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Planet

2007 Schools Wikipedia Selection. Related subjects: The Planets

   The eight planets and three dwarf planets of the Solar System. (Sizes
   to scale.)
   Enlarge
   The eight planets and three dwarf planets of the Solar System. (Sizes
   to scale.)

   The International Astronomical Union defines "planet" as a celestial
   body that, within the Solar System,

          (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;

   or within another system,

          (i) is in orbit around a star or stellar remnants;
          (ii) has a mass below the limiting mass for thermonuclear fusion
          of deuterium; and
          (iii) is above the minimum mass/size requirement for planetary
          status in the Solar System.

   Our solar system is thus considered to have eight planets: Mercury,
   Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Under a
   separate resolution, it is also considered to have three dwarf planets:
   Ceres, Pluto, and Eris. There have been more than two hundred planets
   discovered orbiting other stars to date.

   Historically, there has been no formal scientific definition of
   "planet" and without one, the Solar System had been considered to have
   various planets over the years. This changed when a resolution covering
   planets within our solar system was formally adopted by the IAU in
   2006, limiting the number to eight. However, the IAU's position on
   those in other systems remains only a working definition in place since
   2003, and as such, is easily subject to change. The IAU has not yet
   taken a position on free-floating objects of planetary mass outside
   star systems, other than to exclude those in young star clusters.

Etymology

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

   In ancient times, Grecian astronomers noted how certain lights moved
   across the sky in relation to the other stars. These objects were
   believed to orbit the Earth, which was considered to be stationary. The
   "wandering" lights were called "πλανήτης" (planētēs), a Greek term
   meaning "wanderer", and it is from this that the word "planet" was
   derived.

   In near-universal practice in the Western world, the planets in the
   Solar System are named after Graeco-Roman gods, as, in Europe, it was
   the Greeks who originally named them. However, because of the influence
   of the Roman Empire and, later, the Catholic Church, in most countries
   in the West, the planets are known by their Roman (or Latin) names,
   rather than the original 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. When the Romans studied Greek astronomy,
   they gave the planets their own gods' names. In ancient times, there
   were seven 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: the Moon (called
   Luna by the Romans, and Selene by the Greeks), Mercury (called Hermes
   by the Greeks), Venus (Aphrodite), the Sun (called Sol by the Romans,
   Helios by the Greeks), Mars (Ares), Jupiter (Zeus), and Saturn
   (Kronos). Eventually, the Sun and Moon were removed from the list of
   planets in accordance with the heliocentric model. However, 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, believed that the seven gods after whom the planets
   were named took hourly shifts in looking after affairs on Earth, in
   Ptolemaic orbit order listed inwards. As a result, a list of which god
   has charge of the first hour in each day came out as Sun, Moon, Mars,
   Mercury, Jupiter, Venus, Saturn, i.e. the usual weekday name order.
   Sunday, Monday, and Saturday are straightforward translations of these
   Roman names. In English the other days were renamed after Tiw, Wóden,
   Thunor, and Fríge, Anglo-Saxon gods considered similar or equivalent to
   Mars, Mercury, Jupiter, and Venus.

   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," 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.
   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.

History

   Heliocentrism (lower panel) in comparsion to the geocentric model
   (upper panel)
   Enlarge
   Heliocentrism (lower panel) in comparsion 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. When the heliocentric
   model gained sway in the 16th century, it became accepted that a planet
   was actually something that orbited the Sun, and the Earth was indeed a
   planet, although the Sun and the Moon were not. 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 realise 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.

   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.

   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 as 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 a 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).

Former planets

   In ancient times, astronomers accepted as "planets" the seven visible
   objects that moved across the starfield: the Sun, the Moon, Mercury,
   Venus, Mars, Jupiter and Saturn. Since then, many objects have
   qualified as planets for a time:
   Period of planethood "Planet" Solar System Region Present status Notes
   Antiquity to 1600s Sun Centre Star Planet under the geocentric model.
   Antiquity to 1600s Moon Earth's orbit Satellite Planet under the
   geocentric model.
   1801-1864 Ceres Asteroid belt Dwarf planet Dual status as
   planet/asteroid 1852-1864. Asteroid 1852-2006.
   1802-1864 Pallas Asteroid belt Asteroid Dual status 1852-1864.
   1804-1864 Juno Asteroid belt Asteroid Dual status 1852-1864.
   1807-1864 Vesta Asteroid belt Asteroid Dual status 1852-1864.
   1930-2006 Pluto Kuiper belt Dwarf planet Officially accepted by IAU for
   this period.

