GALAXY
A galaxy is a massive, gravitationally bound system that consists of stars andstellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter.[1][2] The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way galaxy. Examples of galaxies range from dwarfs with as few as ten million (107) stars[3] to giants with a hundred trillion (1014) stars,[4] each orbiting their galaxy's own center of mass.
Galaxies contain varying amounts of star systems, star clusters and types ofinterstellar clouds. In between these objects is a sparse interstellar medium of gas, dust, and cosmic rays. Dark matter appears to account for around 90% of the massof most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are thought to be the primary driver of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object.[5]
Galaxies have been historically categorized according to their apparent shape; usually referred to as their visual morphology. A common form is the elliptical galaxy,[6] which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped with dusty, curving arms. Those with irregular or unusual shapes are known as irregular galaxies and typically originate from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in a merging, sometimes induce significantly increased incidents of star formation leading to starburst galaxies. Smaller galaxies lacking a coherent structure are referred to as irregular galaxies.[7]
There are probably more than 170 billion (1.7 × 1011) galaxies in the observable universe.[8][9] Most are 1,000 to 100,000[10] parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs).[11] Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations known as groups and clusters, which, in turn usually form larger superclusters. At the largest scale, these associations are generally arranged into sheets and filaments, which are surrounded by immense voids
Galaxies contain varying amounts of star systems, star clusters and types ofinterstellar clouds. In between these objects is a sparse interstellar medium of gas, dust, and cosmic rays. Dark matter appears to account for around 90% of the massof most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are thought to be the primary driver of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object.[5]
Galaxies have been historically categorized according to their apparent shape; usually referred to as their visual morphology. A common form is the elliptical galaxy,[6] which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped with dusty, curving arms. Those with irregular or unusual shapes are known as irregular galaxies and typically originate from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in a merging, sometimes induce significantly increased incidents of star formation leading to starburst galaxies. Smaller galaxies lacking a coherent structure are referred to as irregular galaxies.[7]
There are probably more than 170 billion (1.7 × 1011) galaxies in the observable universe.[8][9] Most are 1,000 to 100,000[10] parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs).[11] Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations known as groups and clusters, which, in turn usually form larger superclusters. At the largest scale, these associations are generally arranged into sheets and filaments, which are surrounded by immense voids
BLACKHOLE
A black hole is a region of spacetime from which nothing, not even light, can escape.[1] The theory of general relativity predicts that a sufficiently compact masswill deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return. It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics.[2] Quantum mechanics predicts that black holes emit radiation like a black body with a finitetemperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of stellar mass or greater.
Objects whose gravity field is too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was not fully appreciated for another four decades. Long considered a mathematical curiosity, it was during the 1960s that theoretical work showed black holes were a generic prediction of general relativity. The discovery ofneutron stars sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.
Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. There is general consensus that supermassive black holes exist in the centers of most galaxies. In particular, there is strong evidence of a black hole of more than 4 million solar masses at the center of our galaxy, the Milky Way.
Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with light and other electromagnetic radiation. From stellar movement, the mass and location of an invisible companion object can be calculated; in a number of cases the only known object capable of meeting these criteria is a black hole. Astronomers have identified numerous stellar black hole candidates in binary systems by studying the movement of their companion stars in this way.
Objects whose gravity field is too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was not fully appreciated for another four decades. Long considered a mathematical curiosity, it was during the 1960s that theoretical work showed black holes were a generic prediction of general relativity. The discovery ofneutron stars sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.
Black holes of stellar mass are expected to form when very massive stars collapse at the end of their life cycle. After a black hole has formed it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. There is general consensus that supermassive black holes exist in the centers of most galaxies. In particular, there is strong evidence of a black hole of more than 4 million solar masses at the center of our galaxy, the Milky Way.
Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with light and other electromagnetic radiation. From stellar movement, the mass and location of an invisible companion object can be calculated; in a number of cases the only known object capable of meeting these criteria is a black hole. Astronomers have identified numerous stellar black hole candidates in binary systems by studying the movement of their companion stars in this way.
