The Solar System (or Solar system, solar system^[a]) consists of the Sun and those celestial objects bound to it by gravity. The Sun (Sol is the Star at the center of the Solar System. s are significant physical entities, associations or structures which current Science has confirmed to exist in Space. Gravitation is a natural Phenomenon by which objects with Mass attract one another These objects are the eight planets and their 166 known moons;^[1] three dwarf planets and their four known moons; and billions of small bodies, including asteroids, Kuiper belt objects, comets, meteoroids, and interplanetary dust. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded Small Solar System Body ( SSSB) is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — The interplanetary dust cloud is Cosmic dust (small particles floating in space which pervade the space between Planets in the Solar System and in

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. A terrestrial planet, telluric planet or rocky planet is a Planet that is primarily composed of Silicate rocks Within our The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily Beyond the Kuiper belt is the scattered disc, the heliopause, and ultimately the hypothetical Oort cloud. The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered The heliosphere is a bubble in space "blown" into the Interstellar medium (the hydrogen and helium gas that permeates the Galaxy) by the The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU,

In order of their distances from the Sun, the terrestrial planets are:

• Mercury
• Venus
• Earth
• Mars

The outer gas giants (or Jovians) are:

• Jupiter
• Saturn
• Uranus
• Neptune

The three dwarf planets are

• Ceres, the largest object in the asteroid belt;
• Pluto, the largest known object in the Kuiper belt;
• Eris, the largest known object in the scattered disc. The VENUS ( V ictoria E xperimental N etwork U nder the S ea project is a cabled sea floor observatory operated by the University EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. Ceres (ˈsɪəriːz

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. A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. A planetary ring is a ring of Cosmic dust and other small particles Orbiting around a Planet in a flat disc-shaped region All the planets except Earth are named after deities from Greco-Roman mythology. Greek mythology is the body of stories belonging to the ancient Greeks concerning their gods and Heroes the nature of the world and the origins and significance Roman mythology, or more appropriately Latin mythology, refers to the mythological beliefs of the Italic people inhabiting the region of Latium and its The word mythology (from the Greek grc μυθολογία mythología, meaning "a story-telling a legendary lore"

Contents 1 Terminology 2 Layout and structure 3 Sun 3.1 Interplanetary medium 4 Inner Solar System 4.1 Inner planets 4.2 Asteroid belt 5 Mid Solar System 5.1 Outer planets 5.2 Comets 6 Trans-Neptunian region 6.1 Kuiper belt 6.2 Scattered disc 7 Farthest regions 7.1 Heliopause 7.2 Oort cloud 7.3 Boundaries 8 Galactic context 8.1 Neighbourhood 9 Formation and evolution 10 Discovery and exploration 10.1 Telescopic observations 10.2 Observations by spacecraft 10.2.1 Flybys 10.2.2 Orbiters, landers and rovers 10.3 Manned exploration 11 See also 12 Notes 13 References 14 External links

Terminology

See also: Definition of planet

Objects orbiting the Sun are divided into three classes: planets, dwarf planets, and small Solar System bodies. From its beginnings denoting the "wandering stars" of the classical world the definition of " Planet " has been fraught with ambiguity In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star

A planet is any body in orbit around the Sun that has enough mass to form itself into a spherical shape and has cleared its immediate neighbourhood of all smaller objects. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object "Globose" redirects here See also Globose nucleus. A sphere (from Greek σφαίρα - sphaira, "globe 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. Events 49 BC - Julius Caesar 's General Gaius Scribonius Curio is defeated in the Second Battle of the Bagradas River Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded Ceres (ˈsɪəriːz ^[2] 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. TemplateInfobox Planet.--> 90377 Sedna (ˈsɛdnə) is a Trans-Neptunian TemplateInfobox Planet. --> 90482 Orcus (, Orcus originally known by the Provisional designation

The remainder of the objects in orbit around the Sun are small Solar System bodies (SSSBs). Small Solar System Body ( SSSB) is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that ^[3]

Natural satellites, or moons, are those objects in orbit around planets, dwarf planets and SSSBs, rather than the Sun itself. A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary.

Astronomers usually measure distances within the Solar System in astronomical units (AU). The astronomical unit ( AU or au or au or sometimes ua) is a unit of Length based on the distance from the Earth to the One AU is the approximate distance between the Earth and the Sun, or roughly 149,598,000 km (93,000,000 mi). The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand A mile is a unit of Length, usually used to measure Distance, in a number of different systems including Imperial units United States 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 light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by A body's distance from the Sun varies in the course of its year. A year (from Old English gēr) is the time between two recurrences of an event related to the Orbit of the Earth around the Sun Its closest approach to the Sun is called its perihelion, while its farthest distance from the Sun is called its aphelion. In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from

Informally, the Solar System is sometimes divided into separate zones. The inner Solar System includes the four terrestrial planets and the main asteroid belt. A terrestrial planet, telluric planet or rocky planet is a Planet that is primarily composed of Silicate rocks Within our Some define the outer Solar System as comprising everything beyond the asteroids. ^[4] Others define it as the region beyond Neptune, with the four gas giants considered a separate "middle zone". A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily ^[5]

Layout and structure

The principal component of the Solar System is the Sun, a main sequence G2 star that contains 99. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth 86% of the system's known mass and dominates it gravitationally. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object Gravitation is a natural Phenomenon by which objects with Mass attract one another ^[6] 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 ecliptic is the apparent path that the Sun traces out in the sky during the year The planets are very close to the ecliptic while comets and Kuiper belt objects are usually at significantly greater angles to it. A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending

All of the planets and most other objects also orbit with the Sun's rotation (counter-clockwise, as viewed from above the Sun's north pole). There are exceptions, such as Halley's Comet. Halley's Comet, officially designated 1P/Halley and also referred to as Comet Halley after Edmond Halley, is a Comet that can be seen every

Objects travel around the Sun following Kepler's laws of planetary motion. In Astronomy, Kepler's Laws of Planetary Motion are three mathematical laws that describe the motion of Planets in the Solar System. 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. The Titius–Bode law (sometimes termed just Bode's law) is a hypothesis that the bodies in some orbital systems including Sol 's orbit at semi-major axes

Sun

Main article: Sun

The Sun is the Solar System's parent star, and far and away its chief component. The Sun (Sol is the Star at the center of the Solar System. X-radiation (composed of X-rays) is a form of Electromagnetic radiation. The electromagnetic (EM spectrum is the range of all possible Electromagnetic radiation frequencies 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 density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός Radiant energy is the Energy of Electromagnetic waves The quantity of radiant energy may be calculated by integrating Radiant flux (or power Outer space, often simply called space, comprises the relatively empty regions of the Universe outside the escape velocities of Celestial bodies. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter.

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. In Astronomy, a G V star is a Main-sequence Star of spectral type G and luminosity class V The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply Stars are classified by the Hertzsprung-Russell diagram, a graph which plots the brightness of stars against their surface temperatures. The Hertzsprung-Russell diagram (usually referred to by the abbreviation H-R diagram or HRD, also known as a colour-magnitude diagram, or CMD Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature 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. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness However, stars brighter and hotter than the Sun are rare, while stars dimmer and cooler are common. ^[7]

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. ^[8]

Calculations of the ratios of hydrogen and helium within the Sun suggest it is halfway through its life cycle. Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical It will eventually move off the main sequence and become larger, brighter, cooler and redder, becoming a red giant in about five billion years. A red giant is a luminous Giant star of low or intermediate mass (roughly 0 ^[9] At that point its luminosity will be several thousand times its present value. Luminosity has different meanings in several different fields of science

The Sun is a population I star; it was born in the later stages of the universe's evolution. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen The Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy It contains more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars. In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen ^[10] 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 core of the Sun is considered to extend from the center to about 0 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. ^[11]

Interplanetary medium

Main article: Interplanetary medium

Along with light, the Sun radiates a continuous stream of charged particles (a plasma) known as the solar wind. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 In Physics and Chemistry, plasma is an Ionized Gas, in which a certain proportion of Electrons are free rather than being bound The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. This stream of particles spreads outwards at roughly 1. 5 million kilometres per hour,^[12] creating a tenuous atmosphere (the heliosphere) that permeates the Solar System out to at least 100 AU (see heliopause). The heliosphere is a bubble in space "blown" into the Interstellar medium (the hydrogen and helium gas that permeates the Galaxy) by the This is known as the interplanetary medium. The interplanetary medium is the material which fills the solar system and through which all the larger solar system bodies such as Planets Asteroids Geomagnetic storms on the Sun's surface, such as solar flares and coronal mass ejections, disturb the heliosphere, creating space weather. A geomagnetic storm or solar storm is a temporary disturbance of the Earth 's Magnetosphere caused by a disturbance in Space weather. A solar flare is a violent explosion in a star's (like the Sun 's atmosphere releasing as much Energy as 6 × 1025 Joules Solar flares A coronal mass ejection (CME is an ejection of material from the solar Corona, usually observed with a white-light Coronagraph. Space weather is the concept of changing environmental conditions in Outer space. ^[13] The Sun's rotating magnetic field acts on the interplanetary medium to create the heliospheric current sheet, the largest structure in the solar system. The heliospheric current sheet (HCS is the surface within the Solar System where the polarity of the Sun 's Magnetic field changes from ^[14]

