planets (jovians) and the Kuiper Belt. Orbits not to scale.
Objects orbiting the Sun are divided into three classes: planets, dwarf planets, and small Solar System bodies.
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. By this definition, the Solar System has eight known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. From the time of its discovery in 1930 until 2006, Pluto was considered the Solar System\'s ninth planet. But in the late 20th and early 21st centuries, many objects similar to Pluto were discovered in the outer Solar System, most notably Eris, which is slightly larger than Pluto. On August 24, 2006, the International Astronomical Union defined the term "planet" for the first time, excluding Pluto and reclassifying it under the new category of dwarf planet along with Eris and Ceres.Akwagyiram, Alexis (2005-08-02). Farewell Pluto?. BBC News. Retrieved on 2006-03-05. A dwarf planet is not required to clear its neighbourhood of other celestial bodies. Other objects that may become classified as dwarf planets are Sedna, Orcus, and Quaoar.
The remainder of the objects in orbit around the Sun are small Solar System bodies (SSSBs)."The Final IAU Resolution on the definition of "planet" ready for voting", IAU, 2006-08-24. Retrieved on 2007-03-02.
Natural satellites, or moons, are those objects in orbit around planets, dwarf planets and SSSBs, rather than the Sun itself.
Astronomers usually measure distances within the Solar System in astronomical units (AU). One AU is the approximate distance between the Earth and the Sun, or roughly 149,598,000 km (93,000,000 mi). Pluto is roughly 38 AU from the Sun while Jupiter lies at roughly 5.2 AU. One light-year, the best known unit of interstellar distance, is roughly 63,240 AU. A body\'s distance from the Sun varies in the course of its year. Its closest approach to the Sun is called its perihelion, while its farthest distance from the Sun is called its aphelion.
Informally, the Solar System is sometimes divided into separate zones. The inner Solar System includes the four terrestrial planets and the main asteroid belt. Some define the outer Solar System as comprising everything beyond the asteroids.nineplanets.org. An Overview of the Solar System. Retrieved on 2007-02-15. Others define it as the region beyond Neptune, with the four gas giants considered a separate "middle zone".Amir Alexander (2006). New Horizons Set to Launch on 9-Year Voyage to Pluto and the Kuiper Belt. The Planetary Society. Retrieved on 2006-11-08.
The ecliptic viewed in sunlight from behind the Moon in this Clementine image. From left to right: Mercury, Mars, Saturn.
The principal component of the Solar System is the Sun, a main sequence G2 star that contains 99.86% of the system\'s known mass and dominates it gravitationally.M Woolfson. The origin and evolution of the solar system (PDF). University of York. Retrieved on 2006-07-22. Jupiter and Saturn, the Sun\'s two largest orbiting bodies, account for more than 90% of the system\'s remaining mass.[b]
Most large objects in orbit around the Sun lie near the plane of Earth\'s orbit, known as the ecliptic. The planets are very close to the ecliptic while comets and Kuiper belt objects are usually at significantly greater angles to it.
The orbits of the bodies in the Solar System to scale (clockwise from top left)
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 are exceptions, such as Halley\'s Comet.
Objects travel around the Sun following Kepler\'s laws of planetary motion. Each object orbits along an approximate ellipse with the Sun at one focus of the ellipse. The closer an object is to the Sun, the faster it moves. The orbits of the planets are nearly circular, but many comets, asteroids and objects of the Kuiper belt follow highly-elliptical orbits.
To cope with the vast distances involved, many representations of the Solar System show orbits the same distance apart. In reality, with a few exceptions, 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 Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a correlation between these orbital distances (see Titius-Bode law), but no such theory has been accepted.
Artist\'s conception of a protoplanetary disk
The Solar System is believed to have formed according to the nebular hypothesis, which holds that it emerged from the gravitational collapse of a giant molecular cloud 4.6 billion years ago. This initial cloud was likely several light-years across and probably birthed several stars.Lecture 13: The Nebular Theory of the origin of the Solar System. University of Arizona. Retrieved on 2006-12-27. Studies of ancient meteorites reveal traces of elements only formed in the hearts of very large exploding stars, indicating that the Sun formed within a star cluster, and in range of a number of nearby supernovae explosions. The shock wave from these supernovae may have triggered the formation of the Sun by creating regions of overdensity in the surrounding nebula, allowing gravitational forces to overcome internal gas pressures and cause collapse.Jeff Hester (2004). New Theory Proposed for Solar System Formation. Arizona State University. Retrieved on 2007-01-11.