CHECKOUT... What Happend To STAR When Its Meets The BLACKHOLE
MILKY WAY
The Milky Way is the galaxy that contains the Solar System.[10][a] This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky. The term "Milky Way" is a translation of the Latin for "milky road", Via Lactea, in turn derived from the Greek kyklos galaktikos or "milky circle", "milk" ("γάλα") also being the root for the Greek word for galaxy, γαλαξίας (galaxias).[11][12]
The galaxy has this appearance because it is a disk-shaped structure that is being viewed edge-on. Earth is located within the galactic plane of this disk, around two thirds of the way out from the center, on the inner edge of a spiral-shaped concentration of gas and dust called the Orion–Cygnus Arm. The concept of this faint band of light being made up of stars was proven in 1610 when Galileo Galileiused his telescope to resolve it into individual stars. In the 1920s observations by astronomer Edwin Hubble showed that the Milky Way was just one of around 200 billion galaxies in the observable universe.
The Milky Way is a barred spiral galaxy 100,000-120,000 light-years in diameter containing 200–400 billion stars. The galaxy is estimated to contain at least as many planets, 10 billion of which could be located in the habitable zone of their parent star.[13] Depending on its structure the entire galaxy has a rotational rate of once every 15 to 50 million years. The galaxy is also moving at a velocity of 552 to 630 km per second, depending on the relative frame of reference. It is estimated to be about 13.2 billion years old, nearly as old as the Universe. The Milky Way is part of the Local Group of galaxies
The galaxy has this appearance because it is a disk-shaped structure that is being viewed edge-on. Earth is located within the galactic plane of this disk, around two thirds of the way out from the center, on the inner edge of a spiral-shaped concentration of gas and dust called the Orion–Cygnus Arm. The concept of this faint band of light being made up of stars was proven in 1610 when Galileo Galileiused his telescope to resolve it into individual stars. In the 1920s observations by astronomer Edwin Hubble showed that the Milky Way was just one of around 200 billion galaxies in the observable universe.
The Milky Way is a barred spiral galaxy 100,000-120,000 light-years in diameter containing 200–400 billion stars. The galaxy is estimated to contain at least as many planets, 10 billion of which could be located in the habitable zone of their parent star.[13] Depending on its structure the entire galaxy has a rotational rate of once every 15 to 50 million years. The galaxy is also moving at a velocity of 552 to 630 km per second, depending on the relative frame of reference. It is estimated to be about 13.2 billion years old, nearly as old as the Universe. The Milky Way is part of the Local Group of galaxies
KUIPER BELT
The Kuiper belt ( /ˈkaɪpər/, rhyming with "viper"), sometimes called theEdgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from theSun.[1] It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive.[2][3] Like the asteroid belt, it consists mainly ofsmall bodies, or remnants from the Solar System's formation. While the asteroid belt is composed primarily of rock, ices, and metal, the Kuiper objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water. The classical (low-eccentricity) belt is home to at least three dwarf planets: Pluto,Haumea, and Makemake. Some of the Solar System's moons, such as Neptune'sTriton and Saturn's Phoebe, are also believed to have originated in the region.[4][5]
Since the belt was discovered in 1992,[6] the number of known Kuiper belt objects(KBOs) has increased to over a thousand, and more than 70,000 KBOs over 100 km (62 mi) in diameter are believed to exist.[7] The Kuiper belt was initially believed to be the main repository for periodic comets, those with orbits lasting less than 200 years. However, studies since the mid-1990s have shown that the classical belt is dynamically stable, and that comets' true place of origin is the scattered disc, a dynamically active region created by the outward motion of Neptune 4.5 billion years ago;[8] scattered disc objects such as Eris have extremely eccentric orbits that take them as far as 100 AU from the Sun.[nb 1]
Pluto is the largest known member of the Kuiper belt, if the scattered disc is excluded. Originally considered a planet, Pluto's position as part of the Kuiper belt has caused it to be reclassified as a "dwarf planet". It is compositionally similar to many other objects of the Kuiper belt, and its orbital period is identical to that of the KBOs known as "plutinos". In Pluto's honour, the four currently accepted dwarf planets beyond Neptune's orbit are called "plutoids".
