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Under IAU definitions, there are eight planets in the Solar System ([[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], [[Mars]], [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]]) and also at least three [[dwarf planets]] ([[Ceres (dwarf planet)|Ceres]], [[Pluto]], and [[Eris (dwarf planet)|Eris]]). Many of these planets are orbited by one or more [[natural satellite|moons]], which can be larger than small planets. There have also been more than two hundred planets discovered [[extrasolar planet|orbiting other stars]].<ref name="Encyclopaedia">{{cite web |title=Interactive Extra-solar Planets Catalog |work=The Extrasolar Planets Encyclopaedia |url=http://exoplanet.eu/catalog.php |last=Schneider |first=Jean |date=[[October 30]], [[2006]] |accessdate=2006-10-31}}</ref> Planets are generally divided into two main types: large, low-density [[gas giant]]s and smaller, rocky [[terrestrial planet|terrestrials]].
Under IAU definitions, there are eight planets in the Solar System ([[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], [[Mars]], [[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]]) and also at least three [[dwarf planets]] ([[Ceres (dwarf planet)|Ceres]], [[Pluto]], and [[Eris (dwarf planet)|Eris]]). Many of these planets are orbited by one or more [[natural satellite|moons]], which can be larger than small planets. There have also been more than two hundred planets discovered [[extrasolar planet|orbiting other stars]].<ref name="Encyclopaedia">{{cite web |title=Interactive Extra-solar Planets Catalog |work=The Extrasolar Planets Encyclopaedia |url=http://exoplanet.eu/catalog.php |last=Schneider |first=Jean |date=[[October 30]], [[2006]] |accessdate=2006-10-31}}</ref> Planets are generally divided into two main types: large, low-density [[gas giant]]s and smaller, rocky [[terrestrial planet|terrestrials]].


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[[Image:Olympians.jpg|thumb|left|220 px|The gods of [[Mount Olympus (Mountain)|Olympus]], after whom the Solar System's planets are named]]
[[Image:Olympians.jpg|thumb|left|220 px|The gods of [[Mount Olympus (Mountain)|Olympus]], after whom the Solar System's planets are named]]


Revision as of 01:22, 17 December 2007

Template:Two other uses

artist's depiction of the extrasolar planet HD 209458b orbiting its star

A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its inner core, and has cleared its neighbouring region of planetesimals.[1][2]

The term planet is an ancient one having ties to history, science, myth, and religion. The planets were originally seen as a divine presence; as emissaries of the gods. Even today, many people continue to believe the movement of the planets affects their lives, although such a causation is rejected by the scientific community. As scientific knowledge advanced, the human perception of the planets changed over time, incorporating a number of disparate objects. Even now there is no uncontested definition of what a planet is. In 2006, the IAU officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized, and remains disputed by some scientists.

The planets were initially thought to orbit the Earth in circular motions; after the development of the telescope, the planets were determined to orbit the Sun, and their orbits were found to be elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes and shared such features as ice-caps and seasons. Since the dawn of the space age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics and even hydrology. Since 1992, and the discovery of hundreds of extrasolar planets, scientists are beginning to observe similar features across the galaxy.

Under IAU definitions, there are eight planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and also at least three dwarf planets (Ceres, Pluto, and Eris). Many of these planets are orbited by one or more moons, which can be larger than small planets. There have also been more than two hundred planets discovered orbiting other stars.[3] Planets are generally divided into two main types: large, low-density gas giants and smaller, rocky terrestrials.