The region that would become the Solar System, known as the pre-solar nebula,Irvine, W. M.. The chemical composition of the pre-solar nebula. Amherst College, Massachusetts. Retrieved on 2007-02-15. had a diameter of between 7000 and 20,000 AURawal, J. J. (January 1985). "Further Considerations on Contracting Solar Nebula". Physics and Astronomy 34 (1): 93–100. DOI:10.1007/BF00054038 abstract full text (PDF). and a mass just over that of the Sun (by between 0.1 and 0.001 solar masses).Yoshimi Kitamura; Munetake Momose, Sozo Yokogawa, Ryohei Kawabe, Shigeru Ida and Motohide Tamura (2002-12-10). "Investigation of the Physical Properties of Protoplanetary Disks around T Tauri Stars by a 1 Arcsecond Imaging Survey: Evolution and Diversity of the Disks in Their Accretion Stage". The Astrophysical Journal 581 (1): 357–380. doi:10.1086/344223. Retrieved on 2007-01-09. As the nebula collapsed, conservation of angular momentum made it rotate faster. As the material within the nebula condensed, the atoms within it began to collide with increasing frequency. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc. As gravity, gas pressure, magnetic fields, and rotation acted on the contracting nebula, it began to flatten into a spinning protoplanetary disc with a diameter of roughly 200 AU and a hot, dense protostar at the centre.Greaves, Jane S. (2005-01-07). "Disks Around Stars and the Growth of Planetary Systems". Science 307 (5706): 68–71. DOI:10.1126/science.1101979 abstract full text. Present Understanding of the Origin of Planetary Systems. National Academy of Sciences (2000-04-05). Retrieved on 2007-01-19.
Studies of T Tauri stars, young, pre-fusing solar mass stars believed to be similar to the Sun at this point in its evolution, show that they are often accompanied by discs of pre-planetary matter. These discs extend to several hundred AU and reach only a thousand kelvins at their hottest.Manfred Küker, Thomas Henning and Günther Rüdiger (2003). Magnetic Star-Disk Coupling in Classical T Tauri Systems. Science Magazine. Retrieved on 2006-11-16.
Hubble image of protoplanetary disks in the Orion Nebula, a light-years-wide "stellar nursery" likely very similar to the primordial nebula from which our Sun formed.
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. This increased until hydrostatic equilibrium was achieved, with the thermal energy countering the force of gravitational contraction. At this point the Sun became a full-fledged star.Chrysostomou and Phil W Lucas The formation of stars. Department of Physics Astronomy & Mathematics University of Hertfordshire. Retrieved on 2007-05-02.
From the remaining cloud of gas and dust (the "solar nebula"), the various planets formed. They are believed to have formed by accretion: the planets began as dust grains in orbit around the central protostar; then gathered by direct contact into clumps between one and ten metres in diameter; then collided to form larger bodies (planetesimals) of roughly 5 km in size; then gradually increased by further collisions at roughly 15 cm per year over the course of the next few million years.Peter Goldreich and William R. Ward (1973). The Formation of Planetesimals. The American Astronomical Society. Retrieved on 2006-11-16.
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.Jean-Marc Petit and Alessandro Morbidelli (2001). The Primordial Excitation and Clearing of the Asteroid Belt (PDF). Centre National de la Recherche Scientifique, Observatoire de Nice. Retrieved on 2006-11-19.
Farther out still, beyond the frost line, where more volatile icy compounds could remain solid, Jupiter and Saturn became the gas giants. 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).Mummma, M. J.; M. A. DiSanti, N. Dello Russo, K. Magee-Sauer, E. Gibb, and R. Novak (June 2003). "Remote infrared observations of parent volatiles in comets: A window on the early solar system" (PDF). Advances in Space Research 31 (12): 2563–2575. doi:10.1016/S0273-1177(03)00578-7. Retrieved on 2006-11-16. Edward W. Thommes, Martin J. Duncan and Harold F. Levison. The formation of Uranus and Neptune in the Jupiter–Saturn region of the Solar System. Department of Physics, Queen\'s University, Kingston, Ontario; Space Studies Department, Southwest Research Institute, Boulder, Colorado. Retrieved on 2007-04-02.