The Kuiper belt should not be confused with the hypothesized Oort cloud, which is a thousand times more distant. The objects within the Kuiper belt, together with the members of thescattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to astrans-Neptunian objects (TNOs).[10]
Since the belt was discovered in 1992,[6] the number of known Kuiper belt objects(KBOs) has increased to over a thousand, and more than 70,000 KBOs over 100 km (62 mi) in diameter are believed to exist.[7] The Kuiper belt was initially believed to be the main repository for periodic comets, those with orbits lasting less than 200 years. However, studies since the mid-1990s have shown that the classical belt is dynamically stable, and that comets' true place of origin is the scattered disc, a dynamically active region created by the outward motion of Neptune 4.5 billion years ago;[8] scattered disc objects such as Eris have extremely eccentric orbits that take them as far as 100 AU from the Sun.[nb 1]
Pluto is the largest known member of the Kuiper belt, if the scattered disc is excluded. Originally considered a planet, Pluto's position as part of the Kuiper belt has caused it to be reclassified as a "dwarf planet". It is compositionally similar to many other objects of the Kuiper belt, and its orbital period is identical to that of the KBOs known as "plutinos". In Pluto's honour, the four currently accepted dwarf planets beyond Neptune's orbit are called "plutoids".
The Kuiper belt should not be confused with the hypothesized Oort cloud, which is a thousand times more distant. The objects within the Kuiper belt, together with the members of thescattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to astrans-Neptunian objects (TNOs).[10]
ASTEROID BELT
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. The asteroid belt is also termed the main asteroid belt or main belt because there are other asteroids in the Solar System such as near-Earth asteroids and trojan asteroids. About half the mass of the belt is contained in the four largest asteroids: Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea. These have mean diameters of more than 400 km, while Ceres, the asteroid belt's only identified dwarf planet, is about 950 km in diameter.[1][2][3][4] The remaining bodies range down to the size of a dust particle. The asteroid material is so thinly distributed that numerous unmanned spacecraft have traversed it without incident. Nonetheless, collisions between large asteroids do occur, and these can form anasteroid family whose members have similar orbital characteristics and compositions. Collisions also produce a fine dust that forms a major component of the zodiacal light. Individual asteroids within the asteroid belt are categorized by theirspectra, with most falling into three basic groups: carbonaceous (C-type), silicate (S-type), and metal-rich (M-type).
The asteroid belt formed from the primordial solar nebula as a group ofplanetesimals, the smaller precursors of the planets, which in turn formedprotoplanets. Between Mars and Jupiter, however, gravitational perturbations from the giant planet imbued the protoplanets with too much orbital energy for them toaccrete into a planet. Collisions became too violent, and instead of sticking together, the planetesimals and most of the protoplanets shattered. As a result, most of the asteroid belt's mass has been lost since the formation of the Solar System. Some fragments can eventually find their way into the inner Solar System, leading to meteorite impacts with the inner planets. Asteroid orbits continue to be appreciably perturbed whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. At these orbital distances, aKirkwood gap occurs as they are swept into other orbits.
Other regions of small Solar System bodies include the centaurs, the Kuiper belt and scattered disk, and the Oort cloud.
The asteroid belt formed from the primordial solar nebula as a group ofplanetesimals, the smaller precursors of the planets, which in turn formedprotoplanets. Between Mars and Jupiter, however, gravitational perturbations from the giant planet imbued the protoplanets with too much orbital energy for them toaccrete into a planet. Collisions became too violent, and instead of sticking together, the planetesimals and most of the protoplanets shattered. As a result, most of the asteroid belt's mass has been lost since the formation of the Solar System. Some fragments can eventually find their way into the inner Solar System, leading to meteorite impacts with the inner planets. Asteroid orbits continue to be appreciably perturbed whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. At these orbital distances, aKirkwood gap occurs as they are swept into other orbits.
Other regions of small Solar System bodies include the centaurs, the Kuiper belt and scattered disk, and the Oort cloud.
DWARF PLANET
A dwarf planet, as defined by the International Astronomical Union (IAU), is a celestial body in direct orbit of the Sun[1] that is massive enough that its shape is controlled by gravitational rather than mechanical forces (and thus an ellipsoid in shape), but has not cleared its neighboring regionof planetesimals.[2][3] More explicitly, it is a planetary-mass object—having sufficient mass to overcome its compressive strength and achieve hydrostatic equilibrium—but not a satellite.
The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun,[1] brought about by an increase in discoveries of trans-Neptunian objects that rivaledPluto in size, and finally precipitated by the discovery of an even more massive object, Eris.[4] This classification states that bodies large enough to have cleared the neighbourhood of their orbit are defined as planets, while those that are not massive enough to be rounded by their own gravity are defined as small Solar System bodies. Dwarf planets come in between. The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; it was said to be the "right decision" by Mike Brown,[citation needed] who discovered Eris and the other new dwarf planets accepted by the IAU, but has been rejected by Alan Stern,[5][6][cite petition here] who had coined the term dwarf planet in 1990.[7]
The IAU currently recognizes five dwarf planets in the Solar System: Ceres, Pluto, Haumea,Makemake, and Eris.[8] However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition. Eris has been accepted as a dwarf planet because it is more massive than Pluto. The IAU subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a mathematically delimited minimum diameter of 838 km)[9] are to be named under the assumption that they are dwarf planets. The only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were declared to be dwarf planets.
