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Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind.
Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System so strong in fact that it poses a serious health risk to future manned missions to its moons.
The magnetic fields of the other giant planets are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger.
The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet.
In , a team of astronomers in Hawaii observed an extrasolar planet around the star HD , which appeared to be creating a sunspot on the surface of its parent star.
Several planets or dwarf planets in the Solar System such as Neptune and Pluto have orbital periods that are in resonance with each other or with smaller bodies this is also common in satellite systems.
All except Mercury and Venus have natural satellites , often called "moons". Earth has one, Mars has two, and the giant planets have numerous moons in complex planetary-type systems.
Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life especially Europa.
The four giant planets are also orbited by planetary rings of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny ' moonlets ' whose gravity shapes and maintains their structure.
Although the origins of planetary rings is not precisely known, they are believed to be the result of natural satellites that fell below their parent planet's Roche limit and were torn apart by tidal forces.
No secondary characteristics have been observed around extrasolar planets. The sub-brown dwarf Cha , which has been described as a rogue planet , is believed to be orbited by a tiny protoplanetary disc  and the sub-brown dwarf OTS 44 was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses.
From Wikipedia, the free encyclopedia. Class of astronomical body directly orbiting a star or stellar remnant. This article is about the astronomical object.
For other uses, see Planet disambiguation. History of astronomy , Definition of planet , and Timeline of Solar System astronomy.
Indian astronomy and Hindu cosmology. Astronomy in the medieval Islamic world and Cosmology in medieval Islam. IAU definition of planet.
Weekday names and Naked-eye planet. Supernova remnant ejecta producing planet-forming material. Solar System — sizes but not distances are to scale.
The Sun and the eight planets of the Solar System. The inner planets , Mercury , Venus , Earth , and Mars.
List of gravitationally rounded objects of the Solar System. Orbit and Orbital elements. Kepler's laws of planetary motion.
Atmosphere and Extraterrestrial atmospheres. Natural satellite and Planetary ring. Astronomy portal Solar System portal Space portal.
The official definition applies only to the Solar System, whereas the definition applies to planets around other stars.
The extrasolar planet issue was deemed too complex to resolve at the IAU conference. The term "satellite" had already begun to be used to distinguish such bodies from those around which they orbited "primary planets".
Result of the IAU Resolution votes". Archived from the original on Retrieved 10 May The Extrasolar Planets Encyclopaedia.
Retrieved 11 January Explicit use of et al. The Library of Congress. Retrieved 29 June Journal of Near Eastern Studies.
Astronomy in China, Korea and Japan Walker ed. The History and Practice of Ancient Astronomy. Civilizations of the Ancient Near East.
A History of Horoscopic Astrology. Astrological reports to Assyrian kings. State Archives of Assyria. Enuma Anu Enlil, Tablet The Venus Tablet of Ammisaduqa".
Journal of the American Oriental Society. Mare Kõiva; Andres Kuperjanov, eds. Electronic Journal of Folklore.
Sachs May 2, Philosophical Transactions of the Royal Society. Royal Society of London. Journal for the History of Astronomy.
Bulletin of the Astronomical Society of India. The Biographical Encyclopedia of Astronomers. History of oriental astronomy: Retrieved 11 February Monthly Notices of the Royal Astronomical Society.
Annual Review of Astronomy and Astrophysics. Archived from the original on June 24, University of California and the Carnegie Institution.
The Nature and Meaning of Planethood". Spitzer Finds Cosmic Oddball". What is a Planet? Brown Dwarfs, Gas Giant Planets, and?
The Beginnings of Western Science 2nd ed. The University of Chicago Press. Decouverte de deux Nouvelles Planetes autour de Saturne.
Greek Mythography in the Roman World. The Seven Day Circle: The History and Meaning of the Week. University of Chicago Press.
Journal of the Royal Astronomical Society of Canada. Yoel Natan — via Google Books. Grazer Morgenländischen Symposion GrazKult — via Google Books.
Eisenbrauns — via Google Books. Enhancement of core accretion by a voluminous low-mass envelope". PhD thesis, University of Massachusetts Amherst.
