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Star Racer
Description
You have to drive your space mobile on different tracks and try to defeat all your opponents. You have 3 different space mobiles to choose from and four different racing circuits on distant planets. Clear the checkpoints before the time runs out. Avoid crashing your space mobile or you will lose speed. Go ahead and win all the tracks...
Instructions
� Movements = Arrow Keys
The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece, the two great luminaries the Sun and the Moon were called Helios and Selene; the farthest planet (Saturn) was called Phainon, the shiner; followed by Phaethon (Jupiter), "bright"; the red planet (Mars) was known as Pyroeis, the "fiery"; the brightest (Venus) was known as Phosphoros, the light bringer; and the fleeting final planet (Mercury) was called Stilbon, the gleamer. The Greeks also made each planet sacred to one among their pantheon of gods, the Olympians: Helios and Selene were the names of both planets and gods; Phainon was sacred to Cronus, the Titan who fathered the Olympians; Phaethon was sacred to Zeus, Cronus's son who deposed him as king; Pyroeis was given to Ares, son of Zeus and god of war; Phosphoros was ruled by Aphrodite, the goddess of love; and Hermes, messenger of the gods and god of learning and wit, ruled over Stilbon.[19]
The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphoros after their goddess of love, Ishtar; Pyroeis after their god of war, Nergal, Stilbon after their god of wisdom Nabu, and Phaethon after their chief god, Marduk.[66] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.[19] The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. However, unlike Ares, Nergal was also god of pestilence and the underworld.[67]
Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church, use the Roman (Latin) names rather than the Greek ones. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.[68] When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius (for Hermes), Venus (Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus (Cronus). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained with Neptūnus (Poseidon). Uranus is unique in that it is named for a Greek deity rather than his Roman counterpart.
Some Romans, following a belief possibly originating in Mesopotamia but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet).[69] Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Since each day was named by the god that started it, this is also the order of the days of the week in the Roman calendar after the Nundinal cycle was rejected – and still preserved in many modern languages.[70] In English, Saturday, Sunday, and Monday are straightforward translations of these Roman names. The other days were renamed after Tiw, (Tuesday) Wóden (Wednesday), Thunor (Thursday), and Fríge (Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus, respectively.
Earth is the only planet whose name in English is not derived from Greco-Roman mythology. Since it was only generally accepted as a planet in the 17th century,[36] there is no tradition of naming it after a god. (The same is true, in English at least, of the Sun and the Moon, though they are no longer generally considered planets.) The name originates from the 8th century Anglo-Saxon word erda, which means ground or soil and was first used in writing as the name of the sphere of the Earth perhaps around 1300.[71][72] As with its equivalents in the other Germanic languages, it derives ultimately from the Proto-Germanic word ertho, "ground",[72] as can be seen in the English earth, the German Erde, the Dutch aarde, and the Scandinavian jord. Many of the Romance languages retain the old Roman word terra (or some variation of it) that was used with the meaning of "dry land" as opposed to "sea".[73] However, the non-Romance languages use their own native words. The Greeks retain their original name, Γή (Ge).
Non-European cultures use other planetary-naming systems. India uses a system based on the Navagraha, which incorporates the seven traditional planets (Surya for the Sun, Chandra for the Moon, and Budha, Shukra, Mangala, Bṛhaspati and Shani for Mercury, Venus, Mars, Jupiter and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China and the countries of eastern Asia historically subject to Chinese cultural influence (such as Japan, Korea and Vietnam) use a naming system based on the five Chinese elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth (Saturn).[
Formation
Main article: Nebular hypothesis
It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets.[74] After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag.[75]
An artist's impression of protoplanetary disk
When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting–Robertson drag and other effects.[76][77] Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.[78] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies.
The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core.[79] Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets.[80] (Smaller planets will lose any atmosphere they gain through various escape mechanisms.)
With the discovery and observation of planetary systems around stars other than our own, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity – an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 (helium) – is now believed to determine the likelihood that a star will have planets.[81] Hence, it is thought that a metal-rich population I star will likely possess a more substantial planetary system than a metal-poor, population II star.
