A planet in common parlance is a large object in orbit around a star that is not a star itself. The name comes from the Greek term πλανήτης, planētēs, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relation to the other stars. The International Astronomical Union lists nine planets in our solar system, based on historical consensus. Since the term planet has no scientific definition, many astronomers contest that figure, with some saying it should be lowered to eight (removing Pluto from the list), and others claiming it should be raised to fifteen, about twenty, or even dozens, depending on the definition used.
It is not known with certainty how planets are formed. The prevailing theory is that, when a protostar forms from a condensing nebula, the remnant of the nebula becomes a thin disc of gas and dust that revolves around the protostar. Localised mass concentrations within this disc form increasingly dense pockets of matter, which then collapse inward under gravity to form planets. When the star's core ignites its solar wind blows away the remaining material, leaving a solar system like our own.
This theory is being greatly challenged with the discovery of extrasolar systems that vary greatly from our own.
Within the solar system
- Main article: Solar system.
All of the accepted planets in the solar system are named after Roman gods, except for Uranus, which is named after a Greek god, and the Earth which was not seen as a planet by the ancients (instead considered the centre of the universe). The designated planetary names are near universal in the Western world, but some non-European languages, such as Chinese, use their own. Moons are also named after gods and characters from classical mythology or (in the case of Uranus) after Shakespearean characters. Asteroids can be named, at the discretion of their discoverers, after anybody or anything (subject to approval by the International Astronomical Union's panel on nomenclature). The process of naming planets and their features is known as planetary nomenclature.
- Mercury (astronomical symbol ☿)
- Venus (♀)
- Earth (♁) - with one confirmed satellite (Luna, also known as the Moon)
- Mars (♂) - with two confirmed natural satellites (Deimos, Phobos)
- Jupiter (♃) - with sixty-three confirmed natural satellites
- Saturn (♄) - with forty-six confirmed natural satellites
- Uranus (Astronomical symbol for Uranus - astronomical symbol; ♅ - astrological symbol) - with twenty-seven confirmed natural satellites
- Neptune (♆) - with thirteen confirmed natural satellites
- Pluto (♇) - with three confirmed natural satellites (Charon, S/2005 P 1, S/2005 P 2)
When Ceres was found orbiting between Mars and Jupiter in 1801, it was initially touted as a planet, but after many smaller objects were found with a similar orbit, it was classified as an asteroid. However, due to its large size (relative to the other asteroids), and its roughly spherical shape, Ceres would be considered a planet by some astronomers' definitions.
Similarly, since 1992 many objects have been found in the predicted Kuiper Belt that exists beyond Neptune. Several of the largest of these have challenged the planetary status quo, as they are both spherical and larger than the bodies in the Mars-Jupiter asteroid belt, and are similar in size, orbit and composition to Pluto. However, as yet none have been accepted as planets by the IAU. The most significant of these are (in increasing distance from the Sun) Orcus, Ixion, Template:Mpl, Varuna, Template:Mpl, Template:Mpl, Template:Mpl,Quaoar, Template:Mpl, Template:Mpl, Template:Mpl, Template:Mpl and Sedna. (It should be noted that Sedna is often considered to be beyond the Kuiper Belt and actually a member of the inner Oort Cloud.)
Like Ceres before it, Sedna was widely touted as a planet when it was discovered in 2003, as it was the largest object found since Pluto. However, mainly due to its size still being smaller than Pluto's, it did not achieve planetary status from the IAU. However, the discovery in 2005 of Template:Mpl, with a size and mass larger than Pluto seems to have forced the issue. As of September 2005 it has not yet been accepted as a planet, but the IAU is expected to announce a definition of a planet by the end of the year, which will either see Template:Mp become a planet, or have Pluto stripped of its status.
Definition of a planet
- Main article: Definition of planet
Much like "continent", "planet" is a word without a precise definition, with history and culture, playing as much of a role as geology and astrophysics. Recent definitions have been vague and imprecise; The American Heritage Dictionary, for instance, formerly defined a planet as:
- A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
However, for some time that definition has been viewed by many as inadequate. The eight largest planets (which are also the eight nearest to the Sun) are universally recognised as such, but there is controversy over Pluto and other objects rounded by their own gravity. For this reason these eight are often universally referred to by astronomers as "major planets".
Since the discoveries of many of the objects in the Kuiper belt and around other stars, there has been a concerted push amongst scientists to come up with a precise definition of what constitutes a planet. In 1999, the IAU set up a working group to develop a scientifically plausible recommendation, but as of August, 2005 they had not reached a conclusion. After the discovery of Template:Mpl, a member of the committee, Alan Stern, has said that the group wanted "to get something done, pronto". He also informed journalists that a "consensus" in the group was moving towards the following definition:
- A planet is a body that directly orbits a star, is large enough to be round because of self gravity, and is not so large that it triggers nuclear fusion in its interior.
Note that this definition also covers disputes at the upper end of a planet's size, which provides the extra benefit of forming a barrier between planets and brown dwarfs. Many consider this definition the best option as it sets up divisions based on physical characteristics rather than an arbitrary size limit. It is also somewhat universal in its application where other definitions have been crafted mainly to sort our own solar system into simple categories (such as placing the size limit as just under Mars, Mercury or Pluto).