Lay summary — Kenyon, Scott J. Harvard-Smithsonian Center for Astrophysics. Carnegie Institution for Science. The Extrasolar Planets Encyclopedia.
Retrieved 1 November Retrieved February 26, Radius as a Proxy for Composition". How common are habitable planets? Proceedings of the National Academy of Sciences.
Retrieved 22 February May , "Planetesimals to Brown Dwarfs: Alan; Levison, Harold F. Astronomical Society of the Pacific, 12 , pp.
Retrieved January 26, Spitzer Space Telescope Newsroom. Archived from the original on July 11, Disk and accretion at the planetary border".
Not a Planetary-Mass Binary". Chaos And Stability in Planetary Systems. Period Estimated from Radar Measurements".
California Institute of Technology. Publications of the Astronomical Society of the Pacific. Department of Physics, University of Oregon. Planetary and Space Science.
University of Arizona Press, pp. Lay summary — Center for Astrophysics press release Space Telescope Science Institute.
Lay summary — NASA press release Encyclopedia of the Solar System. New Scientist Print Edition. Bulletin of the American Astronomical Society. The Solar System Third ed.
Solar System objects by size by discovery date Minor planets names Gravitationally rounded objects Possible dwarf planets Natural satellites Comets.
Exoplanet Methods of detecting exoplanets Planetary system. Accretion Merging stars Nebular hypothesis Planetary migration.
Astrometry Direct imaging list Microlensing list Polarimetry Pulsar timing list Radial velocity list Transit method list Transit-timing variation.
En stjärna bildas, när jättelika instabila gas och stoftmoln genom sin egen gravitation drar sig samman, komprimeras och bildar en protostjärna.
De lokala koncentrationerna av materia komprimeras och bildar i sin tur protoplaneter. Där finns nämligen gasen kvar och kan kondensera till planeter.
Internationella Astronomiska Unionen IAU har nyligen etablerat en entydig definition av vad som ska räknas som planet och vad som inte ska göra det, vilket medförde att Pluto sedan 24 augusti i stället räknas som en dvärgplanet.
Mellan och upptäcktes de fyra första asteroiderna, Ceres, Pallas, Juno och Vesta. Numera vet man att nästan alla asteroider är kollisionsfragment.
Stenplaneter eller jordlika planeter kallas de planeter som har en fast yta i form av en skorpa av sten. Dessa är de inre, eller jordlika, planeterna Merkurius , Venus , Jorden och Mars.
Uppbyggnaden hos de jordlika planeterna antas vara liknande, nämligen med en fast skorpa, en mantel av flytande materia och längst in en kärna av flytande metall.
Särskilt Venus och Jorden har en aktiv geologi där vulkanutbrott och jordbävningar ständigt förändrar planetens yta. It has a rocky core.
Neptune was the first planet to be predicted to exist by using math, before it was detected. Irregularities in the orbit of Uranus led French astronomer Alexis Bouvard to suggest some other might be exerting a gravitational tug.
German astronomer Johann Galle used calculations to help find Neptune in a telescope. Neptune is about 17 times as massive as Earth.
Once the ninth planet from the sun, Pluto is unlike other planets in many respects. It is smaller than Earth's moon. Its orbit carries it inside the orbit of Neptune and then way out beyond that orbit.
From until early , Pluto had actually been the eighth planet from the sun. Pluto will stay beyond Neptune for years. New Horizons' Pluto Flyby: Latest News, Images and Video ].
The orbit of Neptune is 2. The strange world's orbit is about times farther from the sun than the Earth's orbit is from the star.
Scientists have not actually seen Planet Nine directly. Its existence was inferred by its gravitational effects on other objects in the Kuiper Belt, a region at the fringe of the solar system that is home to icy objects left over from the birth of the sun and planets.
The research is based on mathematical models and computer simulations using observations of six other smaller Kuiper Belt Objects with orbits that aligned in a similar matter.
Rob has been producing internet content since the mids. He served as Managing Editor of LiveScience since its launch in He now oversees news operations for the TechMediaNetwork's growing suite of technology, science and business news sites.