Once the young Sun began producing energy, the solar wind (see below) blew the gas and dust in the protoplanetary disk into interstellar space and ended the growth of the planets. T Tauri stars have far stronger stellar winds than more stable, older stars.Elmegreen, B. G. (November 1979). "On the disruption of a protoplanetary disk nebula by a T Tauri like solar wind" (PDF). Astronomy and Astrophysics 80 (1): 77–78. Retrieved on 2007-02-11. Heng Hao (November 1979). "Disc-Protoplanet interactions" (PDF). Astronomy and Astrophysics 80 (1): 77–78. Retrieved on 2006-11-19.
Artist\'s conception of the future evolution of our Sun. Left: main sequence; middle: red giant; right: white dwarf
Astronomers estimate that the Solar System as we know it today will last until the Sun begins its journey off of the main sequence. As the Sun burns through its supply of hydrogen fuel, it gets hotter in order to be able to burn the remaining fuel, and so burns it even faster. As a result, the Sun is growing brighter at a rate of roughly ten percent every 1.1 billion years.JEFF HECHT (1994). Science: Fiery future for planet Earth. NewScientist. Retrieved on 2007-10-29.
Around 6.4 billion years from now, the Sun\'s core will become hot enough to cause hydrogen fusion to occur in its less dense upper layers. This will cause the Sun to expand to roughly 100 times its current diameter, and become a red giant.The fading: red giants and white dwarfs. Retrieved on 2006-12-29. At this point, the sun will have cooled and dulled, because of its vastly increased surface area.
Eventually, the Sun\'s outer layers will fall away, leaving a white dwarf, an extraordinarily dense object, half its original mass but only the size of the Earth.Pogge, Richard W. (1997). The Once & Future Sun (lecture notes). New Vistas in Astronomy. Retrieved on 2005-12-07.
The Sun as seen from Earth
The Sun is the Solar System\'s parent star, and far and away its chief component. Its large mass gives it an interior density high enough to sustain nuclear fusion, which releases enormous amounts of energy, mostly radiated into space as electromagnetic radiation such as visible light.
The Sun is classified as a moderately large yellow dwarf, but this name is misleading as, compared to stars in our galaxy, the Sun is rather large and bright. Stars are classified by the Hertzsprung-Russell diagram, a graph which plots the brightness of stars against their surface temperatures. Generally, hotter stars are brighter. Stars following this pattern are said to be on the main sequence; the Sun lies right in the middle of it. However, stars brighter and hotter than the Sun are rare, while stars dimmer and cooler are common.Smart, R. L.; Carollo, D.; Lattanzi, M. G.; McLean, B.; Spagna, A. (2001). The Second Guide Star Catalogue and Cool Stars. Perkins Observatory. Retrieved on 2006-12-26.
The Hertzsprung-Russell diagram; the main sequence is from bottom right to top left.
It is believed that the Sun\'s position on the main sequence puts it in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is growing brighter; early in its history it was 75 percent as bright as it is today.Kasting, J.F.; Ackerman, T.P. (1986). "Climatic Consequences of Very High Carbon Dioxide Levels in the Earth\'s Early Atmosphere". Science 234: 1383–1385.
Calculations of the ratios of hydrogen and helium within the Sun suggest it is halfway through its life cycle. It will eventually move off the main sequence and become larger, brighter, cooler and redder, becoming a red giant in about five billion years.Richard W. Pogge (1997). The Once and Future Sun. Perkins Observatory. Retrieved on 2006-06-23. At that point its luminosity will be several thousand times its present value.
