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A planet is generally considered to be a relatively large mass of accreted matter 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. Based on historical consensus, the International Astronomical Union (IAU) lists nine planets in our solar system. However, since the term "planet" has no precise scientific definition, many astronomers contest that figure. Some say it should be lowered to eight by removing Pluto from the list, while others claim it should be raised to ten, fifteen, twenty, or even higher.

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Planetary formation
Within our solar system
Extrasolar planets
Definition and classification of planets

Planetary formation - Contents

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 don't condense under gravity to form a protostar. Instead, these remnants become a thin disc of dust and gas revolving around the protostar and begin to condense about local concentrations of mass within the disc. These concentrations become ever more dense until they collapse inward under gravity to form protoplanets. 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. Meanwhile, protoplanets that have avoided collisions may become moons of larger planets.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.

Within our solar system - Contents

The process of naming planets and their features is known as planetary nomenclature. All the currently accepted planets in the solar system are named after Roman gods, except for Uranus (named after a Greek god) and the Earth, which was not seen as a planet by the ancients but rather the centre of the universe. The original number of planets was seven: Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn. These were all seen as regular "wanderers" in the sky. Developments in Astronomy removed the Sun and the Moon, and added the currently accepted planetary members of the Solar System.The designated planetary names are near-universal in the Western world, but some non-European languages, such as Chinese, use their own. A western exception is, naturally, Greece which uses the equivalent Greek gods' names: Hermes (Mercury), Aphrodite (Venus), Gaia (Earth), Ares (Mars), Zeus (Jupiter), Cronus (Saturn), Ouranos (Uranus), Poseidon (Neptune), Pluton - not the expected Hades - (Pluto). Moons are also named after gods and characters from classical mythology, or, in the case of Uranus, after characters from English literature. Asteroids can be named after anybody or anything at the discretion of their discoverers, subject to approval by the IAU's nomenclature panel.

Accepted planets
Planets in approximate scale of size, but not distance. A portion of the solar disc is shown at the top.
Planets in approximate scale of size, but not distance. A portion of the solar disc is shown at the top.
According to the authority of the IAU, there are nine planets in our solar system. In increasing distance from the Sun they are (with the astronomical symbol in brackets and their natural satellites):
  1. Mercury ()
  2. Venus ()
  3. Earth () with one confirmed natural satellite, Luna (the Moon)
  4. Mars () with two confirmed natural satellites, Phobos and Deimos
  5. Jupiter () with sixty-three confirmed natural satellites
  6. Saturn () with forty-six confirmed natural satellites
  7. Uranus (Astronomical symbol for Uranus) with twenty-seven confirmed natural satellites
  8. Neptune () with thirteen confirmed natural satellites
  9. Pluto () with three confirmed natural satellites, Charon, S/2005 P 1 and S/2005 P 2
However, there is some pressure for Pluto to be reclassified as a Kuiper Belt object, especially in light of the discovery of 2003 UB313 (temporarily nicknamed "Xena"). This object, however, has not yet received a definitive classification from the IAU.

Other candidates
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 order of increasing distance from the Sun, Orcus, 2003 EL61 ("Santa"), Quaoar, 2005 FY9 ("Easter Bunny"), 2003 UB313 and Sedna. Sedna is however often considered to be beyond the Kuiper Belt; being either a member of the scattered disc or 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 2003 UB313, with a size and mass larger than Pluto seems to have forced the issue. As of January 2006 it has not yet been accepted as a planet, but the IAU is expected to announce a definition of a planet soon, which will likely either see 2003 UB313 become a planet, or have Pluto stripped of its status.

Extrasolar planets - Contents

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 [1], 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 [2] [3] 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 satellite of a brown dwarf has blurred the distinction between "planet" and "star". 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 IAU 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 IAU, 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
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 IAU has not defined the term "planet".

Definition and classification of planets - Contents

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, and for this reason are often universally referred to as "major planets", but there is controversy over Pluto and other smaller objects. It is also worthwhile to remember that, before the invention of the telescope, the classical planets were seven in number: Mercury through Saturn, minus the Earth, plus the Sun and Moon (in certain cultures this is the reason for a seven day week).

Suggested wide definitions
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 2003 UB313 (informally called "Xena"), 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. Gibor Basri, head of astronomy at the University of Berkeley, has suggested a similar definition and has also proposed the terms " fusor" (any object that achieves fusion in its core) and " planemo" (an object that is round from self-gravity but not a fusor) to help improve the astronomical nomenclature. Under Basri's definition:
A planet is a planemo orbiting a fusor
These definitions have the advantage of creating a group including larger moons (which share many characteristics with the smaller planets) and also covering large free-roaming objects, which some astronomers think should be included in the definition of a planet. Basri has also suggested 'liberal use of adjectives' such as "major", "beltway", "dwarf", "giant", "super" and "historical". [4] Others have suggested categories of planet/planemo based on composition such as "rock" (composed mainly of silicate), "gas" (composed mainly of hydrogen and helium), and "ice" (composed mainly of oxygen and carbon).

Suggested narrow definitions
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: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 (including Pluto) 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 (albeit 'flawed') application. Upon his discovery of 2003 UB313, Brown indicated he had become a convert to this way of thinking, and proposed that whatever definition of planet be adopted, it should include both Pluto and any Kuiper Belt object found to be larger than Pluto. [6]

Further categories
Astronomers distinguish between minor planets, such as asteroids, comets, and trans-Neptunian objects; and major (or true) planets.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 a factor of approx. 2.2. (This is not a unique situation in the solar system. The Sun's gravity is also stronger than the primary's on Jupiter's moon S/2003 J 2; Uranus' moon S/2001 U 2; Neptune's moons S/2002 N 4 and Psamathe; and several asteroid moons. However, Luna is the sole case of this phenomenon affecting an object of planetary mass.)
The Pluto- Charon system has been considered to be a binary planet because the mass ratio between these two bodies is small (about 8.6). So much so, that they both orbit a common centre of mass which lies beyond Pluto's surface.
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