Earth's magnetic field protects its atmosphere from interacting with the solar wind. Earth 's magnetic field (and the surface magnetic field) is approximately a Magnetic dipole, with one pole near the North pole (see Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space. ^[15] The interaction of the solar wind with Earth's magnetic field creates the aurorae seen near the magnetic poles. Earth 's magnetic field (and the surface magnetic field) is approximately a Magnetic dipole, with one pole near the North pole (see

Cosmic rays originate outside the Solar System. For the 1962 Bruce Conner film see Cosmic Ray (film Cosmic rays are energetic particles originating from space that impinge on The heliosphere partially shields the Solar System, and planetary magnetic fields (for those planets that 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. ^[16]

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. The interplanetary dust cloud is Cosmic dust (small particles floating in space which pervade the space between Planets in the Solar System and in The zodiacal light is a faint roughly triangular whitish glow seen in the night sky which appears to extend up from the vicinity of the Sun along the Ecliptic It was likely formed by collisions within the asteroid belt brought on by interactions with the planets. ^[17] The second extends from about 10 AU to about 40 AU, and was probably created by similar collisions within the Kuiper belt. ^[18][19]

Inner Solar System

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. For the Artificial intelligence Androids of the 1990s Science fiction series Space Above and Beyond, see Silicate (AI

Inner planets

Main article: Terrestrial planet

The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems. The VENUS ( V ictoria E xperimental N etwork U nder the S ea project is a cabled sea floor observatory operated by the University EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 A terrestrial planet, telluric planet or rocky planet is a Planet that is primarily composed of Silicate rocks Within our In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. A planetary ring is a ring of Cosmic dust and other small particles Orbiting around a Planet in a flat disc-shaped region They are composed largely of minerals with high melting points, such as the silicates which form their crusts and mantles, and metals such as iron and nickel, which form their cores. For the Artificial intelligence Androids of the 1990s Science fiction series Space Above and Beyond, see Silicate (AI In Geology, a crust is the outermost solid shell of a planet or moon The mantle is a part of an Astronomical object. The interior of the Earth, similar to the other Terrestrial planets, is Chemically divided Iron (ˈаɪɚn is a Chemical element with the symbol Fe (ferrum and Atomic number 26 Nickel (ˈnɪkəl is a metallic Chemical element with the symbol Ni and Atomic number 28 The planetary core consists of the innermost layer(s of a Planet. 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. An atmosphere (from Greek ατμός - atmos, " Vapor " + σφαίρα - sphaira, " Sphere " In the broadest sense the term impact crater can be applied to any depression natural or manmade resulting from the high velocity impact of a projectile with larger body This article discusses the geologic usage for the philosophical or architectural usage see Architectonics ' Or see Plate tectonics. A rift valley is a linear-shaped lowland between highlands or mountain ranges created by the action of a geologic Rift or fault. Plate tectonics and hotspots Divergent plate boundaries At the 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. The terms " inferior planet " and " superior planet " were originally used in the Ptolemaic Cosmology to differentiate those planets e. Mercury and Venus).

Mercury

Mercury (0. 4 AU) is the closest planet to the Sun and the smallest planet (0. 055 Earth masses). Mercury has no natural satellites, and its only known geological features besides impact craters are lobed ridges or rupes, probably produced by a period of contraction early in its history. A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. Rupes is the Latin word for 'cliff' It is used in Planetary geology to refer to Escarpments on other Planets such as Mercury, and ^[20] Mercury's almost negligible atmosphere consists of atoms blasted off its surface by the solar wind. ^[21] 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 young Sun's energy. ^[22][23]

Venus

Venus (0. The VENUS ( V ictoria E xperimental N etwork U nder the S ea project is a cabled sea floor observatory operated by the University 7 AU) is close in size to Earth, (0. 815 Earth masses) and like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere and evidence of internal geological activity. However, it is much drier than Earth and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C, most likely due to the amount of greenhouse gases in the atmosphere. The Celsius Temperature scale was previously known as the centigrade scale. Greenhouse gases are gaseous constituents of the atmosphere bothnatural and anthropogenic that absorb and emit radiation at specific wavelengths within the spectrum of thermal infrared ^[24] No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is regularly replenished by volcanic eruptions. ^[25]

Earth

Earth (1 AU) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only planet known to have life. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Life is a state that distinguishes Organisms from non-living objects such as non-life and dead organisms being manifested by growth through Metabolism Its liquid hydrosphere is unique among the terrestrial planets, and it is also the only planet where plate tectonics has been observed. A hydrosphere (from Greek ύδωρ - hydor, " Water " + σφαίρα - sphaira, " Sphere " Plate tectonics (from Greek τέκτων tektōn "builder" or "mason" describes the large scale motions of Earth 's Lithosphere Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the ^[26] It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System.

Mars

Mars (1. 5 AU) is smaller than Earth and Venus (0. 107 Earth masses). It possesses a tenuous atmosphere of mostly carbon dioxide. Carbon dioxide ( Chemical formula:) is a Chemical compound composed of two Oxygen Atoms covalently bonded to a single Its surface, peppered with vast volcanoes such as Olympus Mons and rift valleys such as Valles Marineris, shows geological activity that may have persisted until very recently. Olympus Mons ( Latin for "Mount Olympus" is the highest known Volcano and Mountain in the Solar System. Valles Marineris ( Latin for Mariner Valleys, named after the Mariner 9 Mars orbiter Its red color comes from rust in its iron-rich soil. ^[27] Mars has two tiny natural satellites (Deimos and Phobos) thought to be captured asteroids. TemplateInfobox Planet.--> Deimos (ˈdaɪməs; also /ˈdiːməs/ Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but ^[28]

Asteroid belt

Main article: Asteroid belt

Asteroids are mostly small Solar System bodies composed mainly of rocky and metallic non-volatile minerals. The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but

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. Ceres (ˈsɪəriːz TemplateInfobox Planet. --> 4 Vesta (ˈvɛstə Vesta is the second most massive object in the Asteroid belt Hydrostatic equilibrium occurs when compression due to Gravity is balanced by a Pressure gradient which creates a Pressure gradient force in the opposite

The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. ^[29] Despite this, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth. ^[30] The main belt is very sparsely populated; spacecraft routinely pass through without incident. A space probe is a scientific Space exploration mission in which a Robotic spacecraft leaves the Gravity well of Earth and approaches the Asteroids with diameters between 10 and 10^-4 m are called meteoroids. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International ^[31]

Ceres

Ceres (2. Ceres (ˈsɪəriːz 77 AU) is the largest body in the asteroid belt and is classified as a dwarf planet. A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded 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 19th century, but was reclassified as an asteroid in the 1850s as further observation revealed additional asteroids. ^[32] It was again reclassified in 2006 as a dwarf planet.

Asteroid groups

Asteroids in the main belt are divided into asteroid groups and families based on their orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. An asteroid moon is an Asteroid that Orbits another asteroid as its Natural satellite. 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. Main-belt comets are bodies orbiting within the ( main) asteroid belt which show cometary activity during a part of their orbit ^[33]

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 Hilda family of asteroids consists of Asteroids with a Semi-major axis between 3 In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their

The inner Solar System is also dusted with rogue asteroids, many of which cross the orbits of the inner planets. Near-Earth asteroids (NEAs are Asteroids whose Orbits are close to Earth 's orbit

Mid Solar System

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.

Outer planets

Main article: Gas giant

The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99 percent of the mass known to orbit the Sun. A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily 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. Water is a common Chemical substance that is essential for the survival of all known forms of Life. Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas. Some astronomers suggest they belong in their own category, “ice giants. ”^[34] All four gas giants have rings, although only Saturn's ring system is easily observed from Earth. A planetary ring is a ring of Cosmic dust and other small particles Orbiting around a Planet in a flat disc-shaped region The term outer planet should not be confused with superior planet, which designates planets outside Earth's orbit (the outer planets and Mars). The terms " inferior planet " and " superior planet " were originally used in the Ptolemaic Cosmology to differentiate those planets

Jupiter

Jupiter (5. 2 AU), at 318 Earth masses, masses 2. 5 times all the other planets put together. It is composed largely of hydrogen and helium. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Jupiter has sixty-three known satellites. Jupiter has 62 confirmed moons, giving it the largest retinue of moons with "reasonably secure" orbits of any planet in the Solar System The four largest, Ganymede, Callisto, Io, and Europa, show similarities to the terrestrial planets, such as volcanism and internal heating. TemplateInfobox Planet.--> Ganymede (ˈgænɨmiːd, or as Greek TemplateInfobox Planet.--> Callisto (kəˈlɪstoʊ, or as Greek TemplateInfobox Planet.--> Io (ˈaɪoʊ, or as Greek TemplateInfobox Planet.--> Europa (jʊˈroʊpə; or as ^[35] Ganymede, the largest satellite in the Solar System, is larger than Mercury.