It is suspected that at least another fifty known objects in the Solar System are dwarf planets, and estimates are that up to 200 dwarf planets may be found when the entire region known as theKuiper belt is explored, and that the number might be as high as 2,000 when objects scattered outside the Kuiper belt are considered.[10] Mike Brown published in August 2011 his own list of 390 candidate objects, organized in categories from "nearly certainly" to "possibly" meeting the IAU's criteria, along with his classification methodology.[11] Brown identifies nine known objects – the five mentioned plus 2007 OR10, Sedna, Quaoar, and Orcus – as "virtually certain",[12] with another two dozen highly likely,[12] and there are probably a hundred or so such objects in total.[12]
The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed.
What is a planet? We've been asking that question at least since Greek astronomers came up with the word to describe the bright points of light that seemed to wander among fixed stars. Our solar system's planet count has soared as high as 15 before it was decided that some discoveries were different and should be called asteroids.
Many disagreed in 1930 when Pluto was added as our solar system's ninth planet. The debate flared again in 2005 when Eris-- about the same size as Pluto -- was found deep in a zone beyond Neptune called the Kuiper Belt. Was it the 10th planet? Or are Eris and Pluto examples of an intriguing, new kind of world?
The International Astronomical Union decided in 2006 that a new system of classification was needed to describe these new worlds, which are more developed than asteroids, but different than the known planets. Pluto, Eris and the asteroid Ceresbecame the first dwarf planets. Unlike planets, dwarf planets lack the gravitational muscle to sweep up or scatter objects near their orbits. They end up orbiting the sun in zones of similar objects such as the asteroid and Kuiper belts.
Our solar system's planet count now stands at eight. But the lively debate continues as we continue to explore and make new discoveries.In 2006 the International Astronomical Union (IAU) approved a new classification scheme for planets and smaller objects in our Solar System. Their scheme includes three classes of objects: "small solar system bodies" (including most asteroids and comets), the much larger planets (including Earth, Jupiter, and so on), and the new category of in-between sized "dwarf planets".
There are currently five official dwarf planets. Pluto, formerly the smallest of the nine "traditional" planets, was demoted to dwarf planet status. Ceres, the largest asteroid in the main asteroid belt between Mars and Jupiter, was also declared a dwarf planet. The three other (for now!) dwarf planets are Eris, Makemake, andHaumea. Pluto, Makemake, and Haumea orbit the Sun on the frozen fringes of our Solar System in the Kuiper Belt. Eris, also a Trans-Neptunian Object, is even further from the Sun.
What's the difference between regular planets and dwarf planets? As you might guess, it's partly an issue of size, with dwarf planets being smaller. But just how big does a planet need to be to become a full-fledged planet instead of a dwarf? You might think the minimum size requirement is arbitrary, but the size cutoff is actually based on other properties of the object and its history in the Solar System.
Both planets and dwarf planets orbit the Sun, not other planets (in which case we call them moons). Both must be large enough that their own gravity pulls them into the shapes of spheres; this rules out numerous smaller bodies like most asteroids, many of which have irregular shapes. Planets clear smaller objects out of their orbitsby sucking the small bodies into themselves or flinging them out of orbit. Dwarf planets, with their weaker gravities, are unable to clear out their orbits.
Though there are just five dwarf planets now, their number is expected to grow. Scientists estimate there may be 70 dwarf planets amongst outer solar system objects that have been discovered already. Since we don't know the actual sizes or shapes of many of the objects we've found (because they are so far away), we can't yet determine whether they are actually dwarf planets or not. More observations and better telescopes will help us determine which other objects are dwarf planets. Astronomers speculate that there may be 200 or so dwarf planets out through the distance of the Kuiper Belt, an icy band of frozen planetoids on the edge of our Solar System.