Definition and disputes

   With the discovery during the latter half of the twentieth century of
   more objects within the Solar System and large objects around other
   stars, dispute arose over what should constitute a planet. There was
   particular disagreement over whether round objects that existed in
   belts, and large deuterium fusing objects should qualify.

   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:

          1) Objects with true masses below the limiting mass for
          thermonuclear fusion of deuterium (currently calculated to be 13
          Jupiter masses for objects of solar metallicity) that orbit
          stars or stellar remnants are "planets" (no matter how they
          formed). The minimum mass/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 been widely used by astronomers when publishing
   discoveries in journals since this time, although it remains a
   temporary, working definition until a more permanent one is formally
   adopted. It also did not address the controversy over the lower mass
   limit.

   However, in 2006, the general assembly of the IAU voted to pass a
   resolution that redefined planets within the Solar System as :
   The largest Trans-Neptunian objects that prompted the IAU's decision.
   Enlarge
   The largest Trans-Neptunian objects that prompted the IAU's decision.

          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 neighbourhood 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.

   In the aftermath of the IAU's 2006 vote, there has been criticism of
   the new definition, and some astronomers have even stated that they
   will not use it. 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.

Formation

   It is not known with certainty how planets are formed. The prevailing
   theory is that they are formed from those remnants of a nebula that do
   not condense under gravity to form a protostar. Instead, these remnants
   become a thin, protoplanetary disk of dust and gas revolving around the
   protostar and begin to condense about local concentrations of mass
   within the disc known as planetesimals. These concentrations become
   ever more dense until they collapse inward under gravity to form
   protoplanets. After a planet reaches a diameter larger than the Earth's
   moon, it begins to accumulate an extended atmosphere. This serves to
   increase the capture rate of the planetesimals by a factor of ten.
   An artist's impression of protoplanetary disk.
   Enlarge
   An artist's impression of protoplanetary disk.

   When the protostar has grown such that it ignites to form a star, its
   solar wind blows away most of the disc's remaining material. 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.
   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 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. (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 is now believed to
   determine the likelihood that a star will have planets. 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

Accepted planets

   The terrestrial planets: Mercury, Venus, Earth, Mars. (Sizes to scale.)
   Enlarge
   The terrestrial planets: Mercury, Venus, Earth, Mars. (Sizes to scale.)

   According to the IAU, there are eight planets in the Solar System. In
   increasing distance from the Sun, they are:
    1. ( ☿ ) Mercury, with no confirmed natural satellites
    2. ( ♀ ) Venus, with no confirmed natural satellites
    3. ( ⊕ ) Earth, with one confirmed natural satellite
    4. ( ♂ ) Mars, with two confirmed natural satellites
    5. ( ♃ ) Jupiter, with sixty-three confirmed natural satellites
    6. ( ♄ ) Saturn, with fifty-six confirmed natural satellites
    7. ( ♅ ) Uranus, with twenty-seven confirmed natural satellites
    8. ( ♆ ) Neptune, with thirteen confirmed natural satellites

Categories

   The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune.
   (Sizes to scale.)
   Enlarge
   The four gas giants against the Sun: Jupiter, Saturn, Uranus, Neptune.
   (Sizes to scale.)

   The large 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. If including dwarf planets, Ceres would also be
       counted, with as many as three other asteroids that might be added.
     * 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.
     * Ice dwarfs: Objects that are composed mainly of ice, and do not
       have planetary mass. The dwarf planets Pluto and Eris are ice
       dwarfs, and several dwarf planetary candidates also qualify.

Attributes

   All the planets revolve around the Sun in the same direction -
   counter-clockwise as seen from over the Sun's north pole. The period of
   one revolution of a planet's orbit is known as its year. A planet's
   year depends on its distance from the Sun; the farther a planet is from
   the Sun, not only the longer the distance it must travel, but also the
   slower its speed, as it is less affected by the Sun's gravity.

   The planets also rotate around invisible axes through their centres.
   The period of one rotation of a planet is known as its day. All the
   planets rotate in a counter-clockwise direction, except for Venus,
   which rotates clockwise. There is great variation in the length of day
   between the planets, with Venus taking 243 days to rotate, and the gas
   giants only a few hours.

   Planets also have varying degrees of axial tilt; they orbit at an angle
   to the plane of the Sun's equator. This causes the amount of sunlight
   received by each hemisphere to vary over the course of its year; when
   the northern hemisphere points away from the Sun, 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/nearest from the Sun is
   known as its solstice. Each planet has two in the course of its orbit;
   when a planet's northern hemisphere has its summer solstice, when its
   day is longest, the southern 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.

   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. Earth's atmosphere is greatly
   different to the other planets because of the various life processes
   that have transpired there, while the atmosphere of Mercury has mostly,
   although not entirely, been blasted away by the solar wind.