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The gods of Olympus, after whom the Solar System's planets are named

In ancient times, astronomers noted how certain lights moved across the sky in relation to the other stars. The lights were first called "πλανήται" (planētai),[5] meaning "wanderers", by the ancient Greeks, and it is from this that the word "planet" was derived.[6][7] The Greeks gave the planets names: the farthest was called Phainon, the shiner, while below it was Phaethon, the bright one. The red planet was known as Pyroeis, "fiery", while the brightest was known as Phosphoros, the light bringer, and the fleeting final planet was called Stilbon, the gleamer. However, the Greeks also made each planet sacred to one of their pantheon of gods, the Olympians: Phainon was sacred to Kronos, the Titan who fathered the Olympians, while Phaethon was sacred to Zeus, his son who deposed him as king. Ares, son of Zeus and god of war, was given dominion over Pyroeis, while Aphrodite, goddess of love, ruled over bright Phosphoros, and Hermes ruled over Stilbon.[8]

The Greek practice of grafting of their gods' names onto the planets was almost certainly borrowed from the Babylonians, a contemporary civilisation in what is now Iraq, from whom they had begun to absorb astronomical learning, including constellations and the zodiac, by 600 BCE.[9] The Babylonians named Phosphoros after their goddess of love, Ishtar, Pyroeis after their god of war, Nergal, and Phaethon after their chief god, Marduk.[10] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.[8] There does, however, appear to have been some confusion in translation. For instance, the Babylonian Nergal was a god of war, and the Greeks, seeing this aspect of Nergal's persona, identified him with Ares, their god of war. However, Nergal, unlike Ares, was also a god of the dead and a god of pestilence.[10]

Early printed rendition of a geocentric cosmological model.

Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods; however, because of the influence of the Roman Empire and, later, the Catholic Church, they are known by their Roman (or Latin) names, rather than the Greek. 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.[11] When the Romans studied Greek astronomy, they gave the planets their own gods' names. To the Greeks and Romans, there were five known planets; each presumed to be circling the Earth according to the complex laws laid out by Claudius Ptolemy in the 2nd century. They were, in increasing order from Earth (according to Ptolemy): Mercury (Hermes), Venus (Aphrodite), Mars (Ares), Jupiter (Zeus), and Saturn (Kronos). Although strictly the term "planetai" referred only to those five objects, the term was often expanded to include the Sun and the Moon.[12] When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained: Uranus (Ouranos) and Neptune (Poseidon). The Greeks still use their original names for the planets.

Some Romans, following a belief imported from Mesopotamia into Hellenistic Egypt,[13] believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts began with Jupiter and worked inwards; as a result, a list of which god had charge of the first hour in each day became Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, i.e. the usual weekday name order.[14] Sunday, Monday, and Saturday are straightforward translations of these Roman names. In English the other days were renamed after Tiw, (Tuesday) Wóden (Wednesday), Thunor (Thursday), and Fríge (Friday), Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus respectively.

Since Earth was only generally accepted as a planet in the 17th century, there is no tradition of naming it after a god. Many of the Romance languages (including French, Italian, Spanish and Portuguese), which are descended from Latin, retain the old Roman name of Terra or some variation thereof. However, the non-Romance languages use their own respective native words. Again, the Greeks retain their original name, Γή (Ge or Yi); the Germanic languages, including English, use a variation of an ancient Germanic word ertho, "ground,"[15] as can be seen in the English Earth, the German Erde, the Dutch Aarde, and the Scandinavian Jorde. The same is true for the Sun and the Moon, though they are no longer considered planets.

Some non-European cultures use their own planetary naming systems. India uses a naming system based on the Navagraha, which incorporates the seven traditional planets (Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China, and the countries of eastern Asia subject to Chinese cultural influence, such as Japan, Korea and Vietnam, use a naming system based on the five Chinese elements.[14]

History

See also: List of Solar System bodies formerly regarded as planets
Heliocentrism (lower panel) in comparison to the geocentric model (upper panel)

As scientific knowledge progressed, understanding of the term "planet" changed from something that moved across the sky (in relation to the starfield), to a body that orbited the Earth (or that were believed to do so at the time). When the heliocentric model gained sway in the 16th century, it became accepted that a planet was actually something that directly orbited the Sun. Thus the Earth was itself a planet,[16] while the Sun and Moon were not. At the end of the 17th century, when the first satellites of Saturn were discovered, the terms "planet" and "satellite" were at first used interchangeably, although "satellite" would gradually become more prevalent in the following century.[17] Until the mid-19th century, any newly discovered object orbiting the Sun was listed with the planets by the scientific community, and the number of "planets" swelled rapidly towards the end of that period.