He has a journalism degree from Humboldt State University in California. By Robert Roy Britt November 14, The planets of the solar system as depicted by a NASA computer illustration.
Orbits and sizes are not shown to scale. Mars researchers are focusing both Earth-based and planet orbiting sensors to better understand sources of methane on the red planet.
The shadow of Saturn's moon Mimas dips onto the planet's rings and straddles the Cassini Division in this natural color image taken as Saturn approaches its August equinox.
Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which the planet is tilted.
Pluto and its moons orbit the sun near the edge of our solar system. Learn all about Pluto's weirdly eccentric orbit, four moons and more in this Space.
Robert Roy Britt, Rob has been producing internet content since the mids.Die Gas- und Staubwolke wurde Beste Spielothek in Aathal finden durch die Gezeitenkräfte zerrissen und fragmentiert. In der Staubscheibe um einen jungen Stern slot machine game maker ein Forschungsteam Strukturen beobachtet, die mit hoher Geschwindigkeit vom Zentrum wegfliegen. Sie gibt das Verhältnis der Masse eines Körpers zu der Masse der sonstigen Objekte in seiner Umlaufbahn an, sofern es sich dabei um keine Monde oder resonant umlaufende Himmelskörper handelt. This flying iceberg still conserves the materials from which the planets originated 4. Tom Dooley und die junge, aktive Der drucker shop.
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Pythagoras or Parmenides is said to have been the first to identify the evening star Hesperos and morning star Phosphoros as one and the same Aphrodite , Greek corresponding to Latin Venus ,  though this had long been known by the Babylonians.
In the 3rd century BC, Aristarchus of Samos proposed a heliocentric system, according to which Earth and the planets revolved around the Sun.
The geocentric system remained dominant until the Scientific Revolution. By the 1st century BC, during the Hellenistic period , the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets.
These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness, and account for most of the astronomical movements observed from Earth with the naked eye.
These theories would reach their fullest expression in the Almagest written by Ptolemy in the 2nd century CE.
So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries.
They were, in increasing order from Earth in Ptolemy's order and using modern names: Cicero , in his De Natura Deorum , enumerated the planets known during the 1st century BCE using the names for them in use at the time: In CE, the Indian astronomer Aryabhata propounded a planetary model that explicitly incorporated Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars.
He also believed that the orbits of planets are elliptical. In , Nilakantha Somayaji of the Kerala school of astronomy and mathematics , in his Tantrasangraha , revised Aryabhata's model.
Most astronomers of the Kerala school who followed him accepted his planetary model. In the 11th century, the transit of Venus was observed by Avicenna , who established that Venus was, at least sometimes, below the Sun.
With the advent of the Scientific Revolution , use of the term "planet" changed from something that moved across the sky in relation to the star field ; to a body that orbited Earth or that was believed to do so at the time ; and by the 18th century to something that directly orbited the Sun when the heliocentric model of Copernicus , Galileo and Kepler gained sway.
Thus, Earth became included in the list of planets,  whereas the Sun and Moon were excluded. At first, when the first satellites of Jupiter and Saturn were discovered in the 17th century, the terms "planet" and "satellite" were used interchangeably — although the latter would gradually become more prevalent in the following century.
In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century such as Ceres , Pallas , Juno , and Vesta were very different from the traditional ones.
These bodies shared the same region of space between Mars and Jupiter the asteroid belt , and had a much smaller mass; as a result they were reclassified as " asteroids ".
In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun.
Because there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in , there was no apparent need to have a formal definition.
In the 20th century, Pluto was discovered. After initial observations led to the belief that it was larger than Earth,  the object was immediately accepted as the ninth planet.
Further monitoring found the body was actually much smaller: Then, on October 6, , Michel Mayor and Didier Queloz of the Geneva Observatory announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star 51 Pegasi.
The discovery of extrasolar planets led to another ambiguity in defining a planet: Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as brown dwarfs.
Brown dwarfs are generally considered stars due to their ability to fuse deuterium , a heavier isotope of hydrogen.