Solar System
Planets and dwarf planets of the Solar System (Sizes to scale, distances not to scale)
The inner planets. From left to right: Mercury, Venus, Earth and Mars in true-color. (Sizes to scale, distances not to scale)
The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune (Sizes to scale, distances not to scale)
Main article: Solar System
See also: List of gravitationally rounded objects of the Solar System
According to the IAU, there are eight planets and five recognized dwarf planets in the Solar System. In increasing distance from the Sun, the planets are:
☿ Mercury
♀ Venus
⊕ Earth
♂ Mars
♃ Jupiter
♄ Saturn
♅ Uranus
♆ Neptune
Jupiter is the largest, at 318 Earth masses, while Mercury is smallest, at 0.055 Earth masses.
The planets of the Solar System can be divided into categories based on their composition:
Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock: Mercury, Venus, Earth and Mars. At 0.055 Earth masses, Mercury is the smallest terrestrial planet (and smallest planet) in the Solar System, while Earth is the largest terrestrial planet.
Gas giants (Jovians): Planets largely composed of gaseous material and significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Jupiter, at 318 Earth masses, is the largest planet in the Solar System, while Saturn is one third as big, at 95 Earth masses.
Ice giants, comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass (only 14 and 17 Earth masses), and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice.
Dwarf planets: Before 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 five dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto, Haumea, Makemake 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. By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto, Haumea, and Makemake are members of the Kuiper belt and Eris is a member of the scattered disc. Scientists such as Mike Brown believe that there are probably over one hundred trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.
In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[43] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. Of the 908 extrasolar planets discovered by 6 July 2013,[5] most have masses which are comparable to or larger than Jupiter's, though masses ranging from just below that of Mercury to many times Jupiter's mass have been observed.[5] The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12.[84]
There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses,[5] such as those orbiting the stars Mu Arae, 55 Cancri and GJ 436.[85]
Another new category are the so-called "super-Earths", possibly terrestrial planets larger than Earth but smaller than Neptune or Uranus. To date, about twenty possible super-Earths (depending on mass limits) have been found, including OGLE-2005-BLG-390Lb and MOA-2007-BLG-192Lb, frigid icy worlds discovered through gravitational microlensing,[86][87] Kepler 10b, a planet with a diameter roughly 1.4 times that of Earth, (making it the smallest super-Earth yet measured)[88] and five of the six planets orbiting the nearby red dwarf Gliese 581. Gliese 581 d is roughly 7.7 times Earth's mass,[89] while Gliese 581 c is five times Earth's mass and was initially thought to be the first terrestrial planet found within a star's habitable zone.[90] However, more detailed studies revealed that it was slightly too close to its star to be habitable, and that the farther planet in the system, Gliese 581 d, though it is much colder than Earth, could potentially be habitable if its atmosphere contained sufficient amounts of greenhouse gases.[91] Another super-Earth, Kepler-22b, was later confirmed to be orbiting comfortably within the habitable zone of its star.[92] On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-size extrasolar planets, Kepler-20e[6] and Kepler-20f,[7] orbiting a Sun-like star, Kepler-20.[8][9][10]
Comparison of Kepler-20e[6] and Kepler-20f[7] with Venus and Earth.
It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. Also, a class of hot Jupiters may exist called Chthonian planets, that orbit so close to their star that their atmospheres have been blown away completely by stellar radiation. While many hot Jupiters have been found in the process of losing their atmospheres, as of 2008, no genuine Chthonian planets have been discovered.[93]
Size comparison of HR 8799 c (gray) with Jupiter. Most exoplanets discovered thus far are larger than Jupiter.
More detailed observation of extrasolar planets will require a new generation of instruments, including space telescopes. Currently the COROT and Kepler spacecraft are searching for stellar luminosity variations due to transiting planets. Several projects have also been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. These include the proposed NASA's, Terrestrial Planet Finder, and Space Interferometry Mission programs, and the CNES' PEGASE.[94] The New Worlds Mission is an occulting device that may work in conjunction with the James Webb Space Telescope. However, funding for some of these projects remains uncertain. The first spectra of extrasolar planets were reported in February 2007 (HD 209458 b and HD 189733 b).[95][96] The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.[97]
Planetary-mass objects
A planetary-mass object, PMO, or planemo is a celestial object with a mass that falls within the range of the definition of a planet: massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain core fusion like a star.[98] By definition, all planets are planetary-mass objects, but the purpose of the term is to describe objects which do not conform to typical expectations for a planet. These include dwarf planets, the larger moons, free-floating planets not orbiting a star, such as rogue planets ejected from their system, and objects that formed through cloud-collapse rather than accretion (sometimes called sub-brown dwarfs).