During the 1800s, astronomers began to realize most recent discoveries were unlike the traditional planets. They shared the same region of space, between Mars and Jupiter, and had a far smaller mass. Bodies such as Ceres, Pallas, and Vesta, which had been classed as planets for almost half a century, became classified with the new designation "asteroid." From this point, a "planet" came to be understood, in the absence of any formal definition, as any "large" body that orbited the Sun. There was no apparent need to create a set limit, as 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 1846.[18]

However, in the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth, the recently-created IAU accepted the object as a planet. Further monitoring found the body was actually much smaller, but, as it was still larger than all known asteroids and seemingly did not exist within a larger population, it kept its status for some seventy years.[19]

In the 1990s and early 2000s, there was a flood of discoveries of similar objects in the same region of the Solar System. Like Ceres and the asteroids before it, Pluto was found to be just one small body in a population of thousands. A growing number of astronomers argued for it to be declassified as a planet, since many similar objects approaching its size were found. The discovery of Eris, a more massive object widely publicised as the tenth planet, brought things to a head. The IAU set about creating the definition of planet, and eventually produced one in 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus & Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).[20]

Definition and disputes

Main article: Definition of planet

With the discovery during the latter half of the twentieth century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There was particular disagreement 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.

Error: Image is invalid or non-existent.

In 2003, The International Astronomical Union (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated a working definition:[2]

  1. Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance as the Sun)[21] that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
  2. Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.
  3. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

This definition has since been widely used by astronomers when publishing discoveries in journals,[22] although it remains a temporary yet effective, working definition until a more permanent one is formally adopted. It also did not address the dispute over the lower mass limit and steered clear of the controversy regarding objects within the Solar System.

This matter was finally addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, the assembly voted to pass a resolution that defined planets within the Solar System as:[1]

A celestial body that is (a) 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 which fulfill the first two conditions but not the third (such as Pluto and Eris) are classified as dwarf planets, providing 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. After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.

This definition is based in modern theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer Steven Soter:

"The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Asteroids and comets, including KBOs, differ from planets in that they can collide with each other and with planets."[23]

In the aftermath of the IAU's 2006 vote, there has been criticism of the new definition,[24] and some astronomers have even stated that they will not use it.[25] Part of the dispute centres around the belief that point (c) (clearing its orbit) should not have been listed, and that those objects now categorised as dwarf planets should actually be part of a broader planetary definition. The next IAU conference is not until 2009, when modifications could be made to the definition, also possibly including extrasolar planets.

Beyond the scientific community, Pluto has held a strong cultural significance for many in the general public considering its planetary status during most of the 20th century, in a similar way to Ceres and its kin in the 1800s. More recently, the discovery of Eris was widely reported in the media as the "tenth planet". The reclassification of all three objects as dwarf planets has attracted much media and public attention.[26]

Formation

Main article: Planetary formation

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 more dense until they collapse inward under gravity to form protoplanets.[27] 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.[28]

File:Ra4-protoplanetary-disk.jpg
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.[29][30] 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.[31][32] 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 solar system 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. 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.[33] (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—a astronomical term describing the abundance of isotopes with an atomic number greater than 2 (Helium)—is now believed to determine the likelihood that a star will have planets.[34] Hence it is thought less likely that a metal-poor, population II star will possess a more substantial planetary system than a metal-rich population I star.