   Many of the planets have natural satellites, called "moons", regardless
   of their size. 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.
   Planetary attributes
   Name Equatorial^*
   diameter Mass^* Orbital
   radius ( AU) Orbital period
   (years) Inclination to
   Sun's equator (°) Orbital
   eccentricity Day
   (days) Moons Rings Atmosphere
   Terrestrials Mercury 0.39 0.06 0.39 0.24  3.38    0.206 58.64 none none
   minimal
   Venus 0.95 0.82 0.72 0.62  3.86    0.007 -243.02 none none CO[2], N[2]
   Earth^** 1.00 1.00 1.00 1.00  7.25    0.017 1.00 1 none N[2], O[2]
   Mars 0.53 0.11 1.52 1.88  5.65    0.093 1.03 2 none CO[2], N[2]
   Gas giants Jupiter 11.21 317.8 5.20 11.86  6.09    0.048 0.41 63 yes
   H[2], He
   Saturn 9.41 95.2 9.54 29.46  5.51    0.054 0.43 56 yes H[2], He
   Uranus 3.98 14.6 19.22 84.01  6.48    0.047 -0.72 27 yes H[2], He
   Neptune 3.81 17.2 30.06 164.8  6.43    0.009 0.67 13 yes H[2], He

   *Measured relative to the Earth. **See Earth article for absolute
   values.

Dwarf planets

   Before the August 2006 decision, several objects were proposed by
   astronomers, including at one stage by the IAU, as planets. However in
   2006 several of these objects were reclassified as dwarf planets,
   objects distinct from planets. Currently 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 Mass^* Orbital
   radius ( AU) Orbital period
   (years) Inclination
   to ecliptic (°) Orbital
   eccentricity Day
   (days) Moons Rings Atmosphere
   Terrestrials Ceres 0.08 0.0002 2.76 4.60  10.59    0.080 0.38 none no
   none
   Ice dwarfs Pluto 0.18 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 no temporary

   *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.

Beyond the Solar System

   Of the 210 extrasolar planets (those outside the Solar System)
   discovered to date (October 2006) most have masses which are about the
   same as, or larger than, Jupiter's the planets orbiting the stars Mu
   Arae, 55 Cancri and GJ 436 which are approximately Neptune-sized, and a
   planet orbiting Gliese 876 that is estimated to be about 6 to 8 times
   as massive as the Earth and is probably rocky in composition.

   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.

   Several projects have been proposed to create an array of space
   telescopes to search for extrasolar planets with masses comparable to
   the Earth. The NASA Terrestrial Planet Finder was one such program, but
   (as of 2006-02-06) this program has been put on indefinite hold. The
   ESA is considering a comparable mission called Darwin. 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.

   In 2005, astronomers detected a planet in a triple star system, a
   finding that challenges current theories of planetary formation. The
   planet, a gas giant slightly larger than Jupiter, orbits the main star
   of the HD 188753 system, in the constellation Cygnus, and is hence
   known as HD 188753 Ab. The stellar trio (yellow, orange, and red) is
   about 149 light-years from Earth. The planet, which is at least 14%
   larger than Jupiter, orbits the main star (HD 188753 A) once every 80
   hours or so (3.3 days), at a distance of about 8 Gm, a twentieth of the
   distance between Earth and the Sun. The other two stars whirl tightly
   around each other in 156 days, and circle the main star every 25.7
   years at a distance from the main star that would put them between
   Saturn and Uranus in the Solar System. The latter stars invalidate the
   leading hot Jupiter formation theory, which holds that these planets
   form at "normal" distances and then migrate inward through some
   debatable mechanism. This could not have occurred here; the outer star
   pair would have disrupted outer planet formation.

Interstellar

   Interstellar "planets" are planet-like objects in interstellar space,
   not gravitationally linked to any star. For a brief time in 2006,
   astronomers believed they had found a binary system of interstellar
   planets, which they termed Oph 162225-240515. However, recent analysis
   of the objects has determined that their masses are each greater than
   13 Jupiter-masses, making the pair, once considered to be planemos,
   brown dwarfs. The existence of actual interstellar planets is
   considered likely based on computer simulations of the origin and
   evolution of planetary systems, which often include the ejection of
   bodies of significant mass.

   The term is a controversial one. Some astronomers argue that only
   objects that orbit stars qualify as planets and thus an "interstellar
   planet" is an oxymoron, instead using the terms "planetary body",
   "planetary mass object" or " planemo". . The IAU's working definition
   on extrasolar planets takes no position on the issue. The discoverers
   of the bodies mentioned above decided to avoid the debate over what
   constitutes a planet by referring to the objects as planemos.

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