The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun,[1] brought about by an increase in discoveries of trans-Neptunian objects that rivaledPluto in size, and finally precipitated by the discovery of an even more massive object, Eris.[4] This classification states that bodies large enough to have cleared the neighbourhood of their orbit are defined as planets, while those that are not massive enough to be rounded by their own gravity are defined as small Solar System bodies. Dwarf planets come in between. The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized; it was said to be the "right decision" by Mike Brown,[citation needed] who discovered Eris and the other new dwarf planets accepted by the IAU, but has been rejected by Alan Stern,[5][6][cite petition here] who had coined the term dwarf planet in 1990.[7]
The IAU currently recognizes five dwarf planets in the Solar System: Ceres, Pluto, Haumea,Makemake, and Eris.[8] However, only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they fit the definition. Eris has been accepted as a dwarf planet because it is more massive than Pluto. The IAU subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a mathematically delimited minimum diameter of 838 km)[9] are to be named under the assumption that they are dwarf planets. The only two such objects known at the time, Makemake and Haumea, went through this naming procedure and were declared to be dwarf planets.
It is suspected that at least another fifty known objects in the Solar System are dwarf planets, and estimates are that up to 200 dwarf planets may be found when the entire region known as theKuiper belt is explored, and that the number might be as high as 2,000 when objects scattered outside the Kuiper belt are considered.[10] Mike Brown published in August 2011 his own list of 390 candidate objects, organized in categories from "nearly certainly" to "possibly" meeting the IAU's criteria, along with his classification methodology.[11] Brown identifies nine known objects – the five mentioned plus 2007 OR10, Sedna, Quaoar, and Orcus – as "virtually certain",[12] with another two dozen highly likely,[12] and there are probably a hundred or so such objects in total.[12]
The classification of bodies in other planetary systems with the characteristics of dwarf planets has not been addressed.
What is a planet? We've been asking that question at least since Greek astronomers came up with the word to describe the bright points of light that seemed to wander among fixed stars. Our solar system's planet count has soared as high as 15 before it was decided that some discoveries were different and should be called asteroids.
Many disagreed in 1930 when Pluto was added as our solar system's ninth planet. The debate flared again in 2005 when Eris-- about the same size as Pluto -- was found deep in a zone beyond Neptune called the Kuiper Belt. Was it the 10th planet? Or are Eris and Pluto examples of an intriguing, new kind of world?
The International Astronomical Union decided in 2006 that a new system of classification was needed to describe these new worlds, which are more developed than asteroids, but different than the known planets. Pluto, Eris and the asteroid Ceresbecame the first dwarf planets. Unlike planets, dwarf planets lack the gravitational muscle to sweep up or scatter objects near their orbits. They end up orbiting the sun in zones of similar objects such as the asteroid and Kuiper belts.
Our solar system's planet count now stands at eight. But the lively debate continues as we continue to explore and make new discoveries.In 2006 the International Astronomical Union (IAU) approved a new classification scheme for planets and smaller objects in our Solar System. Their scheme includes three classes of objects: "small solar system bodies" (including most asteroids and comets), the much larger planets (including Earth, Jupiter, and so on), and the new category of in-between sized "dwarf planets".
There are currently five official dwarf planets. Pluto, formerly the smallest of the nine "traditional" planets, was demoted to dwarf planet status. Ceres, the largest asteroid in the main asteroid belt between Mars and Jupiter, was also declared a dwarf planet. The three other (for now!) dwarf planets are Eris, Makemake, andHaumea. Pluto, Makemake, and Haumea orbit the Sun on the frozen fringes of our Solar System in the Kuiper Belt. Eris, also a Trans-Neptunian Object, is even further from the Sun.
What's the difference between regular planets and dwarf planets? As you might guess, it's partly an issue of size, with dwarf planets being smaller. But just how big does a planet need to be to become a full-fledged planet instead of a dwarf? You might think the minimum size requirement is arbitrary, but the size cutoff is actually based on other properties of the object and its history in the Solar System.
Both planets and dwarf planets orbit the Sun, not other planets (in which case we call them moons). Both must be large enough that their own gravity pulls them into the shapes of spheres; this rules out numerous smaller bodies like most asteroids, many of which have irregular shapes. Planets clear smaller objects out of their orbitsby sucking the small bodies into themselves or flinging them out of orbit. Dwarf planets, with their weaker gravities, are unable to clear out their orbits.
Though there are just five dwarf planets now, their number is expected to grow. Scientists estimate there may be 70 dwarf planets amongst outer solar system objects that have been discovered already. Since we don't know the actual sizes or shapes of many of the objects we've found (because they are so far away), we can't yet determine whether they are actually dwarf planets or not. More observations and better telescopes will help us determine which other objects are dwarf planets. Astronomers speculate that there may be 200 or so dwarf planets out through the distance of the Kuiper Belt, an icy band of frozen planetoids on the edge of our Solar System.