Within the Solar System

The terrestrial planets: Mercury, Venus, Earth, Mars (Sizes to scale)
The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune. (Sizes to scale.)
Main article: Solar System

According to the IAU's current definitions there are eight planets in the Solar System. The largest is Jupiter, at 318 Earth masses. The smallest is Mercury, at 0.055 Earth masses. In increasing distance from the Sun, the planets are:

  1. ☿ Mercury
  2. ♀ Venus
  3. ⊕ Earth
  4. ♂ Mars
  5. ♃ Jupiter
  6. ♄ Saturn
  7. ♅ Uranus
  8. ♆ Neptune

The larger bodies of the Solar System can be divided into categories based on their composition:

  • Terrestrials: Planets (and possibly dwarf planets) that are similar to Earth — with bodies largely composed of rock: Mercury, Venus, Earth and Mars.
  • Gas giants: Planets with a composition largely made up of gaseous material and are significantly more massive than terrestrials: Jupiter, Saturn, Uranus, Neptune. Ice giants are a sub-class of gas giants, distinguished from gas giants by their depletion in hydrogen and helium, and a significant composition of rock and ice: Uranus and Neptune.
Planetary attributes
Name Equatorial
diameter[a] 		Mass[a] Orbital
radius (AU) 		Orbital period
(years) 		Inclination
to Sun's equator (°) 		Orbital
eccentricity 		Rotation period
(days) 		Moons Rings Atmosphere
Terrestrials Mercury 0.382 0.06 0.39 0.24 3.38 0.206 58.64 — no minimal
Venus 0.949 0.82 0.72 0.62 3.86 0.007 -243.02 — no CO2, N2
Earth[b] 1.00 1.00 1.00 1.00 7.25 0.017 1.00 1 no N2, O2
Mars 0.532 0.11 1.52 1.88 5.65 0.093 1.03 2 no CO2, N2
Gas giants Jupiter 11.209 317.8 5.20 11.86 6.09 0.048 0.41 63 yes H2, He
Saturn 9.449 95.2 9.54 29.46 5.51 0.054 0.43 60 yes H2, He
Uranus 4.007 14.6 19.22 84.01 6.48 0.047 -0.72 27 yes H2, He
Neptune 3.883 17.2 30.06 164.8 6.43 0.009 0.67 13 yes H2, He
a Measured relative to the Earth.
b See Earth article for absolute values.

Dwarf planets

Main article: Dwarf planet

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 three dwarf planets in the Solar System are recognized by the IAU: Ceres, Pluto and Eris. Several other objects in both the asteroid belt and the Kuiper belt are under consideration, with as many as 50 that could eventually qualify. There may be as many as 200 that could be discovered once the Kuiper Belt has been fully explored. Dwarf planets share many of the same characteristics as planets, although notable differences remain—namely that they are not dominant in their orbits. Their attributes are:

Dwarf planetary attributes
Name Equatorial
diameter[c] 		Mass[c] Orbital
radius (AU) 		Orbital period
(years) 		Inclination
to ecliptic (°) 		Orbital
eccentricity 		Rotation period
(days) 		Moons Rings Atmosphere
Ceres 0.08 0.0002 2.76 4.60 10.59 0.080 0.38 0 no none
Pluto 0.19 0.0022 39.48 248.09 17.14 0.249 -6.39 3 no temporary
Eris 0.19 0.0025 67.67 ~557 44.19 0.442 ~0.3 1 ? temporary
c Measured relative to the Earth.

By definition, all dwarf planets are members of larger populations. Ceres is the largest body in the asteroid belt, while Pluto is a member of the Kuiper belt and Eris is a member of the scattered disc. According to Mike Brown there may soon be over forty trans-Neptunian objects that qualify as dwarf planets under the IAU's recent definition.[35]

Beyond the Solar System

Extrasolar planets

Main article: Extrasolar planet

Since the 1988 discovery of Gamma Cephei Ab, a number of confirmed discoveries have been made of planets orbiting stars other than the Sun. Of the 267 extrasolar planets discovered by November 2007, most have masses which are comparable to or larger than Jupiter's.[36] Exceptions include a number of planets discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12,[37] the planets orbiting the stars Mu Arae, 55 Cancri and GJ 436 which are approximately Neptune-sized,[38] and a planetary system containing at least two planets orbiting Gliese 876.[39]