Depending how it is interpreted, objects counted as planets under such a new system would include some or all of the objects listed above, with potentially many more yet to be found.
However, there are alternate suggestions which would instead reduce the number of planets in the system. Upon his discovery of Sedna, Mike Brown of Caltech suggested a definition which would exclude both Sedna and Pluto from being classified as planets, proposing the following:
- A planet is any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit 
This definition generally plays down the importance of size, but instead focuses on the formation of the proposed planet. Under this definition, no Kuiper Belt objects would be considered planets.
Brown's wish to "demote" Pluto prompted many to criticize him for setting out to create a purely scientific definition for a term which had an existing popular application, "flawed" as it might be. Upon his discovery of Template:Mp, Brown indicated he had become a convert to this way of thinking, and instead proposed that whatever definition of planet be adopted include both Pluto and any Kuiper Belt object found to be larger than Pluto. 
Planets within Earth's solar system can be divided into categories according to composition.
- Terrestrial or rocky: Planets that are similar to Earth — with bodies largely composed of rock: Mercury, Venus, Earth, Mars
- Jovian or gas giant: Those with a composition largely made up of gaseous material: Jupiter, Saturn, Uranus, Neptune. Uranian planets, or ice giants, are a sub-class of gas giants, distinguished from true Jovians by their depletion in hydrogen and helium and a significant composition of rock and ice.
- Icy: Sometimes a third category is added to include bodies like Pluto, whose composition is primarily ice; this category of "icy" bodies also includes many non-planetary bodies such as the icy moons of the outer planets of our solar system (e.g. Triton).
Many consider the Earth and its Moon to be a double planet, for several reasons:
- The Moon, as measured by its diameter, is 1.5 times larger than Pluto.
- The gravitational force of the Sun on the Moon is larger than the gravitational force of the Earth on the Moon (by about a factor of 2.2)
The latter fact is not unique in the solar system, but is unusual for such a large satellite. Other satellites for which the Sun's gravity is actually stronger than the primary's:
- The outermost moon of Jupiter (S/2003 J 2; factor of 1.5)
- The outermost moon of Uranus (S/2001 U 2; factor of 1.2)
- The two outermost moons of Neptune (S/2002 N 4 and Psamathe; factor of 2.1)
- Several asteroid moons (e.g. (22) Kalliope I Linus by a factor of 1.6; (45) Eugenia I Petit-Prince by a factor of 2.8; (243) Ida I Dactyl by a factor of 1.3; and, most remarkably, S/2001 (66391) 1 by a large factor of 625.)
However, the Earth's moon aside, these objects are much too small to be considered planets.
- Main article: Extrasolar planet.
Of the 173 extrasolar planets (those outside our solar system) discovered to date (October 2005) most have masses which are about the same or larger than Jupiter's.
Exceptions include a number of planets discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12, 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 origin.
It is far from clear if the newly discovered large planets would resemble the gas giants in our 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 our 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.
The National Aeronautics and Space Administration of the United States has a program underway to develop a Terrestrial Planet Finder artificial satellite, which would be capable of detecting the planets with masses comparable to terrestrial planets. The frequency of occurrence of these planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy.
Astronomers have recently   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 our own Solar System. The latter stars invalidate the leading hot Jupiter formation theory, which holds these planets form at "normal" distances and then migrate inward through some debatable mechanism. This could not have occurred here, the outer star pair disrupting outer planet formation.
Brown dwarf "planets"
The discovery of a planet-sized satelite of a brown dwarf has blurred the distinction between "planet" and "moon." A brown dwarf, though a star in theory, in practice is often described as in between a planet and a star. It is formally defined by the International Astronomical Union by its official statement that "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."
To the International Astronomical Union, the question of whether an object in orbit around a brown dwarf is a "planet" or a "moon" was simply not relevant, as it does not use the term "moon," only "satellite" and as yet has no official definition for "planet."
Interstellar planets are rogues in interstellar space, not gravitationally linked to any given solar system. No interstellar planet is known to date, but their existence is considered a likely hypothesis based on computer simulations of the origin and evolution of planetary systems, which often include the ejection of bodies of significant mass.
Such objects are not formally called planets, however, since the International Astronomical Union has not defined the term "planet".
- Definition of planet
- Planetary habitability
- Planetary science
- Planets in science fiction
- Prograde and retrograde motion
- Skies of other planets
- Template:Citepaper version
- NinePlanets.org - tour of the solar system
- International Astronomical Union
- Solar System Live (an interactive orrery)
- Solar System Viewer (animation)
- Pictures of the solar system
- Renderings of the planets
- NASA Planet Quest
- Working definition of "planet" from IAU WGESP — the lower bound remained a matter of consensus in February 2003
- Dan Green's page on planet classification
- Gravity Rules: The Nature and Meaning of Planethood; S. Alan Stern; March 22, 2004
- On the status of Pluto; IAU, February 3, 1999
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