Solar System & Planets K-12 Experiments
Solar System & Planets
The Solar System or solar system[1] comprises the Sun and the retinue of celestial objects gravitationally bound to it: the eight planets, their 162 known moons,[2] three currently identified dwarf planets and their four known moons, and thousands of small bodies. This last category includes asteroids, meteoroids, comets, and interplanetary dust. The principal component of the Solar System is the Sun or Sol, (astronomical symbol In broad terms, the charted regions of the Solar System consist of the Sun, four rocky bodies close to it called the terrestrial planets, an inner belt of rocky asteroids, four gas giant planets, and an outer belt of small, icy bodies known as the Kuiper belt. In order of their distances from the Sun, the planets are Mercury ( From 1930 to 2006, Pluto ( For many years, the Solar System was the only known example of planets in orbit around a star. The discovery in recent years of many extrasolar planets has led to the term "solar system" being applied generically to all the newly discovered systems. Technically, however, it should strictly refer to Earth's system only, as the word "solar" is derived from the Sun's Latin name, Sol. Other such systems are usually referred to by the names of their parent star: "the Alpha Centauri system" or "the 51 Pegasi system", or generically as a "star system".
Layout
Most objects in orbit round the Sun lie within the same shallow plane, called the ecliptic, which is roughly parallel to the Sun's equator. The planets lie very close to the ecliptic, while comets and kuiper belt objects often lie at significant angles to it. All of the planets, and most other objects, also orbit with the Sun's rotation in a counter-clockwise direction as viewed from a point above the Sun's north pole. There is a direct relationship between how far away a planet is from the Sun, and how quickly it orbits. Mercury, with the smallest orbital circumference, travels the fastest, while Neptune, being much farther from the Sun, travels more slowly. A planet's distance from the Sun varies in the course of its year. Its closest approach to the Sun is known as its perihelion, while its farthest point from the Sun is called its aphelion. Though planets follow nearly circular orbits, with perihelions roughly equal to their aphelions, many comets, asteroids and objects of the Kuiper belt follow highly elliptical orbits, with large differences between perihelion and aphelion. Astronomers most often measure distances within the solar system in astronomical units, or AU. One AU is the average distance between the Earth and the Sun, or roughly 149 598 000 km (93,000,000 mi). Pluto is roughly 39 AU from the Sun, while Jupiter lies at roughly 5.2 AU. Informally, the Solar System is sometimes divided into separate "zones"; the first zone, known as the inner Solar System, comprises the inner planets and the main asteroid belt. The outer solar system is sometimes defined as everything beyond the asteroids; however, it is also the name often given to the region beyond Neptune, with the gas giants as a separate "middle zone." 
One common misconception with regards to the Solar System is that the orbits of the major objects (planets, Pluto, and asteroids) are equidistant. Due to the vast distances involved, many representations of the Solar System tend to simplify these orbits, with equal spacing between each object. However, with certain exceptions, it can generally be stated that the farther a planet or belt is from the Sun, the greater the distance between it and the previous orbit. For example, Venus is approximately 0.33 AU farther out than Mercury, whereas Jupiter lies 1.9 AU from the farthest extent of the asteroid belt, and Neptune's orbit is roughly 20 AU farther out than that of Uranus. Attempts have been made to determine a correlation between these distances (see Bode's Law) but to date there is no accepted theory that explains the respective orbital distances. Planets, dwarf planets, and small solar system bodies
In a decision passed by the International Astronomical Union General Assembly on August 24, 2006, the objects in the Solar System were divided into three separate groups: planets, dwarf planets and small solar system bodies. Under this classification, a planet is any body in orbit around the Sun that a) has enough mass to form itself into a spherical shape and b) has cleared its immediate neighborhood of all smaller objects. Eight objects in the Solar System currently meet this definition; they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Dwarf planet is a newly defined classification for astronomical objects. The key difference between planets and dwarf planets is that while both are required to orbit the Sun and be of large enough mass that their own gravity pulls them into a nearly round shape, dwarf planets are not required to clear their neighborhood of other celestial bodies. Three objects in the solar system are currently included in this category; they are Pluto (formerly considered a planet), the asteroid Ceres, and the scattered disc object Eris. The IAU will begin evaluating other known objects to see if they fit within the definition of dwarf planets. The most likely candidates are some of the larger asteroids and several Trans-Neptunian Objects such as Sedna, Orcus, and Quaoar. The remainder of the objects in the Solar System were classified as small solar system bodies. A small solar system body (SSSB) is a term defined in 2006 by the International Astronomical Union to describe Solar System objects which are neither planets nor dwarf planets.