Although objects more massive than 75 times that of Jupiter fuse hydrogen, objects of only 13 Jupiter masses can fuse deuterium.
Deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.
With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet.
There were particular disagreements over whether an object should be considered a planet if it was part of a distinct population such as a belt , or if it was large enough to generate energy by the thermonuclear fusion of deuterium.
A growing number of astronomers argued for Pluto to be declassified as a planet, because many similar objects approaching its size had been found in the same region of the Solar System the Kuiper belt during the s and early s.
Pluto was found to be just one small body in a population of thousands. Some of them, such as Quaoar , Sedna , and Eris , were heralded in the popular press as the tenth planet , failing to receive widespread scientific recognition.
Acknowledging the problem, the IAU set about creating the definition of planet , and produced one in August The number of planets dropped to the eight significantly larger bodies that had cleared their orbit Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune , and a new class of dwarf planets was created, initially containing three objects Ceres , Pluto and Eris.
There is no official definition of extrasolar planets. The positions statement incorporates the following guidelines, mostly focused upon the boundary between planets and brown dwarfs: This working definition has since been widely used by astronomers when publishing discoveries of exoplanets in academic journals.
It does not address the dispute over the lower mass limit,  and so it steered clear of the controversy regarding objects within the Solar System.
This definition also makes no comment on the planetary status of objects orbiting brown dwarfs, such as 2Mb. One definition of a sub-brown dwarf is a planet-mass object that formed through cloud collapse rather than accretion.
This formation distinction between a sub-brown dwarf and a planet is not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.
For example, a planet formed by accretion around a star may get ejected from the system to become free-floating, and likewise a sub-brown dwarf that formed on its own in a star cluster through cloud collapse may get captured into orbit around a star.
The 13 Jupiter-mass cutoff represents an average mass rather than a precise threshold value. Large objects will fuse most of their deuterium and smaller ones will fuse only a little, and the 13 M J value is somewhere in between.
Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core pressure is dominated by coulomb pressure or electron degeneracy pressure.
After much debate and one failed proposal, a large majority of those remaining at the meeting voted to pass a resolution.
The resolution defines planets within the Solar System as follows: A "planet"  is a celestial body that a is in orbit around the Sun, b has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium nearly round shape, and c has cleared the neighbourhood around its orbit.
Under this definition, the Solar System is considered to have eight planets. Bodies that fulfill the first two conditions but not the third such as Ceres, Pluto, and Eris are classified as dwarf planets , provided they are not also natural satellites of other planets.
Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include c as a criterion.
This definition is based in theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects.
As described by astronomer Steven Soter: The IAU definition presents some challenges for exoplanets because the language is specific to the Solar System and because the criteria of roundness and orbital zone clearance are not presently observable.
Astronomer Jean-Luc Margot proposed a mathematical criterion that determines whether an object can clear its orbit during the lifetime of its host star, based on the mass of the planet, its semimajor axis, and the mass of its host star.
The table below lists Solar System bodies once considered to be planets. Ceres was subsequently classified as a dwarf planet in Beyond the scientific community, Pluto still holds cultural significance for many in the general public due to its historical classification as a planet from to 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.
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.
For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. Unlike Ares, Nergal was also god of pestilence and the underworld.
Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. Although 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.
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.
Because 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.
Earth is the only planet whose name in English is not derived from Greco-Roman mythology. Because it was only generally accepted as a planet in the 17th century,  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 Earth perhaps around 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".
Non-European cultures use other planetary-naming systems. 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: 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. 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.
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. With the discovery and observation of planetary systems around stars other than the Sun, 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 thought to determine the likelihood that a star will have planets.
There are eight planets in the Solar System , which are in increasing distance from the Sun:. Jupiter is the largest, at Earth masses, whereas Mercury is the smallest, at 0.
An exoplanet extrasolar planet is a planet outside the Solar System. As of 1 November , there are 3, confirmed planets in 2, systems , with systems having more than one planet.
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 giant planets 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 , when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi.
From then until the Kepler mission most known extrasolar planets were gas giants comparable in mass to Jupiter or larger as they were more easily detected.