Saturn

Saturn (9. 5 AU), famous for its extensive ring system, has similarities to Jupiter, such as its atmospheric composition. Saturn has the most extensive Planetary ring system of any planet in the Solar System. Saturn is far less massive, being only 95 Earth masses. Saturn has sixty known satellites (and three unconfirmed); two of which, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Saturn has 60 confirmed moons. These include 22 regular satellites, which all have Prograde orbits that are not greatly inclined with respect TemplateInfobox Planet.--> Titan (ˈtaɪtən, or as TemplateInfobox Planet.--> This article is about the moon of A cryovolcano is literally an Icy Volcano. Cryovolcanoes form on Icy moons and possibly on other low- Temperature Astronomical objects ^[36] Titan is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.

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 is over ninety degrees to the ecliptic. In Astronomy, axial tilt is the Inclination angle of a planet's rotational axis in relation to its orbital plane. The ecliptic is the apparent path that the Sun traces out in the sky during the year It has a much colder core than the other gas giants, and radiates very little heat into space. ^[37] Uranus has twenty-seven known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel and Miranda. Uranus has twenty-seven named moons. Five of them are massive enough to have achieved Hydrostatic equilibrium and so would be considered Dwarf planets Titania (, also) is the largest moon of Uranus and the eighth largest moon in the Solar System. TemplateInfobox Planet.--> Oberon (ˈoʊbərɒn) is the outermost Umbriel (ˈʌmbriəl) is a moon of Uranus discovered on October 24, 1851 by William Lassell. TemplateInfobox Planet.--> Ariel (ˈɛəriəl) is a moon TemplateInfobox Planet.--> Miranda (mɨˈrændə) is the smallest

Neptune

Neptune (30 AU), though slightly smaller than Uranus, is more massive (equivalent to 17 Earths) and therefore more dense. Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different It radiates more internal heat, but not as much as Jupiter or Saturn. ^[38] Neptune has thirteen known satellites. Neptune has thirteen known moons. The largest by far is Triton, discovered by William Lassell just seventeen days after the discovery of Neptune itself The largest, Triton, is geologically active, with geysers of liquid nitrogen. TemplateInfobox Planet.--> Triton (ˈtraɪtən, or as in Greek A geyser is a Hot spring characterized by intermittent discharge of water ejected turbulently and accomplished by a vapour phase Liquid nitrogen (liquid density at the Triple point is 0707 g/mL is the liquid produced industrially in large quantities by Fractional distillation of ^[39] Triton is the only large satellite with a retrograde orbit. Direct motion is the motion of a Planetary body in a direction similar to that of other bodies within its system and is sometimes called prograde motion. Neptune is accompanied in its orbit by a number of minor planets, termed Neptune Trojans, that are in 1:1 resonance with it. Minor planet is a term used since the 19th century to describe objects such as Asteroids that are in Orbit around the Sun but are not Planets As of May 2008, there are six known Neptune trojans (named by analogy to the Trojan asteroids which have the same orbital period as Neptune. In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their

Comets

Main article: Comet

Comets are small Solar System bodies, usually only a few kilometres across, composed largely of volatile ices. A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from In Celestial mechanics, an apsis, plural apsides (ˈæpsɨdɪːz is the point of greatest or least distance of the Elliptical orbit of an object from 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. Sublimation of an element or compound is a transition from the Solid to Gas phase with no intermediate liquid stage An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge In Astronomy, a coma (from the Latin word for "hair" is the nebulous envelope around the nucleus of a Comet.

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. The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending Comet Hale-Bopp ( formally designated C/1995 O1) was probably the most widely observed Comet of the twentieth century and one of the brightest The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, Many comet groups, such as the Kreutz Sungrazers, formed from the breakup of a single parent. The Kreutz Sungrazers (ˈkrɔɪts are Sungrazing comets of a particular family ^[40] Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult. A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — ^[41] Old comets that have had most of their volatiles driven out by solar warming are often categorised as asteroids. ^[42]

Centaurs

The centaurs, which 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. ^[43] The first centaur discovered, 2060 Chiron, has been called a comet since it develops a coma just as comets do when they approach the Sun. TemplateInfobox Planet. --> 2060 Chiron (, or as in Χείρων) is a Planetoid in the ^[44] Some astronomers classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc. ^[45]

Trans-Neptunian region

The area beyond Neptune, or the "trans-Neptunian region", is still largely unexplored. A trans-Neptunian object (TNO is any object in the Solar system that Orbits the sun at a greater distance on average than Neptune. A timeline of Solar System exploration listed by date of Spacecraft launch 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. The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter.

Kuiper belt

Main article: Kuiper belt

The Kuiper belt, the region's first formation, is a great ring of debris similar to the asteroid belt, but composed mainly of ice. The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending It extends between 30 and 50 AU from the Sun. 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. This article is about 20000 Varuna, a minor planet For other uses of the word see Varuna (disambiguation 20000 Varuna ( TemplateInfobox Planet. --> 90482 Orcus (, Orcus originally known by the Provisional designation 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. ^[46] 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 the "classical" belt and the resonances. In astronomy a classical Kuiper Belt object, also called a cubewano (ˌkjuːbiːˈwɑːnoʊ is a Kuiper belt object that Orbits beyond Neptune In Astronomy, a resonant trans-Neptunian object is a Trans-Neptunian object (TNO in mean motion Orbital resonance with Neptune Resonances are orbits linked to that of Neptune (e. g. twice for every three Neptune orbits, or once for every two). The first resonance 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. ^[47] Members of the classical Kuiper belt are classified as cubewanos, after the first of their kind to be discovered, (15760) 1992 QB₁. In astronomy a classical Kuiper Belt object, also called a cubewano (ˌkjuːbiːˈwɑːnoʊ is a Kuiper belt object that Orbits beyond Neptune ^[48]

Pluto and Charon

Pluto (39 AU average), a dwarf planet, is the largest known object in the Kuiper belt. When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal definition of planet. From its beginnings denoting the "wandering stars" of the classical world the definition of " Planet " has been fraught with ambiguity 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.

It is unclear whether Charon, Pluto's largest moon, will continue to be classified as such or as a dwarf planet itself. Charon (ˈʃærən; also, as in Χάρων) discovered in 1978 is either the largest Moon of Pluto or the smaller member of a double Both Pluto and Charon orbit a barycenter of gravity above their surfaces, making Pluto-Charon a binary system. A binary system is an astronomical term referring to two objects in space (usually Stars but also Planets galaxies or Asteroids which Two much smaller moons, Nix and Hydra, orbit Pluto and Charon. Nix (ˈnɪks, or as in Greek Νιξ is a Natural satellite of Pluto. Hydra (ˈhaɪdrə, or as in Greek Ύδρα is the outer-most Natural satellite of Pluto.

Pluto lies in the resonant belt and has a 3:2 resonance with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. In Celestial mechanics, an orbital resonance occurs when two Orbiting bodies exert a regular periodic gravitational influence on each other usually due to their Kuiper belt objects whose orbits share this resonance are called plutinos. In Astronomy, a plutino is a Trans-Neptunian object in 23 mean motion resonance with Neptune. ^[49]

Scattered disc

Main article: Scattered disc

The scattered disc overlaps the Kuiper belt but extends much further outwards. The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered This region is thought to be the source of short-period comets. Scattered disc objects are believed to have been ejected into erratic orbits by the gravitational influence of Neptune's early outward migration. The formation and evolution of the Solar System is estimated to have begun 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. "^[50]

Eris

Eris (68 AU average) is the largest known scattered disc object, and caused a debate about what constitutes a planet, since it is at least 5% larger than Pluto with an estimated diameter of 2400 km (1500 mi). Dysnomia (,, or as in Greek Δυσνομία officially (136199 Eris I Dysnomia, is the only known moon of the Dwarf planet Eris From its beginnings denoting the "wandering stars" of the classical world the definition of " Planet " has been fraught with ambiguity It is the largest of the known dwarf planets. ^[51] It has one moon, Dysnomia. Dysnomia (,, or as in Greek Δυσνομία officially (136199 Eris I Dysnomia, is the only known moon of the Dwarf planet Eris Like 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 steeply inclined to the ecliptic plane.