As of 2006, the IAU considers the following bodies to be SSSB's:
Within the Solar SystemAccepted planets According to the IAU, there are eight planets in the Solar System. In increasing distance from the Sun, they are:
Categories
The large bodies of the Solar System can be divided into categories based on their composition:
Attributes of planetsAll the planets revolve around the Sun in the same direction - counter-clockwise as seen from over the Sun's north pole. The period of one revolution of a planet's orbit is known as its year. A planet's year depends on its distance from the Sun; the farther a planet is from the Sun, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the Sun's gravity. The planets also rotate around invisible axes through their centres. The period of one rotation of a planet is known as its day. All the planets rotate in a counter-clockwise direction, except for Venus, which rotates clockwise. There is great variation in the length of day between the planets, with Venus taking 243 days to rotate, and the gas giants only a few hours. Planets also have varying degrees of axial tilt; they orbit at an angle to the plane of the Sun's equator. This causes the amount of sunlight received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from the Sun, the southern hemisphere points towards it, and vice versa. Each planet therefore possesses seasons; changes to the climate over the course of its year. The point at which each hemisphere is farthest/nearest from the Sun is known as its solstice. Each planet has two in the course of its orbit; when a planet's northern hemisphere has its summer solstice, when its day is longest, the southern has its winter solstice, when its day is shortest. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices. All of the planets have atmospheres as their large masses mean gravity is strong enough to keep gaseous particles close to the surface. The larger gas giants are massive enough to keep large amounts of the light gases Hydrogen and Helium close by, although these gases mostly float into space around the smaller planets. Earth's atmosphere is greatly different to the other planets, due to the various life processes that have transpired there, while the atmosphere of Mercury has mostly, although not entirely, been blasted away by the solar wind. Many of the planets have natural satellites, called "moons", regardless of their size. The gas giants all have numerous moons in complex planetary systems. Many gas giant moons have similar features to the terrestrial planets and dwarf planets, and some have been studied for signs of life.
*Measured relative to the Earth. **See Earth article for absolute values. Dwarf planetsBefore the August 2006 decision, several objects were proposed by astronomers, including at one stage by the IAU, as planets. However in 2006 several of these objects were reclassified as dwarf planets, objects distinct from planets. Currently three dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto and Eris. Several other objects in both the asteroid belt and the Kuiper belt are under consideration, with as many as 50 that could eventually qualify. There may be as many as 200 that could be discovered once the Kuiper Belt has been fully explored. Dwarf planets share many of the same characteristics as planets, although notable differences remain - namely that they are not dominant in their orbits. Their attributes are:
*Measured relative to the Earth. By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto is a member of the Kuiper belt and Eris is a member of the scattered disc. According to Mike Brown there may soon be over forty trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition. [1] Formation
Using radiometric dating, scientists can estimate that the solar system is 4.6 billion years old. The oldest rocks on Earth are approximately 3.9 billion years old. Rocks this old are rare, as the Earth's surface is constantly being reshaped by erosion, volcanism and plate tectonics. To estimate the age of the solar system scientists must use meteorites, which were formed during the early condensation of the solar nebula. The oldest meteorites (such as the Canyon Diablo meteorite) are found to have an age of 4.6 billion years, hence the solar system must be at least 4.6 billion years old.[7] The current hypothesis of Solar System formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace.[8] The nebular theory holds that the Solar System was formed from the gravitational collapse of a gaseous cloud called the solar nebula. It had a diameter of 100 AU and was 2–3 times the mass of the Sun. Over time, a disturbance (possibly a nearby supernova) squeezed the nebula, pushing matter inward until gravitational forces overcame the internal gas pressure and it began to collapse. As the nebula collapsed, conservation of angular momentum meant that it spun faster, and became warmer. As the competing forces associated with gravity, gas pressure, magnetic fields, and rotation acted on it, the contracting nebula began to flatten into a spinning protoplanetary disk with a gradually contracting protostar at the center. From this cloud and its gas and dust, the various planets formed. The inner solar system was too warm for volatile molecules like water and methane to condense, and so the planetesimals which formed there were relatively small (comprising only 0.6% the mass of the disc) and composed largely of compounds with high melting points, such as silicates and metals. These rocky bodies eventually became the terrestrial planets. Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the asteroid belt. Farther out still, beyond the frost line, Jupiter and Saturn developed as large gas giants, while Uranus and Neptune captured much less gas and are known as ice giants because their cores are believed to be made mostly of ice, that is, hydrogen compounds. The gas giants were massive enough to retain a "primary atmosphere" of hydrogen and helium captured from the surrounding solar nebula. The terrestrial planets eventually lost their retained hydrogen and helium, and subsequently generated their own "secondary atmospheres" via volcanism, comet impacts, and, also in Earth's case, the evolution of life. After 100 million years, the pressure and density of hydrogen in the centre of the collapsing nebula became great enough for the protosun to begin thermonuclear fusion, which increased until hydrostatic equilibrium was achieved. The young Sun's solar wind then cleared away all the gas and dust in the protoplanetary disk, blowing it into interstellar space, thus ending the growth of the planets. Sun
The Sun is the Solar System's parent star, and far and away its chief component. It is classed as a moderately large yellow dwarf. However, this name is misleading, as on the scale of stars in our galaxy, the Sun is rather large and bright. Stars are classified based on their position on the Hertzsprung-Russell diagram, a graph which plots the brightness of stars against their surface temperature. Generally speaking, the hotter a star is, the brighter it is. Stars which follow this pattern are said to be on the main sequence, and the Sun lies right in the middle of it. This has led many astronomy textbooks to label the Sun as "average;" however, stars brighter and hotter than it are rare, whereas stars dimmer and cooler than it are common. The vast majority of stars are dim red dwarfs, though they are under-represented in star catalogues as we can observe only those few that are very near the Sun in space. The Sun's position on the main sequence means, according to current theories of stellar evolution, that it is in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion, and been forced, as older red giants must, to fuse more inefficient elements such as helium and carbon. The Sun is growing increasingly bright as it ages. Early in its history, it was roughly 75 percent as bright as it is today.[9] Calculations of the ratios of hydrogen and helium within the Sun suggest it is roughly halfway through its life cycle, and will eventually begin moving off the main sequence, becoming larger, brighter and redder, until, about five billion years from now, it too will become a red giant. The Sun is a population I star, meaning that it is fairly new in galactic terms, having been born in the later stages of the universe's evolution. As such, it contains far more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars such as those found in globular clusters. Since elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, the first generation of stars had to die before the universe could be enriched with them. For this reason, the very oldest stars contain very little "metal", while stars born later have more. This high "metallicity" is thought to have been crucial in the Sun's developing a planetary system, because planets form from accretion of metals.[10] The Sun radiates a continuous stream of charged particles, a plasma known as solar wind, ejecting it outwards at speeds greater than 2 million kilometres per hour,[11] creating a very tenuous "atmosphere" (the heliosphere), that permeates the solar system for at least 100 AU. This environment is known as the interplanetary medium. Small quantities of cosmic dust (some of it arguably interstellar in origin) are also present in the interplanetary medium and are responsible for the phenomenon of zodiacal light. The influence of the Sun's rotating magnetic field on the interplanetary medium creates the largest structure in the solar system, the heliospheric current sheet.