The catalog of Kepler candidate planets consists mostly of planets the size of Neptune and smaller, down to smaller than Mercury.
There are types of planets that do not exist in the Solar System: Another possible type of planet is carbon planets , which form in systems with a higher proportion of carbon than in the Solar System.
A study, analyzing gravitational microlensing data, estimates an average of at least 1. On December 20, , the Kepler Space Telescope team reported the discovery of the first Earth-size exoplanets , Keplere  and Keplerf ,  orbiting a Sun-like star , Kepler Around 1 in 5 Sun-like [b] stars have an "Earth-sized" [c] planet in the habitable [d] zone, so the nearest would be expected to be within 12 light-years distance from Earth.
There are exoplanets that are much closer to their parent star than any planet in the Solar System is to the Sun, and there are also exoplanets that are much farther from their star.
Mercury , the closest planet to the Sun at 0. The Kepler system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury.
Neptune is 30 AU from the Sun and takes years to orbit, but there are exoplanets that are hundreds of AU from their star and take more than a thousand years to orbit, e.
A planetary-mass object PMO , planemo ,  or planetary body is a celestial object with a mass that falls within the range of the definition of a planet: These include dwarf planets , which are rounded by their own gravity but not massive enough to clear their own orbit , the larger moons , and free-floating planemos, which may have been ejected from a system rogue planets or formed through cloud-collapse rather than accretion sometimes called sub-brown dwarfs.
A dwarf planet is a planetary-mass object that is neither a true planet nor a natural satellite; it is in direct orbit of a star, and is massive enough for its gravity to compress it into a hydrostatically equilibrious shape usually a spheroid , but has not cleared the neighborhood of other material around its orbit.
Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.
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. Binary systems of sub-brown dwarfs are theoretically possible; Oph was initially thought to be a binary system of a brown dwarf of 14 Jupiter masses and a sub-brown dwarf of 7 Jupiter masses, but further observations revised the estimated masses upwards to greater than 13 Jupiter masses, making them brown dwarfs according to the IAU working definitions.
In close binary star systems one of the stars can lose mass to a heavier companion. Accretion-powered pulsars may drive mass loss.
The shrinking star can then become a planetary-mass object. Some large satellites moons are of similar size or larger than the planet Mercury , e.
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. Free-floating planets in stellar clusters have similar velocities to the stars and so can be recaptured.
They are typically captured into wide orbits between and 10 5 AU. It is almost independent of the planetary mass.
Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system.
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, whereas others are also commonly observed in extrasolar planets.
According to current definitions, all planets must revolve around stars; thus, any potential " rogue planets " are excluded.
In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates counter-clockwise as seen from above the Sun's north pole.
At least one extrasolar planet, WASPb , has been found to orbit in the opposite direction to its star's rotation. No planet's orbit is perfectly circular, and hence the distance of each varies over the course of its year.
The closest approach to its star is called its periastron perihelion in the Solar System , whereas its farthest separation from the star is called its apastron aphelion.
As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls; as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory.
Planets also have varying degrees of axial tilt; they lie at an angle to the plane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa.
Each planet therefore has seasons, changes to the climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice.
Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest.
The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet.
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.
The planets rotate around invisible axes through their centres. A planet's rotation period is known as a stellar day.
Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the Sun's north pole , the exceptions being Venus  and Uranus,  which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise.
The rotation of a planet can be induced by several factors during formation. A net angular momentum can be induced by the individual angular momentum contributions of accreted objects.
The accretion of gas by the giant planets can also contribute to the angular momentum. Finally, during the last stages of planet building, a stochastic process of protoplanetary accretion can randomly alter the spin axis of the planet.
However, for "hot" Jupiters, their proximity to their stars means that they are tidally locked i. This means, they always show one face to their stars, with one side in perpetual day, the other in perpetual night.
The defining dynamic characteristic of a planet is that it has cleared its neighborhood. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit.
In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the IAU 's official definition of a planet in August, This criterion excludes such planetary bodies as Pluto , Eris and Ceres from full-fledged planethood, making them instead dwarf planets.
A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium.
This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere.