Farthest regions

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. A Hill sphere is roughly the volume around an Astronomical body (such as a Planet) where it dominates in attraction of Satellites to that body rather

Heliopause

The heliosphere is divided into two separate regions. The heliosphere is a bubble in space "blown" into the Interstellar medium (the hydrogen and helium gas that permeates the Galaxy) by the 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. The heliosphere is a bubble in space "blown" into the Interstellar medium (the hydrogen and helium gas that permeates the Galaxy) by the 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 heliosphere is a bubble in space "blown" into the Interstellar medium (the hydrogen and helium gas that permeates the Galaxy) by the 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. The heliosphere is a bubble in space "blown" into the Interstellar medium (the hydrogen and helium gas that permeates the Galaxy) by the ^[52]

The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium,^[53] as well as solar magnetic fields prevailing to the south, e. Fluid dynamics is the sub-discipline of Fluid mechanics dealing with fluid flow: Fluids ( Liquids and Gases in motion 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. A bow shock is a boundary between a Magnetosphere and an ambient medium The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply ^[54]

No spacecraft have yet passed beyond the heliopause, so it is impossible to know for certain the conditions in local interstellar space. It is expected that NASA's Voyager spacecraft will pass the heliopause some time in the next decade, and transmit valuable data on radiation levels and solar wind back to the Earth. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program See also Voyager 1 and Voyager 2. The Voyager program consists of a pair of unmanned scientific probes Voyager 1 and ^[55] How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A dedicated mission beyond the heliosphere has been suggested. ^[56][57]

Oort cloud

Main article: 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 roughly 50,000 AU (around 1 light-year (LY)), and possibly to as far as 100,000 AU (1. A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by 87 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, the tidal force exerted by the Milky Way. A galactic tide is a Tidal force subjected on objects by the Gravitational field of a Galaxy such as the Milky Way. The tidal force is a secondary effect of the Force of Gravity and is responsible for the Tides It arises because the gravitational acceleration experienced The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply ^[58][59]

Sedna and the inner Oort cloud

90377 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. TemplateInfobox Planet.--> 90377 Sedna (ˈsɛdnə) is a Trans-Neptunian Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper belt as its perihelion is too distant to have been affected by Neptune's migration. Education Brown is a Huntsville Alabama native and graduated from Virgil Grissom High School in 1983 The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending He and other astronomers consider it to be the first in an entirely new population, which also may include the object 2000 CR₁₀₅, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years. ^[60] Brown terms this population the "Inner Oort cloud," as it may have formed through a similar process, although it is far closer to the Sun. ^[61] Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.

Boundaries

See also: Hypothetical planet

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). This list of stars nearest to the Earth is ordered by increasing distance out to a maximum of 5 Parsecs (16 The outer extent of the Oort cloud, by contrast, may not extend farther than 50,000 AU. ^[62] 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. ^[63] Objects may yet be discovered in the Solar System's uncharted regions.

Galactic context

The Solar System is located in the Milky Way galaxy, a barred spiral galaxy with a diameter of about 100,000 light-years containing about 200 billion stars. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter A barred spiral galaxy is a Spiral galaxy with a central bar-shaped structure composed of Stars Bars are found in approximately half of all spiral galaxies A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by ^[64] Our Sun resides in one of the Milky Way's outer spiral arms, known as the Orion Arm or Local Spur. The Orion Arm is a minor Spiral arm of the Milky Way galaxy The Solar System and Earth are within the Orion Arm ^[65] The Sun lies between 25,000 and 28,000 light years from the Galactic Centre, and its speed within the galaxy is about 220 kilometres per second, so that it completes one revolution every 225–250 million years. The Galactic Center is the rotational center of the Milky Way Galaxy. This revolution is known as the Solar System's galactic year. The galactic year, also known as a cosmic year, is the duration of time required for the Solar system to Orbit once around the center of the Milky ^[66]

The Solar System's location in the galaxy is very likely a factor in the evolution of life on Earth. eVolution is the third Album by eLDee, it was due to be released in 2008 Life is a state that distinguishes Organisms from non-living objects such as non-life and dead organisms being manifested by growth through Metabolism Its orbit is close to being circular and is at roughly the same speed as that of the spiral arms, which means it passes through them only rarely. Since spiral arms are home to a far larger concentration of potentially dangerous supernovae, this has given Earth long periods of interstellar stability for life to evolve. A supernova (plural supernovae or supernovas) is a stellar Explosion. ^[67] The Solar System also lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort Cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life. ^[67] Even at the Solar System's current location, some scientists have hypothesised that recent supernovae may have adversely affected life in the last 35,000 years by flinging pieces of expelled stellar core towards the Sun in the form of radioactive dust grains and larger, comet-like bodies. ^[68]

Neighbourhood

The immediate galactic neighbourhood of the Solar System is known as the Local Interstellar Cloud or 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 Local Bubble is a cavity in the Interstellar medium (ISM of the Orion Arm of the Milky Way. The Local Interstellar Cloud, casually called the Local Fluff, is the Interstellar cloud (roughly 30 light years across through which our Solar system The Local Bubble is a cavity in the Interstellar medium (ISM of the Orion Arm of the Milky Way. The bubble is suffused with high-temperature plasma that suggests it is the product of several recent supernovae. In Physics and Chemistry, plasma is an Ionized Gas, in which a certain proportion of Electrons are free rather than being bound ^[69]

The solar apex, the direction of the Sun's path through interstellar space, is near the constellation of Hercules in the direction of the current location of the bright star Vega. The solar apex, or the Apex of the Sun's Way refers to the direction that the Sun travels through space Hercules (ˈhɝkjəliːz ˈhɝkjuːliːz is the fifth largest of the 88 modern Constellations It was also one of Ptolemy 's 48 constellations Vega (α Lyr / α Lyrae / Alpha Lyrae ( or) is the brightest Star in the Constellation Lyra, the fifth brightest star in the night ^[70]

There are relatively few stars within ten light years (95 trillion km) of the Sun. This list of stars nearest to the Earth is ordered by increasing distance out to a maximum of 5 Parsecs (16 The closest is the triple star system Alpha Centauri, which is about 4. Alpha Centauri (α Centauri / α Cen also known as Rigil Kentaurus, Rigil Kent, or Toliman, is the brightest Star in the southern Constellation 4 light years away. Alpha Centauri A and B are a closely tied pair of Sun-like stars, while the small red dwarf Alpha Centauri C (also known as Proxima Centauri) orbits the pair at a distance of 0. According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool Star, of the Main sequence, either late K Proxima Centauri ( Latin la ''proximus proxima proximum'' meaning 'next to' or 'nearest to' is a Red dwarf Star approximately 4 2 light years. The stars next closest to the Sun are the red dwarfs Barnard's Star (at 5. Barnard's Star ( is a very low-mass Red dwarf Star approximately 6 Light-years away in the Constellation of Ophiuchus (the 9 light years), Wolf 359 (7. 8 light years) and Lalande 21185 (8. Lalande 21185, often abbreviated as LAL 21185, is a Red dwarf star approximately 8 Light-years away in the Constellation of Ursa Major 3 light years). The largest star within ten light years is Sirius, a bright main sequence star roughly twice the Sun's mass and orbited by a white dwarf called Sirius B. Sirius is the brightest star in the night sky with a visual Apparent magnitude of &minus1 The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. It lies 8. 6 light years away. The remaining systems within ten light years are the binary red dwarf system Luyten 726-8 (8. Luyten 726-8 is a binary Star system that is one of Earth's nearest neighbors the closest star in the constellation Cetus 7 light years) and the solitary red dwarf Ross 154 (9. Ross 154 ( V1216 Sgr) is a Red dwarf Star approximately 30 pc or 9 7 light years). ^[71] Our closest solitary sunlike star is Tau Ceti, which lies 11. Tau Ceti (τ Cet / τ Ceti ˌtaʊ ˈsiːtaɪ is a Star in the Constellation Cetus that is similar to the Sun in Mass and 9 light years away. It has roughly 80 percent the Sun's mass, but only 60 percent its luminosity. ^[72] The closest known extrasolar planet to the Sun lies around the star Epsilon Eridani, a star slightly dimmer and redder than the Sun, which lies 10. An extrasolar planet, or exoplanet, is a Planet beyond the Solar System, orbiting around other Stars As of September 2008 312 Epsilon Eridani (ε Eri / ε Eridani is a main-sequence K2 class Star. 5 light years away. Its one confirmed planet, Epsilon Eridani b, is roughly 1. Epsilon Eridani b, often cataloged as HD 22049 b, is an Extrasolar planet approximately 10 Light-years away in the Constellation of 5 times Jupiter's mass and orbits its star every 6. 9 years. ^[73]

