Dieser Artikel enthält eine tabellarische Übersicht der Planeten des Sonnensystems. Inklination, 7,°, 3,°, —, 1,°, 1,°, 2,°, 0,°, 1,°. Länge des aufsteigenden Knotens (J), 48,°, 76,°. Die großartigsten Szenen der neuen BBC Studios-Naturserie „Seven Worlds, One Planet” in 4K-Ultra-HD auf einer riesigen LED-Wand – live begleitet von einem. Das Sonnensystem ist das Planetensystem, das die Sonne, die sie umkreisenden Planeten 30,1 AE) mit Dichten zwischen 0,7 g/cm³ und 1,66 g/cm³.
SonnensystemUranus ist der siebte Planet in unserem Sonnensystem. Bis heute wurde der 7. und der 8. Planet unseres Sonnensystems nur von einer einzigen Sonde. Die großartigsten Szenen der neuen BBC Studios-Naturserie „Seven Worlds, One Planet” in 4K-Ultra-HD auf einer riesigen LED-Wand – live begleitet von einem. Dazu zählte man damals nach dieser Definition auch Sonne und Mond. Diese „Heiligen 7“ wurden als mächtige Götter angesehen, denen jeweils einer der sieben.
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Planets in the Solar System are divided into two main types: large low-density giant planets , and smaller rocky terrestrials.
There are eight planets in the Solar System according to the IAU definition. Six of the planets are orbited by one or more natural satellites.
Several thousands of planets around other stars " extrasolar planets " or "exoplanets" have been discovered in the Milky Way. As of 1 January , 4, known extrasolar planets in 3, planetary systems including multiple planetary systems , ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter have been discovered, out of which more than planets are the same size as Earth , nine of which are at the same relative distance from their star as Earth from the Sun, i.
The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age.
The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy.
The five classical planets of the Solar System , being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology , religious cosmology , and ancient astronomy.
In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the " fixed stars ", which maintained a constant relative position in the sky.
The reasons for this perception were that stars and planets appeared to revolve around Earth each day  and the apparently common-sense perceptions that Earth was solid and stable and that it was not moving but at rest.
The first civilization known to have a functional theory of the planets were the Babylonians , who lived in Mesopotamia in the first and second millennia BC.
The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa , a 7th-century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as the second millennium BC.
APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year.
These would remain the only known planets until the invention of the telescope in early modern times. The ancient Greeks initially did not attach as much significance to the planets as the Babylonians.
The Pythagoreans , in the 6th and 5th centuries BC appear to have developed their own independent planetary theory, which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe.
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 : the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.
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: in , Ray Lyttleton suggested that Pluto may be an escaped satellite of Neptune ,  and Fred Whipple suggested in that Pluto may be a comet.
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: the point at which a planet becomes a star. 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. In , the International Astronomical Union IAU Working Group on Extrasolar Planets issued a position statement, but this position statement was never proposed as an official IAU resolution and was never voted on by IAU members.
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.
The matter of the lower limit was addressed during the meeting of the IAU's General Assembly. 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 but no longer considered as such by the IAU, as well as whether they would be considered planets under Stern's and definitions.
Ceres was subsequently classified by the IAU as a dwarf planet in The reporting of newly discovered large Kuiper belt objects as planets - particularly Eris - triggered the August IAU decision on what a planet is.
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 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.
The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon from the farthest to the closest planet. 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 : water Mercury , metal Venus , fire Mars , wood Jupiter and earth Saturn.
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.
According to the IAU definition , 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.
The number of geophysical planets in the solar system is unknown - previously considered to be potentially in the hundreds, but now only estimated at only the low double digits.
An exoplanet extrasolar planet is a planet outside the Solar System. As of 1 January , there are 4, confirmed exoplanets in 3, 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: super-Earths and mini-Neptunes , which could be rocky like Earth or a mixture of volatiles and gas like Neptune—a radius of 1.
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 20 December , 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 stars have an "Earth-sized" [d] planet in the habitable [e] 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: massive enough to achieve hydrostatic equilibrium to be rounded under its own gravity , but not enough to sustain core fusion like a star.
These include dwarf planets , which are rounded by their own gravity but not massive enough to clear their own orbit , planetary-mass 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.
Planetary scientist and New Horizons principal investigator Alan Stern , who proposed the term 'dwarf planet', has argued that location should not matter and that only geophysical attributes should be taken into account, and that dwarf planets are thus a subtype of planet.
The IAU accepted the term rather than the more neutral 'planetoid' but decided to classify dwarf planets as a separate category of object.
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. An example is a Jupiter-mass object orbiting the pulsar PSR J Some large satellites moons are of similar size or larger than the planet Mercury , e.
Jupiter's Galilean moons and Titan. Proponents of the geophysical definition of planets argue that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet.
Alan Stern proposes the term satellite planet for a planet-sized satellite. Rogue 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 60 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.
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. This article is about the astronomical object. For planets in astrology, see Planets in astrology.
For other uses, see Planet disambiguation. Class of astronomical body directly orbiting a star or stellar remnant. Further information: History of astronomy , Definition of planet , and Timeline of Solar System astronomy.
Main article: Babylonian astronomy. See also: Greek astronomy. Main articles: Indian astronomy and Hindu cosmology. Main articles: Astronomy in the medieval Islamic world and Cosmology in medieval Islam.
See also: Heliocentrism. Main article: IAU definition of planet. See also: List of former planets. See also: Weekday names and Naked-eye planet.
Main article: Nebular hypothesis. 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. The four giant planets Jupiter , Saturn , Uranus , and Neptune against the Sun and some sunspots.
Main article: Solar System. See also: List of gravitationally rounded objects of the Solar System. Main article: Exoplanet. Main article: Geophysical definition of 'planet'.
Main article: Dwarf planet. Main article: Rogue planet. See also: Five-planet Nice model. Main article: Sub-brown dwarf.
Main article: Satellite planet. Main articles: Orbit and Orbital elements. Main article: Axial tilt. Main article: Clearing the neighbourhood. Main article: Planetary mass.Ihre Einheit ist das Kilogramm. Sieben Kontinente Ein Planet. Oberhausen — König-Pilsener-Arena. Play at Planet 7 Casino. The #1 Trusted Online Casino. Over Top-rated Games Join Now. Compra tus juguetes en la tienda online de Toy Planet. Más de jugueterías en toda España. Tenemos las mejores marcas y ofertas increíbles. ¡Entra! A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and – according to the International Astronomical Union but not all planetary scientists – has cleared its neighbouring region of planetesimals.. The term planet is ancient, with ties to history, astrology, science.