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. There is also a class of hot Jupiters that orbit so close to their star that their atmospheres are slowly blown away in a comet-like tail: the Chthonian planets.[40]

More detailed observation of extrasolar planets will require a new generation of instruments, including space telescopes. Currently the CoRoT spacecraft is 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 Kepler Mission, Terrestrial Planet Finder, and Space Interferometry Mission programs, the ESA's Darwin, and the CNES' PEGASE.[41] 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 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.[42]

Interstellar "planets"

Main article: Interstellar planetary mass object

Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.[43] Some scientists have argued that such objects found roaming in deep space should be classed as "planets". However, others have suggested that they could be low-mass stars.[44] The IAU's working definition on extrasolar planets takes no position on the issue.

In 2005, astronomers announced the discovery of Cha 110913-773444, the smallest brown dwarf found to date, at only seven times Jupiter's mass. Since it was not found in orbit around a fusing star, it is a sub-brown dwarf according to the IAU's working definition. However, some astronomers believe it should be referred to as a planet.[45]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[46] of the objects has determined that their masses are each greater than 13 Jupiter-masses, making the pair brown dwarfs.[47]

Attributes

Although each planet has unique physical characteristics, a number of broad commonalities do exist between 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 common to extrasolar planets.

Dynamic characteristics

See also: Kepler's laws of planetary motion

Orbit

The orbit of the planet Neptune compared to that of Pluto. Note the elongation of Pluto's orbit in relation to Neptune's (eccentricity), as well as its large angle to the ecliptic (inclination)

All planets revolve around stars. In the Solar System, all the planets orbit in sync with the Sun's rotation. It is not yet known whether all extrasolar planets follow this pattern. The period of one revolution of a planet's orbit is known as its sidereal period or year.[48] A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, as it is less affected by the star's gravity. Because no planet's orbit is perfectly circular, the distance of each varies over the course of its year. Its closest distance to its is called its periastron (perihelion in the Solar System), while its farthest distance from the star is called its apastron (aphelion in the Solar System). As a planet approaches periastron, its speed increases as the pull of its star's gravity strengthens; as it reaches apastron, its speed decreases.[49]

Each planet's orbit is delineated by a set of elements:

  • The eccentricity of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, while planets with a high eccentricities have more elliptical orbits. The planets in our Solar System have very low eccentricities, and thus nearly circular orbits.[48] Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.[50][51]
an illustration of the semi-major axis
  • The semi-major axis is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit (see image). This distance is not necessarily the same as its apasteron, as no planet's orbit has its star at its exact centre.[48]
  • In our Solar System, the inclination of a planet tells how far above or below the plane of Earth's orbit (called the ecliptic) a planet's orbit lies. The eight planets of our Solar System all lie very close to the ecliptic; comets and Kuiper belt objects like Pluto are at far more extreme angles to it.[52] The points at which a planet crosses above and below the ecliptic are called its ascending and descending nodes.[48] Other orbital elements used to describe the orientation of a planet's orbit within our Solar System include the argument of periapsis and longitude of the ascending node.[48]

Rotation

The planets also rotate around invisible axes through their centres. A planet's rotation period is known as its day. All planets in the Solar System rotate in a counter-clockwise direction, except for Venus, which rotates clockwise[53] (Uranus is generally said to be rotating clockwise as well[54] though because of its extreme axial tilt, it can be said to be rotating either clockwise or anti-clockwise, depending on whether one states it to be inclined 82° from the ecliptic in one direction, or 98° in the opposite direction).[55] There is great variation in the length of day between the planets, with Venus taking 243 Earth days to rotate, and the gas giants only a few hours.[56] The rotational periods of extrasolar planets are not known; however their close proximity to their stars means that hot Jupiters are tidelocked; their orbits are in sync with their rotations. This means they only ever show one face to their stars, with one side in perpetual day, the other in perpetual night.[57]