The Sun is a population I star; it was born in the later stages of the universe\'s evolution. It contains more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars.T. S. van Albada, Norman Baker (1973). "On the Two Oosterhoff Groups of Globular Clusters". Astrophysical Journal 185: 477–498. Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, while stars born later have more. This high metallicity is thought to have been crucial to the Sun\'s developing a planetary system, because planets form from accretion of metals. Charles H. Lineweaver (2001-03-09). An Estimate of the Age Distribution of Terrestrial Planets in the Universe: Quantifying Metallicity as a Selection Effect. University of New South Wales. Retrieved on 2006-07-23.
The heliospheric current sheet
Along with light, the Sun radiates a continuous stream of charged particles (a plasma) known as the solar wind. This stream of particles spreads outwards at roughly 1.5 million kilometres per hour,Solar Physics: The Solar Wind. Marshall Space Flight Center (2006-07-16). Retrieved on 2006-10-03. creating a tenuous atmosphere (the heliosphere) that permeates the Solar System out to at least 100 AU (see heliopause). This is known as the interplanetary medium. The Sun\'s 11-year sunspot cycle and frequent solar flares and coronal mass ejections disturb the heliosphere, creating space weather.Phillips, Tony (2001-02-15). The Sun Does a Flip. Science@NASA. Retrieved on 2007-02-04. The Sun\'s rotating magnetic field acts on the interplanetary medium to create the heliospheric current sheet, the largest structure in the solar system.Artist\'s Conception of the Heliospheric Current Sheet. Wilcox Solar Observatory. Retrieved on 2006-06-22.
Aurora australis seen from orbit.
Earth\'s magnetic field protects its atmosphere from interacting with the solar wind. Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.Lundin, Richard (2001-03-09). "Erosion by the Solar Wind". Science 291 (5510): 1909. DOI:10.1126/science.1059763 abstract full text. The interaction of the solar wind with Earth\'s magnetic field creates the aurorae seen near the magnetic poles.
Cosmic rays originate outside the Solar System. The heliosphere partially shields the Solar System, and planetary magnetic fields (for planets which have them) also provide some protection. The density of cosmic rays in the interstellar medium and the strength of the Sun\'s magnetic field change on very long timescales, so the level of cosmic radiation in the Solar System varies, though by how much is unknown.Langner, U. W.; M.S. Potgieter (2005). "Effects of the position of the solar wind termination shock and the heliopause on the heliospheric modulation of cosmic rays". Advances in Space Research 35 (12): 2084–2090. doi:10.1016/j.asr.2004.12.005. Retrieved on 2007-02-11.
The interplanetary medium is home to at least two disc-like regions of cosmic dust. The first, the zodiacal dust cloud, lies in the inner Solar System and causes zodiacal light. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets.Long-term Evolution of the Zodiacal Cloud (1998). Retrieved on 2007-02-03. The second extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the Kuiper belt.ESA scientist discovers a way to shortlist stars that might have planets. ESA Science and Technology (2003). Retrieved on 2007-02-03.Landgraf, M.; Liou, J.-C.; Zook, H. A.; Grün, E. (May 2002). "Origins of Solar System Dust beyond Jupiter". The Astronomical Journal 123 (5): 2857–2861. doi:10.1086/339704. Retrieved on 2007-02-09.
The inner Solar System is the traditional name for the region comprising the terrestrial planets and asteroids. Composed mainly of silicates and metals, the objects of the inner Solar System huddle very closely to the Sun; the radius of this entire region is shorter than the distance between Jupiter and Saturn. This region was, in old parlance, denoted inner space; the area outside the asteroid belt was denoted outer space.
The inner planets. From left to right: Mercury, Venus, Earth, and Mars (sizes to scale)
The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of minerals with high melting points, such as the silicates which form their solid crusts and semi-liquid mantles, and metals such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have substantial atmospheres; all have impact craters and 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 Earth is (i.e. Mercury and Venus).
of New Mexico, Albuquerque, NM. Retrieved on 2006-11-19.
Image of the main asteroid belt and the Trojan asteroids
Asteroids are mostly small Solar System bodies composed mainly 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 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 kilometres across to microscopic. All asteroids save the largest, Ceres, are classified as small Solar System bodies, but some asteroids such as Vesta and Hygieia may be reclassed as dwarf planets if they are shown to have achieved hydrostatic equilibrium.