Formation and evolution

Main article: Formation and evolution of the Solar System

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. In Cosmogony, the nebular hypothesis is the most widely accepted model explaining the Formation and evolution of the Solar System. See also Solar nebula A molecular cloud, sometimes called a stellar nursery if Star formation is occurring within is a type of Interstellar 6 billion years ago. This initial cloud was likely several light-years across and probably birthed several stars. ^[74] 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. A meteorite is a natural object originating in Outer space that survives an impact with the Earth 's surface A chemical element is a type of Atom that is distinguished by its Atomic number; that is by the number of Protons in its nucleus. Star clusters are groups of Stars which are gravitationally bound A supernova (plural supernovae or supernovas) is a stellar Explosion. 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. For the music album by Converter see Shock Front For the 1977 horror film see Shock Waves A shock wave (also called This page is about the physical properties of gas as a state of matter ^[75]

Solar System's Most
Abundant Isotopes^[76]

Isotope	Nuclei per Million
Hydrogen-1	705,700
Hydrogen-2	23
Helium-4	275,200
Helium-3	35
Oxygen-16	5,920
Carbon-12	3,032
Carbon-13	37
Neon-20	1,548
Neon-22	208
Iron-56	1,169
Iron-54	72
Iron-57	28
Nitrogen-14	1,105
Silicon-28	653
Silicon-29	34
Silicon-30	23
Magnesium-24	513
Magnesium-26	79
Magnesium-25	69
Sulfur-32	396
Argon-36	77
Calcium-40	60
Aluminum-27	58
Nickel-58	49
Sodium-23	33

The region that would become the Solar System, known as the pre-solar nebula,^[77] had a diameter of between 7000 and 20,000 AU^[74][78] and a mass just over that of the Sun (by between 0. Isotopes (Greek isos = "equal" tópos = "site place" are any of the different types of atoms ( Nuclides The nucleus of an Atom is the very dense region consisting of Nucleons ( Protons and Neutrons, at the center of an atom A hydrogen atom is an atom of the chemical element Hydrogen. The electrically neutral Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth Helium-4 ( or) is a non- Radioactive and light Isotope of Helium. This article is about the elemental isotope For the record label Helium 3 see Muse or A&E Records. There are three stable isotopes of oxygen that lead to Oxygen ( O) having a standard atomic mass of 15 Carbon-12 is the most abundant of the two stable Isotopes of the element Carbon, accounting for 98 Carbon-13 ( 13C) is a natural stable Isotope of Carbon and one of the Environmental isotopes. Neon ( Ne)Standard atomic mass 201797(6 u Table Neon ( Ne)Standard atomic mass 201797(6 u Table Iron-56 is the most common Isotope of Iron. About 91754% of all iron is iron-56 Naturally occurring Iron ( Fe) consists of four Isotopes 5845% of radioactive 54Fe (half-life >3 Naturally occurring Iron ( Fe) consists of four Isotopes 5845% of radioactive 54Fe (half-life >3 Nitrogen-14 is a stable, non- Radioactive Isotope of the Chemical element Nitrogen. Silicon ( Si) has numerous known Isotopes with Mass numbers ranging from 22 to 44 Silicon ( Si) has numerous known Isotopes with Mass numbers ranging from 22 to 44 Silicon ( Si) has numerous known Isotopes with Mass numbers ranging from 22 to 44 Magnesium ( Mg)Standard atomic mass 243050(6 u Table Magnesium ( Mg)Standard atomic mass 243050(6 u Table Magnesium ( Mg)Standard atomic mass 243050(6 u Table Sulfur ( S) has 18 Isotopes four of which are stable 32S (9502% 33S (0 The main Isotopes of Argon ( Ar) found on Earth are 40Ar 36Ar and 38Ar Calcium ( Ca) has four stable Isotopes (40Ca and 42Ca through 44Ca plus two more isotopes (46Ca and 48Ca Aluminium ( Al) has multiple Isotopes Only 27Al ( Stable isotope) and 26Al ( radioactive isotope ''t''1/2 Naturally occurring Nickel ( Ni) is composed of 5 stable Isotopes 58Ni 60Ni 61Ni 62Ni and 64Ni There are thirteen recognized Isotopes of Sodium. 23Na is the only stable isotope The formation and evolution of the Solar System is estimated to have begun 1 and 0. 001 solar masses). ^[79] As the nebula collapsed, conservation of angular momentum made it rotate faster. In Physics, the angular momentum of a particle about an origin is a vector quantity equal to the mass of the particle multiplied by the Cross product of the position As the material within the nebula condensed, the atoms within it began to collide with increasing frequency. Condensation is the change of the physical state of aggregation (or simply state of matter from gaseous phase into liquid phase History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny The centre, where most of the mass collected, became increasingly hotter than the surrounding disc. ^[74] 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^[74] and a hot, dense protostar at the centre. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges A protoplanetary disk (or proplyd) is a rotating Circumstellar disk of dense gas surrounding a young newly formed star a T Tauri star or Herbig star A protostar is a large object that forms by contraction out of the gas of a Giant molecular cloud in the Interstellar medium. ^[80][81]

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. T Tauri stars ( TTS) are a class of variable Stars named after their prototype – T Tauri. ^[79] These discs extend to several hundred AU and reach only a thousand kelvins at their hottest. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic ^[82]

Within 50 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. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus ^[83] The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved, with the thermal energy countering the force of gravitational contraction. Hydrostatic equilibrium occurs when compression due to Gravity is balanced by a Pressure gradient which creates a Pressure gradient force in the opposite At this point the Sun became a full-fledged main sequence star. The main sequence is the name for a continuous and distinctive band of stars that appear on a plot of stellar color versus brightness ^[84]

From the remaining cloud of gas and dust (the "solar nebula"), the various planets formed. The formation and evolution of the Solar System is estimated to have begun 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. In Astrophysics, the term accretion is used for at least two distinct processes Planetesimals are solid objects thought to exist in Protoplanetary disks and in Debris disks A widely accepted theory of planet formation the so-called planetesimal A centimetre ( American spelling: centimeter, symbol cm) is a unit of Length in the Metric system, equal to one hundredth ^[85]

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. In Chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two Atoms in a definite arrangement held together by Water is a common Chemical substance that is essential for the survival of all known forms of Life. Methane is a Chemical compound with the molecular formula. It is the simplest Alkane, and the principal component of Natural gas. 6% the mass of the disc)^[74] and composed largely of compounds with high melting points, such as silicates and metals. A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by Mass. The melting point of a solid is the temperature range at which it changes state from solid to Liquid. For the Artificial intelligence Androids of the 1990s Science fiction series Space Above and Beyond, see Silicate (AI The M acro E xpansion T emplate A ttribute L anguage complements TAL, providing macros which allow the reuse of code across These rocky bodies eventually became the terrestrial planets. A terrestrial planet, telluric planet or rocky planet is a Planet that is primarily composed of Silicate rocks Within our Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the asteroid belt. The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. ^[86]

Farther out still, beyond the frost line, where more volatile icy compounds could remain solid, Jupiter and Saturn became the gas giants. In Astronomy or Planetary science, the frost line, also known as the snow line, refers to a particular distance in the Solar nebula from the A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily 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). ^[87][88]

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. The solar wind is a Stream of charged particles&mdasha plasma &mdashthat are ejected from the upper atmosphere of the Sun. T Tauri stars have far stronger stellar winds than more stable, older stars. A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a Star. ^[89][90]

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. ^[91]

Around 7. 6 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 up to 260 times its current diameter, and become a red giant. A red giant is a luminous Giant star of low or intermediate mass (roughly 0 ^[92] 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. A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. ^[93]

Discovery and exploration

Main articles: Geocentric model and Heliocentrism

For many thousands of years, humanity, with a few notable exceptions, did not believe the Solar System existed. In Astronomy, the geocentric model of the Universe is the superseded theory that the Earth is the center of the universe and other In Astronomy, heliocentrism is the theory that the Sun is at the center of the Solar System. 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 Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy While the Indian mathematician-astronomer Aryabhata and the Greek philosopher Aristarchus of Samos, had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus first developed a mathematically predictive heliocentric system. Indian astronomy —the earliest textual mention of which is given in the religious literature of India (2nd millennium BCE—became an established tradition by the 1st millennium BCE Āryabhaṭa ( Devanāgarī: आर्यभट (AD 476 &ndash 550 is the first in the line of great mathematician-astronomers from the classical age of Indian mathematics Aristarchus (Ἀρίσταρχος 310 BC - ca 230 BC) was a Greek Astronomer and Mathematician, born on the island of His 17th-century successors, Galileo Galilei, Johannes Kepler, and Isaac Newton developed systems of physics which led to the gradual acceptance of the idea not only that the Earth moved round the Sun, but that the planets were governed by the same physical laws that governed the Earth. Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher Johannes Kepler (ˈkɛplɚ ( December 27 1571 &ndash November 15 1630) was a German Mathematician, Astronomer Sir Isaac Newton, FRS (ˈnjuːtən 4 January 1643 31 March 1727) Biography Early years See also Isaac Newton's early life and achievements In more recent times this led to the investigation of geological phenomena such as mountains and craters and seasonal meteorological phenomena such as clouds, dust storms and ice caps on the other planets.