[12] Earth's magnetic field protects its atmosphere from interacting with the solar wind. However, Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space. Inner planetsThe four inner or terrestrial planets are characterised by their dense, rocky composition, few or no moons, and lack of ring systems. They are composed largely of minerals with high melting points such as silicates to form the planets' solid crusts and semi-liquid mantles, and metallic dust grains such as iron, which forms their cores. Three of the four inner planets have atmospheres. All have impact craters, and all but one possess tectonic surface features, such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets which are closer to the Sun than the Earth is (i.e. Mercury and Venus). The four inner planets are: MercuryMercury (0.4 AU), the closest planet to the Sun, is also the least massive of the planets, at only 0.055 Earth masses. Mercury has a very thin atmosphere consisting of atoms blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly escape into space. Thus in contrast to the Earth and Venus whose atmospheres are stable, Mercury's atmosphere is constantly being replenished.[13] Mercury is surrounded by an extremely small amount of helium, hydrogen, oxygen, and sodium. This envelope of gases is so thin that the greatest possible atmospheric pressure (force exerted by the weight of gases) on Mercury would be about 0.000000000002 kgf/cm² (0.00000000003 psi or 0.2 µPa). The atmospheric pressure on the Earth is about 1.03 kgf/cm² (14.7 psi or 101 kPa).[14] It has no natural satellite, and, to date, no observed geological activity save that produced by impacts. Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the Sun's gravity. The MESSENGER probe should aid in resolving this issue when it arrives in Mercury's orbit in 2011. VenusVenus (0.7 AU), the first truly terrestrial planet, is of comparable mass to the Earth (0.815 Earth masses), and, like Earth, possesses a thick silicate mantle around an iron core, as well as a substantial atmosphere and evidence of one-time internal geological activity, such as volcanoes. However, it is much drier than Earth and its atmosphere is 90 times as dense and is composed overwhelmingly (96.5%) of carbon dioxide. Unlike Earth, evidence suggests that Venus's crust is not divided into tectonic plates but instead comprises a single very thick rind.[15] Venus has no natural satellite. It is the hottest planet, despite being farther from the sun than Mercury, with temperatures reaching more than 400 degrees Celsius. This is most likely due to the amount of greenhouse gases in the atmosphere. EarthThe largest and densest of the inner planets, Earth (1 AU) is also the only one to demonstrate unequivocal evidence of current geological activity. Earth is the only planet known to have life. Its liquid hydrosphere, unique among the terrestrials, is probably the reason Earth is also the only planet where multi-plate tectonics has been observed, because water acts as a lubricant for subduction.[16] Its atmosphere is radically different from the other terrestrials, having been altered by the presence of life to contain 21 percent free oxygen. Its satellite, the Moon, is sometimes considered a terrestrial planet in a co-orbit with its partner, because its orbit around the Sun never actually loops back on itself when observed from above.[17] The Moon possesses many features in common with other terrestrial planets, though it lacks an iron core. MarsMars (1.5 AU), at only 0.107 Earth masses, is less massive than either Earth or Venus. It possesses a tenuous atmosphere of carbon dioxide. Its surface, peppered with vast volcanoes and rift valleys such as Valles Marineris, shows that it was once geologically active and recent evidence[18] suggests this may have been true until very recently. Mars possesses two tiny moons (Deimos and Phobos) thought to be captured asteroids. Asteroid belt
Asteroids are mostly small solar system bodies that are composed in significant part of rocky, non-volatile minerals. The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be the remnants of a small terrestrial planet that failed to coalesce due to the gravitational interference of Jupiter. Asteroids range in size from hundreds of kilometers to as small as dust. All asteroids save the largest, Ceres, are classified as small solar system bodies; however, a number of other asteroids, such as Vesta and Hygeia, could potentially be reclassed as dwarf planets if it can be conclusively shown that they are spherical. The asteroid belt contains tens of thousands - and potentially millions - of objects over one kilometre in diameter.[19] However, despite their large numbers, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth.[20] In contrast to its various depictions in science fiction, the main belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with a diameter of less than 50 m are called meteoroids. Ceres