Rogue planets
Main article: Rogue planet
Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.[99] Some scientists have argued that such objects found roaming in deep space should be classed as "planets", although others have suggested that they could be low-mass stars.[100][101]
Sub-brown dwarfs
Main article: Sub-brown dwarf
Stars form via the gravitational collapse of gas clouds, but smaller objects can also form via cloud-collapse. Planetary-mass objects formed this way are sometimes called sub-brown dwarfs. Sub-brown dwarfs may be free-floating such as Cha 110913-773444, or orbiting a larger object such as 2MASS J04414489+2301513.
For a brief time in 2006, astronomers believed they had found a binary system of such objects, Oph 162225-240515, which the discoverers described as "planemos", or "planetary-mass objects". However, recent analysis of the objects has determined that their masses are probably each greater than 13 Jupiter-masses, making the pair brown dwarfs.[102][103][104]
Former stars
In close binary star systems one of the stars can lose mass to a heavier companion. See accretion-powered pulsars. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the pulsar PSR J1719-1438.[105]
Satellite planets and belt planets
Some large satellites are of similar size or larger than the planet Mercury, e.g. Jupiter's Galilean moons and Titan. Alan Stern has argued that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet, and proposes the term satellite planet for a planet-sized satellite. Likewise, dwarf planets in the asteroid belt and Kuiper belt should be considered planets according to Stern.[106]
Attributes
Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whilst others are also commonly observed in extrasolar planets.
You have to drive your space mobile on different tracks and try to defeat all your opponents. You have 3 different space mobiles to choose from and four different racing circuits on distant planets. Clear the checkpoints before the time runs out. Avoid crashing your space mobile or you will lose speed. Go ahead and win all the tracks...
Instructions
� Movements = Arrow Keys
The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece, the two great luminaries the Sun and the Moon were called Helios and Selene; the farthest planet (Saturn) was called Phainon, the shiner; followed by Phaethon (Jupiter), "bright"; the red planet (Mars) was known as Pyroeis, the "fiery"; the brightest (Venus) was known as Phosphoros, the light bringer; and the fleeting final planet (Mercury) was called Stilbon, the gleamer. The Greeks also made each planet sacred to one among their pantheon of gods, the Olympians: Helios and Selene were the names of both planets and gods; Phainon was sacred to Cronus, the Titan who fathered the Olympians; Phaethon was sacred to Zeus, Cronus's son who deposed him as king; Pyroeis was given to Ares, son of Zeus and god of war; Phosphoros was ruled by Aphrodite, the goddess of love; and Hermes, messenger of the gods and god of learning and wit, ruled over Stilbon.[19]
The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphoros after their goddess of love, Ishtar; Pyroeis after their god of war, Nergal, Stilbon after their god of wisdom Nabu, and Phaethon after their chief god, Marduk.[66] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.[19] The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. However, unlike Ares, Nergal was also god of pestilence and the underworld.[67]
Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. While modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church, use the Roman (Latin) names rather than the Greek ones. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.[68] When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius (for Hermes), Venus (Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus (Cronus). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained with Neptūnus (Poseidon). Uranus is unique in that it is named for a Greek deity rather than his Roman counterpart.
Some Romans, following a belief possibly originating in Mesopotamia but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet).[69] Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Since each day was named by the god that started it, this is also the order of the days of the week in the Roman calendar after the Nundinal cycle was rejected – and still preserved in many modern languages.[70] In English, Saturday, Sunday, and Monday are straightforward translations of these Roman names. The other days were renamed after Tiw, (Tuesday) Wóden (Wednesday), Thunor (Thursday), and Fríge (Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus, respectively.