Pre-biotic organic matter
Pre-biotic organic matter from comets and asteroids
SEVERAL authors1–3 have suggested that comets or carbonaceous asteroids contributed large amounts of organic matter to the primitive Earth, and thus possibly played a vital role in the origin of life. But organic matter cannot survive the extremely high temperatures (> 104 K) reached on impact, which atomize the projectile and break all chemical bonds. Only fragments small enough to be gently decelerated by the atmosphere—principally meteors of 10−12–10−6 g—can deliver their organic matter intact4. The amount of such 'soft-landed' organic carbon can be estimated from data for the infall rate of meteoritic matter. At present rates, only ~0.006 g cm−2 intact organic carbon would accumulate in 108 yr, but at the higher rates of ~4 x 109 yr ago, about 20 g cm−2 may have accumulated in the few hundred million years between the last cataclysmic impact and the beginning of life. It may have included some biologically important compounds that did not form by abiotic synthesis on Earth.
SEVERAL authors1–3 have suggested that comets or carbonaceous asteroids contributed large amounts of organic matter to the primitive Earth, and thus possibly played a vital role in the origin of life. But organic matter cannot survive the extremely high temperatures (> 104 K) reached on impact, which atomize the projectile and break all chemical bonds. Only fragments small enough to be gently decelerated by the atmosphere—principally meteors of 10−12–10−6 g—can deliver their organic matter intact4. The amount of such 'soft-landed' organic carbon can be estimated from data for the infall rate of meteoritic matter. At present rates, only ~0.006 g cm−2 intact organic carbon would accumulate in 108 yr, but at the higher rates of ~4 x 109 yr ago, about 20 g cm−2 may have accumulated in the few hundred million years between the last cataclysmic impact and the beginning of life. It may have included some biologically important compounds that did not form by abiotic synthesis on Earth.
EXO PLANETS(EXTRA SOLOR PLANETS)
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of 755 such planets (in 605 planetary systems and 99 multiple planetary systems) have been identified as of January 30, 2012.[1] It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars.[2] In a 2012 study, each star of the 100 billion or so in our Milky Way Galaxy is estimated to host "on average ... at least 1.6 planets."[3][4] Accordingly, at least 160 billion star-bound planets may exist in the Milky Way Galaxy alone.[3][4] Unbound free-floating planetary-mass bodies in the Milky Way may number in the trillions with 100,000 objects larger than Pluto for every main-sequence star.[5]
For centuries, many philosophers and scientists supposed that extrasolar planets existed. But there was no way of knowing how common they were or how similar they might be to the planets of our Solar System. Various detection claims made starting in the nineteenth century were all eventually rejected by astronomers. The first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[6] The first confirmed detection of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Due to improved observational techniques, the rate of detections has increased rapidly since then.[1] Some exoplanets have been directly imaged by telescopes, but the vast majority have been detected through indirect methods such as radial velocitymeasurements.[1]
Most known exoplanets are giant planets believed to resemble Jupiter or Neptune. That reflects a sampling bias, since massive planets are easier to observe. Some relatively lightweight exoplanets, only a few times more massive than Earth (now known by the termSuper-Earth), are known as well; statistical studies now indicate that they actually outnumber giant planets[7][8] while recent discoveries have included Earth-sized and smaller planets and a handful that appear to exhibit other Earth-like properties.[9][10][11] There also exist planetary-mass objects that orbit brown dwarfs, and there exist others that "float free" in space not bound to any star, however the term "planet" isn't always applied to these objects.
The discovery of extrasolar planets has intensified interest in the possibility of extraterrestrial life.[12] Several discoveries have been made in the habitable zone a region around stars thought to be life bearing. Planetary habitability is the measure of a planetary body's potential to sustain life and considers a wide range of factors.