The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter.New study reveals twice as many asteroids as previously believed. ESA (2002). Retrieved on 2006-06-23. Despite this, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth.Krasinsky, G. A.; Pitjeva, E. V.; Vasilyev, M. V.; Yagudina, E. I. (July 2002). "Hidden Mass in the Asteroid Belt". Icarus 158 (1): 98–105. doi:10.1006/icar.2002.6837. The main belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with diameters between 10 and 10-4 m are called meteoroids.Beech, M.; Duncan I. Steel (September 1995). "On the Definition of the Term Meteoroid". Quarterly Journal of the Royal Astronomical Society 36 (3): 281–284. Retrieved on 2006-08-31.
Ceres
Trojan asteroids are located in either of Jupiter\'s L4 or L5 points (gravitationally stable regions leading and trailing a planet in its orbit); the term "Trojan" is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids 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.
The middle region of the Solar System is home to the gas giants and their planet-sized satellites. Many short period comets, including the centaurs, also lie in this region. It has no traditional name; it is occasionally referred to as the "outer Solar System", although recently that term has been more often applied to the region beyond Neptune. The solid objects in this region are composed of a higher proportion of "ices" (water, ammonia, methane) than the rocky denizens of the inner Solar System.
From top to bottom: Neptune, Uranus, Saturn, and Jupiter (not to scale)
The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn\'s atmospheres are largely hydrogen and helium. Uranus and Neptune\'s atmospheres have a higher percentage of “ices”, such as water, ammonia and methane. Some astronomers suggest they belong in their own category, “ice giants.”Jack J. Lissauer, David J. Stevenson (2006). Formation of Giant Planets (PDF). NASA Ames Research Center; California Institute of Technology. Retrieved on 2006-01-16. All four gas giants have rings, although only Saturn\'s ring system is easily observed from Earth. The term outer planet should not be confused with superior planet, which designates planets outside Earth\'s orbit (the outer planets and Mars).
Comet Hale-Bopp
Comets are small Solar System bodies, usually only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye.
Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are believed to originate in the Kuiper belt, while long-period comets, such as Hale-Bopp, are believed to originate in the Oort cloud. Many comet groups, such as the Kreutz Sungrazers, formed from the breakup of a single parent.Sekanina, Zdenek (2001). "Kreutz sungrazers: the ultimate case of cometary fragmentation and disintegration?". Publications of the Astronomical Institute of the Academy of Sciences of the Czech Republic 89 p. 78–93. Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.Królikowska, M. (2001). "A study of the original orbits of hyperbolic comets". Astronomy & Astrophysics 376 (1): 316–324. doi:10.1051/0004-6361:20010945. Retrieved on 2007-01-02. Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids.Fred L. Whipple (1992-04). The activities of comets related to their aging and origin. Retrieved on 2006-12-26.
The area beyond Neptune, or the "trans-Neptunian region", is still largely unexplored. It appears to consist overwhelmingly of small worlds (the largest having a diameter only a fifth that of the Earth and a mass far smaller than that of the Moon) composed mainly of rock and ice. This region is sometimes known as the "outer Solar System", though others use that term to mean the region beyond the asteroid belt.
Plot of all known Kuiper belt objects, set against the four outer planets
The Kuiper belt, the region\'s first formation, is a great ring of debris similar to the asteroid belt, but composed mainly of ice. It extends between 30 and 50 AU from the Sun. This region is thought to be the source of short-period comets. It is composed mainly of small Solar System bodies, but many of the largest Kuiper belt objects, such as Quaoar, Varuna, (136108) 2003 EL61, (136472) 2005 FY9 and Orcus, may be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km, but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of the Earth.Audrey Delsanti and David Jewitt (2006). The Solar System Beyond The Planets (PDF). Institute for Astronomy, University of Hawaii. Retrieved on 2007-01-03. Many Kuiper belt objects have multiple satellites, and most have orbits that take them outside the plane of the ecliptic.