Ceres (2.77 AU) is the largest astronomical body in the asteroid belt and the only known dwarf planet in this region. It has a diameter of slightly under 1000 km, large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the nineteenth century, but was reclassified as an asteroid as further observation revealed additional asteroids.[21] It has since been again reclassified as a dwarf planet. Asteroid groupsAsteroids in the main belt are subdivided into asteroid groups and families based on their specific orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets[22] which may have been the source of Earth's water. Trojan asteroids are located in either of Jupiter's L4 or L5 points, (gravitationally stable regions leading and trailing a planet in its orbit) though the term is also sometimes used for asteroids in any other planetary Lagrange point as well. The inner solar system is also dusted with rogue asteroids, many of which cross the orbits of the inner planets. Outer planetsThe four outer planets, or gas giants, (sometimes called Jovian planets) are so large they collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn are true giants, at 318 and 95 Earth masses, respectively, and composed largely of hydrogen and helium. Uranus and Neptune are both substantially smaller, being only 14 and 17 Earth masses, respectively. Their atmospheres contain a smaller percentage of hydrogen and helium, and a higher percentage of “ices”, such as water, ammonia and methane. For this reason some astronomers suggested that they belong in their own category, “Uranian planets,” or “ice giants.” All four of the gas giants exhibit orbital debris rings, although only the ring system of Saturn is easily observable from Earth. The term outer planet should not be confused with superior planet, which designates those planets which lie outside Earth's orbit (thus consisting of the outer planets plus Mars). JupiterJupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. Its composition of largely hydrogen and helium is not very different from that of the Sun, and the planet has been described as a "failed star". Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Three of its 63 satellites, Ganymede, Io, and Europa share elements in common with the terrestrial planets, such as volcanism and internal heating. Ganymede, the largest satellite in the Solar System, has a diameter larger than Mercury. SaturnSaturn (9.5 AU), famous for its extensive ring system, has many qualities in common with Jupiter, including its atmospheric composition, though it is far less massive, being only 95 Earth masses. Two of its 56 moons, Titan and Enceladus, show signs of geological activity, though they are largely made of ice. Titan, like Ganymede, is larger than Mercury; it is also the only satellite in the solar system with a substantial atmosphere, similar in composition to that of the atmosphere of the early Earth. UranusUranus (19.6 AU) at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt lies at over ninety degrees to the ecliptic. Its core is remarkably cold (compared with the other gas giants; it is still several thousand degrees Celsius) and radiates very little heat into space. Uranus has 27 satellites, the largest being Titania, Oberon, Umbriel, Ariel and Miranda. NeptuneNeptune (30 AU), though slightly smaller than Uranus, it is denser and slightly more massive, at 17 Earth masses, and radiates more internal heat than Uranus, but not as much as Jupiter or Saturn. Its peculiar ring system is composed of a number of dense "arcs" of material separated by gaps. Neptune has 13 moons. The largest, Triton, is geologically active, with geysers of liquid nitrogen, and is the only large satellite to revolve around its host planet in a prograde (clockwise) motion. Kuiper belt
The area beyond Neptune, often referred to as the outer solar system or simply the "trans-Neptunian region", is still largely unexplored. This region's first formation, which actually begins inside the orbit of Neptune, is the Kuiper belt, a great ring of debris, similar to the asteroid belt but composed mainly of ice and far greater in extent, which lies between 30 and 50 AU from the Sun. This region is thought to be the place of origin for short-period comets, such as Halley's comet. Though it is composed mainly of small solar system bodies, many of the largest Kuiper belt objects could soon be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km; however, the total mass of the Kuiper belt is relatively low, perhaps barely equalling the mass of the Earth.[23] Many Kuiper belt objects have multiple satellites and most have orbits that take them outside the plane of the ecliptic. The Kuiper belt can be roughly divided into two regions: the "resonant" belt, consisting of objects whose orbits are in some way linked to that of Neptune (orbiting, for instance, three times for every two Neptune orbits, or twice for every one), which actually begins within the orbit of Neptune itself, and the "classical" belt, consisting of objects that don't have any resonance with Neptune, and which extends from roughly 39.4 AU to 47.7 AU.[24] Pluto and Charon