For centuries, many philosophers and scientists supposed that extrasolar planets existed. But there was no way of knowing how common they were or how similar they might be to the planets of our Solar System. Various detection claims made starting in the nineteenth century were all eventually rejected by astronomers. The first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[6] The first confirmed detection of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Due to improved observational techniques, the rate of detections has increased rapidly since then.[1] Some exoplanets have been directly imaged by telescopes, but the vast majority have been detected through indirect methods such as radial velocitymeasurements.[1]
Most known exoplanets are giant planets believed to resemble Jupiter or Neptune. That reflects a sampling bias, since massive planets are easier to observe. Some relatively lightweight exoplanets, only a few times more massive than Earth (now known by the termSuper-Earth), are known as well; statistical studies now indicate that they actually outnumber giant planets[7][8] while recent discoveries have included Earth-sized and smaller planets and a handful that appear to exhibit other Earth-like properties.[9][10][11] There also exist planetary-mass objects that orbit brown dwarfs, and there exist others that "float free" in space not bound to any star, however the term "planet" isn't always applied to these objects.
The discovery of extrasolar planets has intensified interest in the possibility of extraterrestrial life.[12] Several discoveries have been made in the habitable zone a region around stars thought to be life bearing. Planetary habitability is the measure of a planetary body's potential to sustain life and considers a wide range of factors.
SOLAR SYSTEM
The Solar System[a] consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formedfrom the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass (well over 99%) is in the Sun. Of the many objects that orbit the Sun, most of the mass is contained within eight relatively solitary planets[e]whose orbits are almost circular and lie within a nearly flat disc called the ecliptic plane. The four smaller inner planets, Mercury,Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, thegas giants, are substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are composed mainly of hydrogen and helium; the two outermost planets, Uranus andNeptune, are composed largely of ices, such as water, ammonia and methane, and are often referred to separately as "ice giants".
The Solar System is also home to a number of regions populated by smaller objects. The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. Beyond Neptune's orbit lie theKuiper belt and scattered disc; linked populations of trans-Neptunian objects composed mostly ofices such as water, ammonia and methane. Within these populations, five individual objects,Ceres, Pluto, Haumea, Makemake and Eris, are recognized to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets.[e] In addition to thousands ofsmall bodies[e] in those two regions, various other small body populations, such as comets,centaurs and interplanetary dust, freely travel between regions.
Six of the planets and three of the dwarf planets are orbited by natural satellites,[b] usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other particles.
The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc. The hypothetical Oort cloud, which acts as the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere.
The Solar System is located in the Milky Way galaxy, which contains about 200 billion stars.
The Solar System is also home to a number of regions populated by smaller objects. The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. Beyond Neptune's orbit lie theKuiper belt and scattered disc; linked populations of trans-Neptunian objects composed mostly ofices such as water, ammonia and methane. Within these populations, five individual objects,Ceres, Pluto, Haumea, Makemake and Eris, are recognized to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets.[e] In addition to thousands ofsmall bodies[e] in those two regions, various other small body populations, such as comets,centaurs and interplanetary dust, freely travel between regions.
Six of the planets and three of the dwarf planets are orbited by natural satellites,[b] usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other particles.
The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc. The hypothetical Oort cloud, which acts as the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere.
The Solar System is located in the Milky Way galaxy, which contains about 200 billion stars.
HELIOSPHERE
The heliosphere is a bubble in space "blown" into the interstellar medium (the hydrogen and helium gas that permeates the galaxy) by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself. It was thought for decades that it extends in a long comet-like heliotail, but in 2009 data from the Cassini and IBEX show a different shape.[1][2] However, depiction of the heliotail is still common. Another change is that the heliosheath area is not smooth but filled with magnetic bubbles.NASA 2011
For the first ten billion kilometres of its radius, the solar wind travels at over 1 000 000 km/h.[3][4] As it begins to drop out with theinterstellar medium, it slows down before finally ceasing altogether. The point where the solar wind slows down is the termination shock; then there is the heliosheath area; then the point where the interstellar medium and solar wind pressures balance is called theheliopause; the point where the interstellar medium, traveling in the opposite direction, slows down as it collides with the heliosphere is the bow shock.
As of June 2011, the heliosheath area is thought to be filled with magnetic bubbles (each about 1AU wide), creating a "foamy zone".[5] The theory helps explain in situ heliosphere measurements by the two Voyager probes.
Solar wind
Main articles: Solar wind and interplanetary medium
The solar wind consists of particles (ionized atoms from the solar corona) and fields (in particular, magnetic fields). As the Sun rotates once in approximately 27 days, the magnetic field transported by the solar wind gets wrapped into a spiral. Variations in the Sun's magnetic field are carried outward by the solar wind and can produce magnetic storms in the Earth's own magnetosphere.