Diagram showing the resonant and classical Kuiper belt
The Kuiper belt can be roughly divided into the "resonant" belt and the "classical" belt. The resonant belt consists of objects with orbits linked to that of Neptune (e.g. orbiting twice for every three Neptune orbits, or once for every two). The resonant belt actually begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 AU to 47.7 AU.M. W. Buie, R. L. Millis, L. H. Wasserman, J. L. Elliot, S. D. Kern, K. B. Clancy, E. I. Chiang, A. B. Jordan, K. J. Meech, R. M. Wagner, D. E. Trilling (2005). Procedures, Resources and Selected Results of the Deep Ecliptic Survey. Lowell Observatory, University of Pennsylvania, Large Binocular Telescope Observatory, Massachusetts Institute of Technology, University of Hawaii, University of California at Berkeley. Retrieved on 2006-09-07. Members of the classical Kuiper belt are classified as cubewanos, after the first of their kind to be discovered, (15760) 1992 QB1.E. Dotto1, M.A. Barucci2, and M. Fulchignoni (2006-08-24). Beyond Neptune, the new frontier of the Solar System (PDF). Retrieved on 2006-12-26.
Pluto and its three known moons
Black: scattered; blue: classical; green: resonant
The scattered disc overlaps the Kuiper belt but extends much further outwards. Scattered disc objects are believed to come from the Kuiper belt, having been ejected into erratic orbits by the gravitational influence of Neptune\'s early outward migration. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun. SDOs\' orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as "scattered Kuiper belt objects."David Jewitt (2005). The 1000 km Scale KBOs. University of Hawaii. Retrieved on 2006-07-16.
Eris and its moon Dysnomia
The point at which the Solar System ends and interstellar space begins is not precisely defined, since its outer boundaries are shaped by two separate forces: the solar wind and the Sun\'s gravity. The solar wind is believed to surrender to the interstellar medium at roughly four times Pluto\'s distance. However, the Sun\'s Roche sphere, the effective range of its gravitational influence, is believed to extend up to a thousand times farther.
The Voyagers entering the heliosheath
The heliosphere is divided into two separate regions. The solar wind travels at its maximum velocity out to about 95 AU, or three times the orbit of Pluto. The edge of this region is 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 heliosphere, the heliopause, is the point at which the solar wind finally terminates, and is the beginning of interstellar space.Voyager Enters Solar System\'s Final Frontier. NASA. Retrieved on 2007-04-02.
The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium,Fahr, H. J.; Kausch, T.; Scherer, H. (2000). A 5-fluid hydrodynamic approach to model the Solar System-interstellar medium interaction. Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn. Retrieved on 2006-06-23. as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU (roughly 900 million miles) farther than the southern hemisphere. 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.P. C. Frisch (2002). The Sun\'s Heliosphere & Heliopause. University of Chicago. Retrieved on 2006-06-23.
No spacecraft have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space. How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A dedicated mission beyond the heliosphere has been suggested.R. L. McNutt, Jr. et al. (2006). "Innovative Interstellar Explorer". AIP Conference Proceedings 858: 341–347. Interstellar space, and step on it!. New Scientist (2007-01-05). Retrieved on 2007-02-05.
Artist\'s rendering of the Kuiper Belt and hypothetical Oort cloud
The hypothetical Oort cloud 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 at around 50 AU, and extending out to roughly 50,000 AU (around 1 LY), and possibly to as far as 100,000 AU (1.8 LY). It is believed to be composed of comets which were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events such as collisions, the gravitational effects of a passing star, or the galactic tide.Stern SA, Weissman PR. (2001). Rapid collisional evolution of comets during the formation of the Oort cloud.. Space Studies Department, Southwest Research Institute, Boulder, Colorado. Retrieved on 2006-11-19.Bill Arnett (2006). The Kuiper Belt and the Oort Cloud. nineplanets.org. Retrieved on 2006-06-23.
Telescopic image of Sedna
Sedna and the inner Oort cloud
Much of our Solar System is still unknown. The Sun\'s gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light years (125,000 AU). The outer extent of the Oort cloud, by contrast, may not extend farther than 50,000 AU.T. Encrenaz, JP. Bibring, M. Blanc, MA. Barucci, F. Roques, PH. Zarka (2004). The Solar System: Third edition. Springer, 1. Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury and the Sun.Durda D.D.; Stern S.A.; Colwell W.B.; Parker J.W.; Lev