Pluto (39 AU average), is the largest known object in the Kuiper belt and was previously accepted as the smallest planet in the Solar System. In 2006, it was reclassified as a dwarf planet by the Astronomers Congress organized by the International Astronomers Union (IAU).[25] Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion. Prior to the 2006 redefinitions, Charon was considered a moon of Pluto, but in light of the redefinition it is unclear whether Charon will continue to be classified as a moon of Pluto or as a dwarf planet itself. Charon does not exactly orbit Pluto in a traditional sense; Charon is about one-tenth the mass of Pluto and the center of gravity of the pair is not within Pluto. Both bodies orbit a barycenter of gravity above the surface of Pluto (in empty space), making Pluto-Charon a binary system. Two much smaller moons, Nix and Hydra, orbit Pluto and Charon. Those Kuiper belt objects which, like Pluto, possess a 3:2 orbital resonance with Neptune (ie, they orbit twice for every three Neptunian orbits) are called Plutinos. Other Kuiper belt objects have different resonant orbits (2:1, 4:7, 3:5 etc) and are grouped accordingly. The remaining Kuiper belt objects, in more "classical" orbits, are classified as Cubewanos. Comets
Comets are small solar system bodies (usually only a few kilometres across) composed largely of volatile ices, which possess highly eccentric orbits, generally having a perihelion within the orbit of the inner planets and an aphelion far beyond Pluto. When a comet approaches the Sun, its icy surface begins to sublimate, or boil away, creating a coma; a long tail of gas and dust which is often visible with the naked eye. There are two basic types of comet: short-period comets, with orbits less than 200 years, and long-period comets, with orbits lasting thousands of years. Short-period comets are believed to originate in the Kuiper belt, while long period comets, such as Hale-Bopp (pictured), are believed to originate in the Oort Cloud. Some comets with hyperbolic orbits may originate outside the solar system. Old comets that have had most of their volatiles driven out by solar warming are often categorized as asteroids. Centaurs are icy comet-like bodies that have less-eccentric orbits so that they remain in the region between Jupiter and Neptune. The first centaur to be discovered, 2060 Chiron, has been called a comet since it has been shown to develop a coma just as comets do when they approach the sun.[26] Scattered disc
Overlapping the Kuiper belt but extending much further outwards is the scattered disc. Scattered disc objects are believed to have been originally native to the Kuiper belt, but were ejected into erratic orbits in the outer fringes by the gravitational influence of Neptune's outward migration (see Formation and evolution of the Solar System). Most scattered disc objects have perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun. Their orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers, such as Kuiper belt co-discoverer David Jewitt, consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as "scattered Kuiper belt objects."[27] Eris
Eris (68 AU average) is the largest known scattered disc object and was the cause of the most recent debate about what constitutes a planet since it is at least 5% larger than Pluto with an estimated diameter of 2400 km (1500 mi). It is now the largest of the known dwarf planets.[28] It has one moon, Dysnomia. The object has many similarities with Pluto: its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and is steeply inclined to the ecliptic plane, at 44 degrees, more so than any known object in the solar system except the newly-discovered object 2004 XR190 (also known as "Buffy"[29]) and is believed to consist largely of rock and ice.[30] Farthest regionsThe point at which the solar system ends and interstellar space begins is not precisely defined, since its outer boundaries are delineated by two separate forces: the solar wind and the Sun's gravity. The solar wind extends to a point roughly 130 AU from the Sun, whereupon it surrenders to the surrounding environment of the interstellar medium. It is generally accepted, however, that the Sun's gravity holds sway to the Oort cloud. This great mass of up to a trillion icy objects, currently hypothetical, is believed to be the source for all long-period comets and to surround the solar system like a shell from 50,000 to 100,000 AU beyond the Sun, or almost a quarter the distance to the next star system. The vast majority of the solar system, therefore, is completely unknown; however, recent observations of both the solar system and other star systems have led to an increased understanding of what is or may be lying at its outer edge.[31] 