In March 2005, it was reported that measurements by the Solar Wind Anisotropies (SWAN) instrument onboard the Solar and Heliospheric Observatory (SOHO) have shown that the heliosphere, the solar wind-filled volume which prevents the solar system from becoming embedded in the local (ambient) interstellar medium, is not axisymmetrical, but is distorted, very likely under the effect of the local galactic magnetic field.[6]
COMET
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma (a thin, fuzzy, temporary atmosphere) and sometimes also a tail. These phenomenaare both due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. Comets have been observed since ancient times and have traditionally been considered bad omens.
Comets have a wide range of orbital periods, ranging from a few years to hundreds of thousands of years. Short-period comets originate in the Kuiper belt, or its associated scattered disc,[1] which lie beyond the orbit of Neptune. Longer-period comets are thought to originate in the Oort cloud, a hypothesized spherical cloud of icy bodies in the outer Solar System. Long-period comets plunge towards the Sun from the Oort cloud because of gravitational perturbations caused by either the massive outer planets of the Solar System (Jupiter, Saturn, Uranus, and Neptune), or passing stars. Rare hyperbolic comets pass once through the inner Solar System before being thrown out into interstellar space along hyperbolic trajectories.
Comets are distinguished from asteroids by the presence of a coma or a tail. However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.[2] Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.[3][4]The discovery of main-belt comets and active centaurs has blurred the distinction between asteroids and comets (see asteroid terminology).
As of January 2011 there are a reported 4,185 known comets[5] of which about 1,500 are Kreutz Sungrazers and about 484 are short-period.[6] This number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population: the reservoir of comet-like bodies in the outer Solar System may number one trillion.[7] The number visible to the naked eyeaverages roughly one per year, though many of these are faint and unspectacular.[8] Particularly bright or notable examples are called "Great Comets".
Comets have a wide range of orbital periods, ranging from a few years to hundreds of thousands of years. Short-period comets originate in the Kuiper belt, or its associated scattered disc,[1] which lie beyond the orbit of Neptune. Longer-period comets are thought to originate in the Oort cloud, a hypothesized spherical cloud of icy bodies in the outer Solar System. Long-period comets plunge towards the Sun from the Oort cloud because of gravitational perturbations caused by either the massive outer planets of the Solar System (Jupiter, Saturn, Uranus, and Neptune), or passing stars. Rare hyperbolic comets pass once through the inner Solar System before being thrown out into interstellar space along hyperbolic trajectories.
Comets are distinguished from asteroids by the presence of a coma or a tail. However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids.[2] Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System.[3][4]The discovery of main-belt comets and active centaurs has blurred the distinction between asteroids and comets (see asteroid terminology).
As of January 2011 there are a reported 4,185 known comets[5] of which about 1,500 are Kreutz Sungrazers and about 484 are short-period.[6] This number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population: the reservoir of comet-like bodies in the outer Solar System may number one trillion.[7] The number visible to the naked eyeaverages roughly one per year, though many of these are faint and unspectacular.[8] Particularly bright or notable examples are called "Great Comets".
ASTEROIDS
Asteroids (from Greek ἀστεροειδής - asteroeidēs, "star-like",[1] from ἀστήρ "star" and εἶδος"like, in form") are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disk of a planet and was not observed to have the characteristics of an active comet, but as small objects in the outer Solar System were discovered, their volatile-based surfaces were found to more closely resemble comets, and so were often distinguished from traditional asteroids.[2] Thus the term asteroidhas come increasingly to refer specifically to the small bodies of the inner Solar System out to the orbit of Jupiter, which are usually rocky or metallic. They are grouped with the outer bodies--centaurs, Neptune trojans, and trans-Neptunian objects—as minor planets, which is the term preferred in astronomical circles.[3] This article will restrict the use of the term 'asteroid' to the minor planets of the inner Solar System.
There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun’s solar nebula that never grew large enough to become planets.[4]A large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter or co-orbital with Jupiter (the Jupiter Trojans). However, other orbital families exist with significant populations, including the near-Earth asteroids. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, S-type, andM-type. These were named after and are generally identified with carbon-rich, stony, andmetallic compositions, respectively.
There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun’s solar nebula that never grew large enough to become planets.[4]A large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter or co-orbital with Jupiter (the Jupiter Trojans). However, other orbital families exist with significant populations, including the near-Earth asteroids. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, S-type, andM-type. These were named after and are generally identified with carbon-rich, stony, andmetallic compositions, respectively.