Telescopic observations

See also: Timeline of solar system astronomy

The first exploration of the Solar System was conducted by telescope, when astronomers first began to map those objects too faint to be seen with the naked eye. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study

Galileo Galilei was the first to discover physical details about the individual bodies of the Solar System. Galileo Galilei (15 February 1564 &ndash 8 January 1642 was a Tuscan ( Italian) Physicist, Mathematician, Astronomer, and Philosopher He discovered that the Moon was cratered, that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it. ^[94] Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn. Christiaan Huygens (ˈhaɪgənz in English ˈhœyɣəns in Dutch) ( April 14, 1629 &ndash July 8, 1695) was a Dutch TemplateInfobox Planet.--> Titan (ˈtaɪtən, or as Saturn has the most extensive Planetary ring system of any planet in the Solar System. ^[95] Giovanni Domenico Cassini later discovered four more moons of Saturn, the Cassini division in Saturn's rings, and the Great Red Spot of Jupiter. Giovanni Domenico Cassini ( June 8, 1625 &ndash September 14, 1712) was an Italian Mathematician, Astronomer Sidera Lodoicea (ˈsɪdərə ˌlɒdoʊˈɪʃ(iə is the name given by the astronomer Giovanni Domenico Cassini to the four moons of Saturn discovered by him in the Saturn has the most extensive Planetary ring system of any planet in the Solar System. ^[96]

Edmond Halley realised in 1705 that repeated sightings of a comet were in fact recording the same object, returning regularly once every 75–76 years. A sunspot is a region on the Sun 's surface ( Photosphere) that is marked by intense magnetic activity which inhibits Convection, forming Limb darkening refers to the diminishing of intensity in the image of a star as one moves from the center of the image to the edge or " Limb " of the image Edmond Halley FRS (ˈɛdmənd ˈhɔːlɪ ( November 8, 1656 &ndash January 14, 1742) was an English Astronomer Halley's Comet, officially designated 1P/Halley and also referred to as Comet Halley after Edmond Halley, is a Comet that can be seen every This was the first evidence that anything other than the planets orbited the Sun. ^[97] Around this time (1704), the term "Solar System" first appeared in English. ^[98]

In 1781, William Herschel was looking for binary stars in the constellation of Taurus when he observed what he thought was a new comet. Sir Frederick William Herschel FRS KH ( 15 November 1738 – 25 August 1822) was a German -born British A binary star is a Star system consisting of two Stars orbiting around their Center of mass. Taurus (it looks like a bull (ˈtɔrəs bull, symbol, Unicode ♉ is one of the Constellations of the Zodiac. In fact, its orbit revealed that it was a new planet, Uranus, the first ever discovered. ^[99]

Giuseppe Piazzi discovered Ceres in 1801, a small world between Mars and Jupiter that initially was considered a new planet. Giuseppe Piazzi ( July 7 1746 - July 22 1826) was an Italian Theatine monk Mathematician, and Astronomer Ceres (ˈsɪəriːz However, subsequent discoveries of thousands of other small worlds in the same region led to their eventual reclassification as asteroids. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but ^[100]

By 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. Urbain Jean Joseph Le Verrier ( March 11, 1811 &ndash September 23, 1877) was a French Mathematician who specialized in Celestial ^[101] The excess perihelion precession of Mercury's orbit led Le Verrier to postulate the intra-Mercurian planet Vulcan in 1859, but that would turn out to be a red herring. Vulcan was a small Planet proposed to exist in an Orbit between Mercury and the Sun in a 19th-century hypothesis

While it is debatable when the Solar System was truly "discovered," three 19th century observations determined its nature and place in the universe beyond reasonable doubt. First, in 1838, Friedrich Bessel successfully measured a stellar parallax, an apparent shift in the position of a star created by the Earth's motion around the Sun. Friedrich Wilhelm Bessel (22 July 1784 &ndash 17 March 1846 was a German Mathematician, Astronomer, and systematizer of the Bessel functions Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between This was not only the first direct, experimental proof of heliocentrism, but also revealed, for the first time, the vast distance between our Solar System and the stars. Then, in 1859, Robert Bunsen and Gustav Kirchhoff, using the newly invented spectroscope, examined the spectral signature of the Sun and discovered that it was composed of the same elements as existed on Earth, establishing for the first time a physical link between the Earth and the heavens. Robert Wilhelm Eberhard Bunsen (31 March 1811 &ndash 16 August 1899 was a German Chemist. Gustav Robert Kirchhoff ( March 12, 1824 &ndash October 17, 1887) was a German Physicist who contributed to the fundamental A spectrometer is an Optical instrument used to measure properties of Light over a specific portion of the Electromagnetic spectrum, typically used ^[102] Then, Father Angelo Secchi compared the spectral signature of the Sun with those of other stars, and found them virtually identical. Father Pietro Angelo Secchi SJ (29 June 1818 &ndash 26 February 1878] was an Italian astronomer. The realisation that the Sun was a star led to the hypothesis that other stars could have systems of their own, though this was not to be proven for nearly 140 years.

Further apparent discrepancies in the orbits of the outer planets led Percival Lowell to conclude that yet another planet, "Planet X", must lie beyond Neptune. Percival Lawrence Lowell ( March 13, 1855 – November 12, 1916) was a businessman author mathematician and Astronomer who fueled After his death, his Lowell Observatory conducted a search which ultimately led to Clyde Tombaugh's discovery of Pluto in 1930. Lowell Observatory is an astronomical Observatory in Flagstaff Arizona. Clyde William Tombaugh ( February 4, 1906 &ndash January 17, 1997) was an American Astronomer. 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 reclassified in 2006 as a dwarf planet by the IAU. A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded ^[101]

In 1992, the first evidence of a planetary system other than our own was discovered, orbiting the pulsar PSR B1257+12. Pulsars are highly magnetized rotating Neutron stars that emit a beam of Electromagnetic radiation in the form of radio waves PSR B1257+12, sometimes abbreviated as PSR 1257+12, is a Pulsar located 980 Light-years from the Sun. Three years later, 51 Pegasi b, the first extrasolar planet around a Sunlike star, was discovered. 51 Pegasi b, also unofficially named Bellerophon and or abbreviated as 51 Peg b, is an Extrasolar planet approximately 50 Light-years An extrasolar planet, or exoplanet, is a Planet beyond the Solar System, orbiting around other Stars As of September 2008 312 As of 2008, 221 extrasolar systems have been found. ^[103]

Also in 1992, astronomers David C. Jewitt of the University of Hawaii and Jane Luu of the Massachusetts Institute of Technology discovered (15760) 1992 QB₁. David C Jewitt is a Professor of Astronomy at the University of Hawaii Institute for Astronomy. The University of Hawaii, formally the University of Hawaii System and popularly known as UH, is a public co-educational college and university system that confers Jane Luu (aka Jane X Luu) is a Vietnamese American Astronomer. 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. The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending ^[104][105]

Mike Brown, Chad Trujillo and David Rabinowitz announced the discovery of Eris in 2005, a scattered disc object larger than Pluto and the largest object discovered in orbit round the Sun since Neptune. Education Brown is a Huntsville Alabama native and graduated from Virgil Grissom High School in 1983 Early career Trujillo attended Oak Park and River Forest High School in Oak Park Illinois. David Lincoln Rabinowitz (born 1960) is a researcher at Yale University. The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered ^[106]

Observations by spacecraft

Main article: Timeline of Solar System exploration

Since the start of the Space Age, a great deal of exploration has been performed by robotic spacecraft missions that have been organized and executed by various space agencies. The Space Age is a contemporary period encompassing the activities related to the Space Race, Space exploration, space technology and the cultural developments A robotic spacecraft is a Spacecraft with no humans on board that is usually under Telerobotic control

All planets in the Solar System have now been visited to varying degrees by spacecraft launched from Earth. 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. A lander is a Spacecraft which descends toward and comes to rest on the surface of an Astronomical body. An atmosphere (from Greek ατμός - atmos, " Vapor " + σφαίρα - sphaira, " Sphere "

The first manmade object sent into space was the Soviet satellite Sputnik 1, launched in 1957, which successfully orbited the Earth for over a year. Sputnik 1 ( "Спутник-1", "Satellite-1" ПС-1 ( PS-1, i The American probe Explorer 6, launched in 1959, was the first satellite to image the Earth from space. Explorer 6 (1959-004A or S-2) was a United States Satellite launched on August 7, 1959.