Earth is the only planet whose name in English is not derived from Greco-Roman mythology. Since it was only generally accepted as a planet in the 17th century,[36] there is no tradition of naming it after a god. (The same is true, in English at least, of the Sun and the Moon, though they are no longer generally considered planets.) The name originates from the 8th century Anglo-Saxon word erda, which means ground or soil and was first used in writing as the name of the sphere of the Earth perhaps around 1300.[71][72] As with its equivalents in the other Germanic languages, it derives ultimately from the Proto-Germanic word ertho, "ground",[72] as can be seen in the English earth, the German Erde, the Dutch aarde, and the Scandinavian jord. Many of the Romance languages retain the old Roman word terra (or some variation of it) that was used with the meaning of "dry land" as opposed to "sea".[73] However, the non-Romance languages use their own native words. The Greeks retain their original name, Γή (Ge).
Non-European cultures use other planetary-naming systems. India uses a system based on the Navagraha, which incorporates the seven traditional planets (Surya for the Sun, Chandra for the Moon, and Budha, Shukra, Mangala, Bṛhaspati and Shani for Mercury, Venus, Mars, Jupiter and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China and the countries of eastern Asia historically subject to Chinese cultural influence (such as Japan, Korea and Vietnam) use a naming system based on the five Chinese elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth (Saturn).[
Formation
Main article: Nebular hypothesis
It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets.[74] After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an extended atmosphere, greatly increasing the capture rate of the planetesimals by means of atmospheric drag.[75]
An artist's impression of protoplanetary disk
When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting–Robertson drag and other effects.[76][77] Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.[78] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Meanwhile, protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies.
The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core.[79] Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets.[80] (Smaller planets will lose any atmosphere they gain through various escape mechanisms.)
With the discovery and observation of planetary systems around stars other than our own, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity – an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 (helium) – is now believed to determine the likelihood that a star will have planets.[81] Hence, it is thought that a metal-rich population I star will likely possess a more substantial planetary system than a metal-poor, population II star.
Solar System
Planets and dwarf planets of the Solar System (Sizes to scale, distances not to scale)
The inner planets. From left to right: Mercury, Venus, Earth and Mars in true-color. (Sizes to scale, distances not to scale)
The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune (Sizes to scale, distances not to scale)
Main article: Solar System
See also: List of gravitationally rounded objects of the Solar System
According to the IAU, there are eight planets and five recognized dwarf planets in the Solar System. In increasing distance from the Sun, the planets are:
☿ Mercury
♀ Venus
⊕ Earth
♂ Mars
♃ Jupiter
♄ Saturn
♅ Uranus
♆ Neptune
Jupiter is the largest, at 318 Earth masses, while Mercury is smallest, at 0.055 Earth masses.
The planets of the Solar System can be divided into categories based on their composition:
Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock: Mercury, Venus, Earth and Mars. At 0.055 Earth masses, Mercury is the smallest terrestrial planet (and smallest planet) in the Solar System, while Earth is the largest terrestrial planet.
Gas giants (Jovians): Planets largely composed of gaseous material and significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Jupiter, at 318 Earth masses, is the largest planet in the Solar System, while Saturn is one third as big, at 95 Earth masses.
Ice giants, comprising Uranus and Neptune, are a sub-class of gas giants, distinguished from gas giants by their significantly lower mass (only 14 and 17 Earth masses), and by depletion in hydrogen and helium in their atmospheres together with a significantly higher proportion of rock and ice.
Dwarf planets: Before 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 five dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto, Haumea, Makemake 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. By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto, Haumea, and Makemake are members of the Kuiper belt and Eris is a member of the scattered disc. Scientists such as Mike Brown believe that there are probably over one hundred trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.
In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[43] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that survived the supernova and then decayed into their current orbits.