Axial tilt

Planets also have varying degrees of axial tilt; they lie at an angle to the plane of the 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 possesses seasons; changes to the climate over the course of its year. The point at which each hemisphere is 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. 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.[58] Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to possess negligible to no axial tilt, as a result of their proximity to their stars.[59]

Orbital clearance

The defining dynamic characteristic of a planet is that it has cleared its neighborhood. 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 definiton of a planet in August, 2006. This criterion excludes such planetary bodies as Pluto, Eris and Ceres from full-fledged planethood, making them instead dwarf planets.[1] Although to date this criterion only applies to our Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.[60]

Physical characteristics

Mass

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.[61]

Mass is also the prime attribute by which planets are distinguished from stars. The upper mass limit for planethood, beyond which it achieves conditions suitable for nuclear fusion, is roughly 13 times Jupiter's mass. No objects of such mass exist in our Solar System; however a number of extrasolar planets lie at that threshold. The Extrasolar Planets Encyclopedia lists the largest planets discovered to date as HD 38529 c, at 12.7 Jupiter masses, AB Pic b, at 13.5 Jupiter masses, HD 162020 b at 13.75 Jupiter masses, and HD 13189 b, at 14 Jupiter masses. A number of objects of higher mass are also listed, but since they lie above the fusion threshold, they would be better described as brown dwarfs.[62]

The smallest known planet, excluding dwarf planets and satellites, is PSR B1257+12 a, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury.[62]

Internal differentiation

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 which either is or was a fluid. The terrestrial planets are sealed within hard crusts,[63] but in the gas giants the mantle simply dissolves into the upper cloud layers. The terrestrial planets possess cores of magnetic elements such as iron and nickel, and mantles of silicates. Jupiter and Saturn are believed to possess cores of rock and metal surrounded by mantles of metallic hydrogen.[64] Uranus and Neptune, which are smaller, possess rocky cores surrounded by mantles of water, ammonia, methane and other ices.[65] The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.[63]

Atmospheres

All of the planets have atmospheres as their large masses mean gravity is strong enough to keep gaseous particles close to the surface. The larger gas giants are massive enough to keep large amounts of the light gases Hydrogen and Helium close by, although these gases mostly float into space around the smaller planets.[66] Earth's atmosphere is greatly different to the other planets because the various life processes that have transpired there have introduced massive amounts of free oxygen,[67] while the atmosphere of Mercury has mostly, although not entirely, been blasted away by the solar wind.[68] Planetary atmospheres are affected by the varying degrees of energy received from either the Sun or their interiors, leading to the formation of dynamic weather systems such as hurricanes, (on Earth), planet-wide dust storms (on Mars) and Earth-sized anticyclones (on Jupiter).[58] At least one extrasolar planet, HD 189733b, has been shown to possess such a weather system, similar to the Great Red Spot on Jupiter but twice as large.[69] Hot Jupiters have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets.[70] These planets have vast differences in temperature between their day and night sides which produce supersonic windspeeds.[71]

Secondary characteristics

Planets in our Solar System possess orbital resonances in their own right. Many have natural satellites, often called "moons." Mercury and Venus have no moons, the Earth has one, and Mars has two, but the gas giants all have numerous moons in complex planetary systems. Many gas giant moons have similar features to the terrestrial planets and dwarf planets, and some have been studied for signs of life.[72][73][74]

The four largest planets in the Solar System 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 which fell below their parent planet's Roche limit and were torn apart by tidal forces.[75][76]

No secondary characteristics have been observed around extrasolar planets. However the sub-brown dwarf Cha 110913-773444, which has been described as a rogue planet, is believed to be orbited by a tiny protoplanetary disc.[77]

See also

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Definition and reclassification debate

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