Flybys

The first successful probe to fly by another Solar System body was Luna 1, which sped past the Moon in 1959. Luna 1 (E-1 series also known as Mechta (Мечта lit: Dream) was the first Spacecraft to reach the vicinity Originally meant to impact with the Moon, it instead missed its target and became the first manmade object to orbit the Sun. Mariner 2 was the first probe to fly by another planet, Venus, in 1962. Mariner 2 (Mariner-Venus 1962 a space probe to Venus, was the first successful Spacecraft in the NASA Mariner program. The first successful flyby of Mars was made by Mariner 4 in 1965. Mariner 4 (Mariner-Mars 1964 was the fourth in a series of spacecraft intended for planetary exploration in a flyby mode and performed the first successful Mercury was first encountered by Mariner 10 in 1974. Mariner 10 was a robotic Space probe launched on November 3, 1973 to fly by the planets Mercury and Venus.

The first probe to explore the outer planets was Pioneer 10, which flew by Jupiter in 1973. Pioneer 10 ( Pioneer-F) was the first Spacecraft to travel through the Asteroid belt, which it entered on July 15, 1972 Pioneer 11 was the first to visit Saturn, in 1979. Pioneer 11 was the second mission (after its sister probe Pioneer 10) to investigate Jupiter and the Outer solar system and the first to explore the planet 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. See also Voyager 1 and Voyager 2. The Voyager program consists of a pair of unmanned scientific probes Voyager 1 and Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989. Voyager 2 is an unmanned Interplanetary Spacecraft launched on August 20, 1977. The Voyager probes are now far beyond Neptune's orbit, and are on course to find and study the termination shock, heliosheath, and heliopause. The heliosphere is a bubble in space "blown" into the Interstellar medium (the hydrogen and helium gas that permeates the Galaxy) by the According to NASA, both Voyager probes have encountered the termination shock at a distance of approximately 93 AU from the Sun. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program ^[52][108]

The first flyby of a comet occurred in 1985, when the International Cometary Explorer (ICE) passed by the comet Giacobini-Zinner,^[109] while the first flybys of asteroids were conducted by the Galileo space probe, which imaged both 951 Gaspra (in 1991) and 243 Ida (in 1993) on its way to Jupiter. The International Cometary Explorer (ICE Spacecraft was originally known as International Sun/Earth Explorer 3 (ISEE-3 satellite launched August 12, 1978 Comet Giacobini-Zinner is a Periodic comet in our solar system and was discovered by Michel Giacobini ( Nice, France) in Aquarius Galileo was an Unmanned spacecraft sent by NASA to study the Planet Jupiter and its moons Named after the Astronomer TemplateInfobox Planet. --> 951 Gaspra (ˈɡæsprə is an S-type Asteroid that orbits very TemplateInfobox Planet. --> 243 Ida (ˈaɪdə eye'-də) is a Main belt Asteroid

No Kuiper belt object has yet been visited by a spacecraft. Launched on January 19, 2006, the New Horizons probe is currently en route to becoming the first man-made spacecraft to explore this area. Events 1419 - Hundred Years' War: Rouen surrenders to Henry V of England completing his reconquest of Normandy. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. New Horizons is a Robotic spacecraft mission by NASA currently underway 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. ^[110]

Orbiters, landers and rovers

In 1966, the Moon became the first Solar System body beyond Earth to be orbited by an artificial satellite (Luna 10), followed by Mars in 1971 (Mariner 9), Venus in 1975 (Venera 9), Jupiter in 1995 (Galileo), the asteroid 433 Eros in 2000 (NEAR Shoemaker), and Saturn in 2004 (Cassini–Huygens). This article is about artificial satellites For natural satellites also known as moons see Natural satellite. Mariner 9 (Mariner Mars '71 / Mariner-I was a NASA space probe orbiter that helped in the exploration of Mars and was part of the Mariner program. Venera 9 (Венера-9 was a USSR Unmanned space mission to Venus. Galileo was an Unmanned spacecraft sent by NASA to study the Planet Jupiter and its moons Named after the Astronomer TemplateInfobox Planet. --> 433 Eros (ˈɪərɒs irr'-os) is the first discovered Near-Earth asteroid The Near Earth Asteroid Rendezvous - Shoemaker (NEAR Shoemaker renamed after its launch in honor of planetary scientist Eugene M Cassini–Huygens is a joint NASA / ESA / ASI Robotic spacecraft mission currently studying the planet Saturn and its The MESSENGER probe is currently en route to commence the first orbit of Mercury in 2011, while the Dawn spacecraft is currently set to orbit the asteroid Vesta in 2011 and the dwarf planet Ceres in 2015. The MErcury Surface Space ENvironment GEochemistry and Ranging ( MESSENGER) probe is a NASA Spacecraft, launched August 3, 2004 Dawn, launched on September 27 2007 is a Robotic spacecraft being sent by NASA on a Space exploration mission to the two most massive members TemplateInfobox Planet. --> 4 Vesta (ˈvɛstə Vesta is the second most massive object in the Asteroid belt Ceres (ˈsɪəriːz

The first probe to land on another Solar System body was the Soviet Luna 2 probe, which impacted the Moon in 1959. This is a list of all spacecraft landings on other planets and bodies in the solar system including both intended and unintended crash-landings The Soviet space program consisted of initiatives within the Soviet Union by competing design groups Luna 2 (E-1A series was the second of the Soviet Union's Luna program spacecraft launched in the direction of the Moon. Since then, increasingly distant planets have been reached, with probes landing on or impacting the surfaces of Venus in 1966 (Venera 3), Mars in 1971 (Mars 3, although a fully successful landing didn't occur until Viking 1 in 1976), the asteroid 433 Eros in 2001 (NEAR Shoemaker), and Saturn's moon Titan (Huygens) and the comet Tempel 1 (Deep Impact) in 2005. Venera 3 ( Russian:Венера-3 was a Venera program Space probe that was built and launched by the Soviet Union to explore the surface of The Mars program was a series of Mars unmanned Landers and Orbiters launched by the Soviet Union in the early 1970s. Viking 1 was the first of two spacecraft sent to Mars as part of NASA 's Viking program, and holds the record for the longest Mars surface mission TemplateInfobox Planet. --> 433 Eros (ˈɪərɒs irr'-os) is the first discovered Near-Earth asteroid The Near Earth Asteroid Rendezvous - Shoemaker (NEAR Shoemaker renamed after its launch in honor of planetary scientist Eugene M TemplateInfobox Planet.--> Titan (ˈtaɪtən, or as The Huygens probe, supplied by the European Space Agency (ESA and named after the Dutch 17th century astronomer Christiaan Huygens, is an atmospheric entry 9P/Tempel, also known as Tempel 1 or 9P/Tempel 1, is a periodic Comet. Deep Impact is an ongoing NASA Space probe launched on 12 January 2005 that was designed to study the composition of the interior of the Comet The Galileo orbiter also dropped a probe into Jupiter's atmosphere in 1995; since Jupiter has no physical surface, it was destroyed by increasing temperature and pressure as it descended.

To date, only two worlds in the Solar System, the Moon and Mars, have been visited by mobile rovers. A rover is a Space exploration vehicle designed to move across the surface of a Planet or other Astronomical body. The first rover to visit another celestial body was the Soviet Lunokhod 1, which landed on the Moon in 1970. Lunokhod 1 ( Луноход, moon walker in Russian) was the first of two unmanned lunar rovers landed on the Moon by the The first to visit another planet was Sojourner, which travelled 500 metres across the surface of Mars in 1997. The only manned rover to visit another world was NASA's Lunar rover, which travelled with Apollos 15, 16 and 17 between 1971 and 1972. The Lunar Roving Vehicle (LRV or lunar rover was a type of surface exploration rover used on the Moon during the Apollo program.