The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. Of the 908 extrasolar planets discovered by 6 July 2013,[5] most have masses which are comparable to or larger than Jupiter's, though masses ranging from just below that of Mercury to many times Jupiter's mass have been observed.[5] The smallest extrasolar planets found to date have been discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12.[84]
There have been roughly a dozen extrasolar planets found of between 10 and 20 Earth masses,[5] such as those orbiting the stars Mu Arae, 55 Cancri and GJ 436.[85]
Another new category are the so-called "super-Earths", possibly terrestrial planets larger than Earth but smaller than Neptune or Uranus. To date, about twenty possible super-Earths (depending on mass limits) have been found, including OGLE-2005-BLG-390Lb and MOA-2007-BLG-192Lb, frigid icy worlds discovered through gravitational microlensing,[86][87] Kepler 10b, a planet with a diameter roughly 1.4 times that of Earth, (making it the smallest super-Earth yet measured)[88] and five of the six planets orbiting the nearby red dwarf Gliese 581. Gliese 581 d is roughly 7.7 times Earth's mass,[89] while Gliese 581 c is five times Earth's mass and was initially thought to be the first terrestrial planet found within a star's habitable zone.[90] However, more detailed studies revealed that it was slightly too close to its star to be habitable, and that the farther planet in the system, Gliese 581 d, though it is much colder than Earth, could potentially be habitable if its atmosphere contained sufficient amounts of greenhouse gases.[91] Another super-Earth, Kepler-22b, was later confirmed to be orbiting comfortably within the habitable zone of its star.[92] On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-size extrasolar planets, Kepler-20e[6] and Kepler-20f,[7] orbiting a Sun-like star, Kepler-20.[8][9][10]
Comparison of Kepler-20e[6] and Kepler-20f[7] with Venus and Earth.
It is far from clear if the newly discovered large planets would resemble the gas giants in the Solar System or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in the Solar System, which makes it questionable whether they are the same type of planet at all. Also, a class of hot Jupiters may exist called Chthonian planets, that orbit so close to their star that their atmospheres have been blown away completely by stellar radiation. While many hot Jupiters have been found in the process of losing their atmospheres, as of 2008, no genuine Chthonian planets have been discovered.[93]
Size comparison of HR 8799 c (gray) with Jupiter. Most exoplanets discovered thus far are larger than Jupiter.
More detailed observation of extrasolar planets will require a new generation of instruments, including space telescopes. Currently the COROT and Kepler spacecraft are searching for stellar luminosity variations due to transiting planets. Several projects have also been proposed to create an array of space telescopes to search for extrasolar planets with masses comparable to the Earth. These include the proposed NASA's, Terrestrial Planet Finder, and Space Interferometry Mission programs, and the CNES' PEGASE.[94] The New Worlds Mission is an occulting device that may work in conjunction with the James Webb Space Telescope. However, funding for some of these projects remains uncertain. The first spectra of extrasolar planets were reported in February 2007 (HD 209458 b and HD 189733 b).[95][96] The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.[97]
Planetary-mass objects
A planetary-mass object, PMO, or planemo is a celestial object with a mass that falls within the range of the definition of a planet: massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain core fusion like a star.[98] By definition, all planets are planetary-mass objects, but the purpose of the term is to describe objects which do not conform to typical expectations for a planet. These include dwarf planets, the larger moons, free-floating planets not orbiting a star, such as rogue planets ejected from their system, and objects that formed through cloud-collapse rather than accretion (sometimes called sub-brown dwarfs).
Rogue planets
Main article: Rogue planet
Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.[99] Some scientists have argued that such objects found roaming in deep space should be classed as "planets", although others have suggested that they could be low-mass stars.[100][101]
Sub-brown dwarfs
Main article: Sub-brown dwarf
Stars form via the gravitational collapse of gas clouds, but smaller objects can also form via cloud-collapse. Planetary-mass objects formed this way are sometimes called sub-brown dwarfs. Sub-brown dwarfs may be free-floating such as Cha 110913-773444, or orbiting a larger object such as 2MASS J04414489+2301513.
For a brief time in 2006, astronomers believed they had found a binary system of such objects, Oph 162225-240515, which the discoverers described as "planemos", or "planetary-mass objects". However, recent analysis of the objects has determined that their masses are probably each greater than 13 Jupiter-masses, making the pair brown dwarfs.[102][103][104]
Former stars
In close binary star systems one of the stars can lose mass to a heavier companion. See accretion-powered pulsars. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the pulsar PSR J1719-1438.[105]
Satellite planets and belt planets
Some large satellites are of similar size or larger than the planet Mercury, e.g. Jupiter's Galilean moons and Titan. Alan Stern has argued that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet, and proposes the term satellite planet for a planet-sized satellite. Likewise, dwarf planets in the asteroid belt and Kuiper belt should be considered planets according to Stern.[106]
Attributes
Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whilst others are also commonly observed in extrasolar planets.