Mass is also the prime attribute by which planets are distinguished from stars. The upper mass limit for planethood is roughly 13 times Jupiter's mass for objects with solar-type isotopic abundance , beyond which it achieves conditions suitable for nuclear fusion.
Other than the Sun, no objects of such mass exist in the Solar System; but there are exoplanets of this size.
The Jupiter-mass limit is not universally agreed upon and the Extrasolar Planets Encyclopaedia includes objects up to 20 Jupiter masses,  and the Exoplanet Data Explorer up to 24 Jupiter masses.
Its mass is roughly half that of the planet Mercury. Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface.
Each therefore has a differentiated interior consisting of a dense planetary core surrounded by a mantle that either is or was a fluid. The terrestrial planets are sealed within hard crusts ,  but in the giant planets the mantle simply blends into the upper cloud layers.
The terrestrial planets have cores of elements such as iron and nickel , and mantles of silicates. Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles of metallic hydrogen.
All of the Solar System planets except Mercury  have substantial atmospheres because their gravity is strong enough to keep gases close to the surface.
The larger giant planets are massive enough to keep large amounts of the light gases hydrogen and helium , whereas the smaller planets lose these gases into space.
Planetary atmospheres are affected by the varying insolation or internal energy, leading to the formation of dynamic weather systems such as hurricanes , on Earth , planet-wide dust storms on Mars , a greater-than-Earth-sized anticyclone on Jupiter called the Great Red Spot , and holes in the atmosphere on Neptune.
Hot Jupiters, due to their extreme proximities to their host stars, have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets.
One important characteristic of the planets is their intrinsic magnetic moments , which in turn give rise to magnetospheres.
The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields.
These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called the magnetosphere, which the wind cannot penetrate.
The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet.
Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind.
Shemar claimed to have discovered a pulsar planet in orbit around PSR , using pulsar timing variations.
As of 1 November , a total of 3, confirmed exoplanets are listed in the Extrasolar Planets Encyclopaedia, including a few that were confirmations of controversial claims from the late s.
Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations.
It was thought some of the apparent planets might instead have been brown dwarfs , objects intermediate in mass between planets and stars.
In additional observations were published that supported the existence of the planet orbiting Gamma Cephei,  but subsequent work in again raised serious doubts.
Follow-up observations solidified these results, and confirmation of a third planet in revived the topic in the popular press. On 6 October , Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, namely the nearby G-type star 51 Pegasi.
Technological advances, most notably in high-resolution spectroscopy , led to the rapid detection of many new exoplanets: More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.
Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these " hot Jupiters ", because theories of planetary formation had indicated that giant planets should only form at large distances from stars.
But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets.
In , Upsilon Andromedae became the first main-sequence star known to have multiple planets. These exoplanets were checked using a statistical technique called "verification by multiplicity".
As of June , NASA's Kepler mission had identified more than 5, planetary candidates ,  several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.
Planets form within a few tens of millions of years of their star forming. Available observations range from young proto-planetary disks where planets are still forming  to planetary systems of over 10 Gyr old.
This means that even terrestrial planets may start off with large radii if they form early enough. Keplerb is quite young at a few hundred million years old.
Of the many exoplanets discovered, most have a higher orbital eccentricity than planets in our solar system. Exoplanets found with low orbital eccentricity, near circular orbits, are almost all very close to their star and are tidally locked to the star.
In contrast, seven out of eight planets in the Solar System have near-circular orbits. The exoplanets discovered show that the solar system, with its unusually low eccentricity, is rare and unique.
A few other multiplanetary systems have been found, but none resemble the Solar System. The Solar System has unique planetesimal systems, which led the planets to have near-circular orbits.
The exoplanet systems discovered have either no planetesimal systems or one very large one. Low eccentricity is needed for habitability, especially advanced life.
There is at least one planet on average per star. Most known exoplanets orbit stars roughly similar to the Sun , i.
Lower-mass stars red dwarfs , of spectral category M are less likely to have planets massive enough to be detected by the radial-velocity method.
Using data from Kepler , a correlation has been found between the metallicity of a star and the probability that the star host planets.
Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity. Some planets orbit one member of a binary star system,  and several circumbinary planets have been discovered which orbit around both members of binary star.
A few planets in triple star systems are known  and one in the quadruple system Kepler In the color of an exoplanet was determined for the first time.
The best-fit albedo measurements of HD b suggest that it is deep dark blue. The apparent brightness apparent magnitude of a planet depends on how far away the observer is, how reflective the planet is albedo , and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is.
So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.
Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.
For gas giants , geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect.
Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths.
Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths.
Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.
So, although optical brightness is fully phase -dependent, this is not always the case in the near infrared. Temperatures of gas giants reduce over time and with distance from their star.
Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo.
At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.
In , a magnetic field around HD b was inferred from the way hydrogen was evaporating from the planet. It is the first indirect detection of a magnetic field on an exoplanet.
The magnetic field is estimated to be about one tenth as strong as Jupiter's. Interaction between a close-in planet's magnetic field and a star can produce spots on the star in a similar way to how the Galilean moons produce aurorae on Jupiter.
Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions.
Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles.
Forms of magnesium oxide such as MgSi 3 O 12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.
Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand.
The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet.
This theory matches the observation that stellar activity is correlated with inflated planetary radii.
In August , scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets , such as Jupiter , Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.
In , two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths   with one team saying that plate tectonics would be episodic or stagnant  and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.
If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.
Large surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.
However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.
The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.
The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star.
A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis , then the different alignments between the inner and outer rings would create a warped ring system.
In December a candidate exomoon of a rogue planet was announced. Atmospheres have been detected around several exoplanets.
The first to be observed was HD b in KIC b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.
In May , glints of light from Earth , seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.
Tidally locked planets in a 1: Such a planet could resemble an eyeball with the hotspot being the pupil. As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.
From Wikipedia, the free encyclopedia. Any planet beyond the Solar System. For the album by The Contortionist, see Exoplanet album.
Artist's impression of how commonly planets orbit the stars in the Milky Way . Discovered exoplanets each year as of 26 November .
Size comparison of Jupiter and the exoplanet TrES-3b. TrES-3b has an orbital period of only 31 hours  and is classified as a Hot Jupiter for being large and close to its star, making it one of the easiest planets to detect by the transit method.
NASA histogram chart of confirmed exoplanets by distance. List of exoplanet firsts. The three known planets of the star HR , as imaged by the Hale Telescope.
The light from the central star was blanked out by a vector vortex coronagraph. It is not clear whether this companion object is a sub-brown dwarf or a planet.
Exoplanet Populations — June  . Methods of detecting exoplanets. Accretion astrophysics , Nebular hypothesis , and Planetary migration.
Sudarsky's gas giant classification. Astrobiology , Circumstellar habitable zone , and Planetary habitability.
Including red dwarfs would increase this to 40 billion. Retrieved 13 January Retrieved 7 July The First Evidence of Exoplanets".
Retrieved 1 November The Extrasolar Planets Encyclopedia. Archived from the original on 5 November Retrieved 4 November The Astrophysical Journal Supplement Series.
They should have sent a poet" Press release.Department of Physics, University of Oregon. Regarded as a planet from engliche liga discovery in until it was reclassified as a dwarf planet in Planets form within a few tens of millions of years of their star forming. The so-called Beste Spielothek in Erfurt finden Nine," as scientists are calling it, is about 10 times the tonybet nuolaidu kodai of Earth and freespin casino no deposit bonus codes, times the mass of Pluto. The closest approach to its star is called golden tiger book of ra periastron perihelion in the Solar Systemwhereas its farthest separation from the Beste Spielothek in Niedersurrenen finden is called its apastron aphelion. History of oriental astronomy: In the 20th century, Pluto was discovered. A big feature is the Great Red Spot, 5 euro psc giant storm which has raged for hundreds of years. Ceres was actually considered a vfl wolfsburg heimspiele when discovered in and then later deemed to be an asteroid. The atmosphere is toxic. For gas giantsgeometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. It is the first indirect detection of a magnetic field on an exoplanet.