Manned exploration

Manned exploration of the Solar System is currently confined to Earth's immediate environs. The first human being to reach space (defined as an altitude of over 100 km) and to orbit the Earth was Yuri Gagarin, a Soviet cosmonaut who was launched in Vostok 1 on April 12, 1961. A soviet (сове́т, "council" originally was a workers' local council in late Imperial Russia. Vostok 1 (Восток-1 meaning Orient -1 or East-1 was the first Human spaceflight. Events 467 - Anthemius is elevated to Emperor of the Western Roman Empire. Year 1961 ( MCMLXI) was a Common year starting on Sunday (link will display full calendar of the Gregorian calendar. The first man to walk on the surface of another Solar System body was Neil Armstrong, who stepped onto the Moon on July 21, 1969 during the Apollo 11 mission; five more Moon landings occurred through 1972. Neil Alden Armstrong (born August 5 1930 is a former American Astronaut, Test pilot, University Professor, and United States Events 356 BC - Herostratus sets fire to the Temple of Artemis in Ephesus, one of the Seven Wonders of the World Year 1969 ( MCMLXIX) was a Common year starting on Wednesday (link will display full calendar of the Gregorian calendar. Year 1972 ( MCMLXXII) was a Leap year starting on Saturday (link will display full calendar of the Gregorian calendar. The United States' Space Shuttle, which debuted in 1981, is the only reusable spacecraft to successfully make multiple orbital flights. The United States of America —commonly referred to as the NASA 's Space Shuttle, officially called the Space Transportation System ( STS) is the Spacecraft currently used by the United States Year 1981 ( MCMLXXXI) was a Common year starting on Thursday (link displays the 1981 The five shuttles that have been built have flown a total of 121 missions, with two of the craft destroyed in accidents. The first orbital space station to host more than one crew was NASA's Skylab, which successfully held three crews from 1973 to 1974. A space station is an artificial structure designed for Humans to live in Outer space. The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program Skylab was the first Space station the United States launched into orbit and the second space station ever visited by a human crew The first true human settlement in space was the Soviet space station Mir, which was continuously occupied for close to ten years, from 1989 to 1999. Mir (Мир which can mean both Peace and World, and was the name given to peasant communes in pre-revolutionary Russia was a Soviet (and It was decommissioned in 2001, and its successor, the International Space Station, has maintained a continuous human presence in space since then. In 2004, SpaceShipOne became the first privately funded vehicle to reach space on a suborbital flight. SpaceShipOne is a Spaceplane that completed the first privately funded Human spaceflight on June 21, 2004. That same year, U. S. President George W. Bush announced the Vision for Space Exploration, which called for a replacement for the aging Shuttle, a return to the Moon and, ultimately, a manned mission to Mars. George Walker Bush ( born July 6 1946 is the forty-third and current President of the United States. The Vision for Space Exploration is the United States space policy announced on January 14, 2004 by U

See also

Notes

References

External links

• Solar System Profile by NASA's Solar System Exploration
• NASA's Solar System Simulator
• NASA/JPL Solar System main page
• The Nine Planets – Comprehensive Solar System site by Bill Arnett
• SPACE.com: All About the Solar System
• Illustration of the distance between planets
• Illustration comparing the sizes of the planets with each other, the sun, and other stars
• Solar System Live (an interactive orrery)
• Solar System Viewer (animation)

The Solar System v • d • e
Sun • Heliosphere	Planets ☾ = moon(s) ∅ = rings		Mercury	Venus	Earth ☾	Mars ☾
			Jupiter ☾ ∅	Saturn ☾ ∅	Uranus ☾ ∅	Neptune ☾ ∅
	Dwarf planets		Ceres	Pluto ☾	Eris ☾
	Small Solar System bodies	Asteroids (minor planets)	Groups and families: Vulcanoids · Near-Earth asteroids · Asteroid belt Jupiter Trojans · Centaurs · Neptune Trojans · Asteroid moons · Meteoroids
			See also the list of asteroids, and the meaning and pronunciation of asteroid names. This article is on the mechanical device For the British peerage see Earl of Orrery. The Sun (Sol is the Star at the center of the Solar System. The heliosphere is a bubble in space "blown" into the Interstellar medium (the hydrogen and helium gas that permeates the Galaxy) by the A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. A planetary ring is a ring of Cosmic dust and other small particles Orbiting around a Planet in a flat disc-shaped region The VENUS ( V ictoria E xperimental N etwork U nder the S ea project is a cabled sea floor observatory operated by the University EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 Mars has two tiny moons, Phobos and Deimos, which are thought to be captured Asteroids If viewed from the surface Jupiter has 62 confirmed moons, giving it the largest retinue of moons with "reasonably secure" orbits of any planet in the Solar System The planet Jupiter has a system of rings known as the rings of Jupiter or the Jovian ring system. Saturn has 60 confirmed moons. These include 22 regular satellites, which all have Prograde orbits that are not greatly inclined with respect Saturn has the most extensive Planetary ring system of any planet in the Solar System. Uranus has twenty-seven named moons. Five of them are massive enough to have achieved Hydrostatic equilibrium and so would be considered Dwarf planets The planet Uranus has a system of rings intermediate in complexity between the more extensive set around Saturn and the simpler systems around Jupiter and Neptune ( English|AmE] ] is the eighth and farthest Planet from the Sun in the Solar System. Neptune has thirteen known moons. The largest by far is Triton, discovered by William Lassell just seventeen days after the discovery of Neptune itself Neptune has a faint dusty Planetary ring system, which was discovered in 1989 by Voyager 2 spacecraft A dwarf planet, as defined by the International Astronomical Union (IAU is a Celestial body Orbiting the Sun that is massive enough to be rounded Ceres (ˈsɪəriːz Pluto has three known moons. The largest Charon, is proportionally larger compared to its primary than any other satellite of a known planet or dwarf planet in Dysnomia (,, or as in Greek Δυσνομία officially (136199 Eris I Dysnomia, is the only known moon of the Dwarf planet Eris Small Solar System Body ( SSSB) is a term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but Minor planet is a term used since the 19th century to describe objects such as Asteroids that are in Orbit around the Sun but are not Planets Vulcanoids are a hypothetical group of Asteroids that may orbit in a dynamically stable zone between 0 Near-Earth asteroids (NEAs are Asteroids whose Orbits are close to Earth 's orbit The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. As of May 2008, there are six known Neptune trojans (named by analogy to the Trojan asteroids which have the same orbital period as Neptune. An asteroid moon is an Asteroid that Orbits another asteroid as its Natural satellite. This is a list of numbered Minor planets in sequential order there are 192280 numbered minor planets and many more not yet numbered This is a list of named Minor planets (mostly Asteroids, with links to the Wikipedia articles on the people places characters and concepts that they are named after This page alphabetically lists the first thousand Asteroids to be numbered which are mostly in the main belt
		Trans- Neptunians	Kuiper belt – Plutinos: Orcus · Ixion – Cubewanos: 2002 UX25 · Varuna · 1992 QB1 · 2002 TX300 · 2003 EL61 · Quaoar · 2005 FY9 · 2002 AW197
			Scattered disc: 2002 TC302 · 2004 XR190 · Sedna
		Comets	Lists of periodic and non-periodic comets · Damocloids · Oort cloud
See also Geology of solar terrestrial planets, astronomical objects, the solar system's list of objects, sorted by radius or mass, and the Solar System Portal

A trans-Neptunian object (TNO is any object in the Solar system that Orbits the sun at a greater distance on average than Neptune. The Kuiper belt (ˈkaɪpɚ to rhyme with "viper" sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending In Astronomy, a plutino is a Trans-Neptunian object in 23 mean motion resonance with Neptune. TemplateInfobox Planet. --> 90482 Orcus (, Orcus originally known by the Provisional designation TemplateInfobox Planet. --> 28978 Ixion (ɪkˈsaɪən ik-SYE-ən, sometimes In astronomy a classical Kuiper Belt object, also called a cubewano (ˌkjuːbiːˈwɑːnoʊ is a Kuiper belt object that Orbits beyond Neptune This article is about 20000 Varuna, a minor planet For other uses of the word see Varuna (disambiguation 20000 Varuna ( The scattered disc (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy Minor planets known as scattered (also written 2004 XR190) is a Trans-Neptunian object located in the Scattered disc. TemplateInfobox Planet.--> 90377 Sedna (ˈsɛdnə) is a Trans-Neptunian A comet is a small Solar System body that orbits the Sun and when close enough to the Sun exhibits a visible coma (atmosphere or a tail — Periodic comets are defined for these purposes as those Comets having orbital periods of less than 200 years (also known as "short-period comets" or which Non-periodic comets are defined for these purposes as Comets that have an orbital period of 200 years or more including single-apparition comets that pass through the inner solar Damocloids are Asteroids such as 5335 Damocles and 1996 PW that have Halley family or long-period highly eccentric orbits typical of The Oort cloud ( ort alternatively the Öpik-Oort Cloud) is a hypothetical spherical cloud of Comets believed to lie roughly 50 000 AU, The geology of solar terrestrial planet mainly deals with the geological aspects of four planets of the Solar system namely Mercury, Venus, s are significant physical entities, associations or structures which current Science has confirmed to exist in Space. The following is a list of Solar System objects by orbit, ordered by increasing distance from the Sun This is a list of Solar System objects by radius, arranged in descending order of mean volumetric Radius. This is a list of Solar System objects by mass, in decreasing order

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