SednaSedna is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion and takes 12,050 years to complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper Belt as it has too distant a perihelion to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, one which also may include the object 2000 CR105, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years.[32] Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty. Heliopause
The heliosphere expands outward in a great bubble to about 95 AU, or three times the orbit of Pluto. The edge of this bubble is known as the termination shock; the point at which the solar wind collides with the opposing winds of the interstellar medium. Here the wind slows, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath that looks and behaves very much like a comet's tail; extending outward for a further 40 AU at its stellar-windward side, but tailing many times that distance in the opposite direction. The outer boundary of the sheath, the heliopause, is the point at which the solar wind finally terminates, and one enters the environment of interstellar space.[33] 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.[34] Galactic context
The solar system is located in the Milky Way galaxy, a barred spiral galaxy with a diameter estimated at about 100,000 light years containing approximately 200 billion stars. Our Sun resides in one of the Milky Way's outer spiral arms, known as the Orion Arm or Local Spur.[35] The immediate galactic neighborhood of the solar system is known as the Local Fluff, an area of dense cloud in an otherwise sparse region known as the Local Bubble, an hourglass-shaped cavity in the interstellar medium roughly 300 light-years across. The bubble is suffused with high-temperature plasma that suggests it is the product of several recent supernovae.[36] Estimates place the solar system at between 25,000 and 28,000 light years from the galactic center. Its speed is about 220 kilometres per second, and it completes one revolution every 226 million years. The apex of solar motion--that is, the direction in which the Sun is heading--is near the current location of the bright star Vega.[37] At the galactic location of the solar system, the escape velocity with regard to the gravity of the Milky Way is about 1000 km/s. 
The solar system appears to have a very remarkable orbit. It is both extremely close to being circular, and at nearly the exact distance at which the orbital speed matches the speed of the compression waves that form the spiral arms. The solar system appears to have remained between spiral arms for most of the existence of life on Earth. The radiation from supernovae in spiral arms could theoretically sterilize planetary surfaces, preventing the formation of large animal life on land. By remaining out of the spiral arms, Earth may be unusually free to form large animal life on its surface. The solar system also lies well outside the star-crowded environs of the galactic centre. The opposing gravitational tugs from so many close stars within the galactic centre would have prevented planets from forming.[38] Recent studies of Extrasolar systems neighboring Earth's have shown that our system's configuration might not be common, as the vast majority so far discovered have been found to be markedly different. For instance, many extrasolar planetary systems contain a "hot Jupiter";[39] a planet of comparable size to Jupiter that nonetheless orbits very close to its star, at, for instance, 0.05 AU. It has been hypothesised that while the giant planets in these systems formed in the same place as the gas giants in Earth's solar system did, some sort of migration took place which resulted in the giant planet spiralling in towards the parent star. Any terrestrial planets which had previously existed would presumably either be destroyed or ejected from the system. On the other hand, the apparent prevalence of hot Jupiters could result from a sampling error, as planets of similar size at greater distances from their stars are more difficult to detect. Discovery and exploration | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
