Nama : nur elita XL ips 1

Sumber: http://en.wikipedia.org/wiki/Planet

Planet

A planet (from Ancient Greek αστήρ πλανήτης (astēr planētēs), meaning "wandering star") is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.^[a][1][2] The term planet is ancient, with ties to history, science, mythology, and religion. The planets were originally seen by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union (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 since it excludes many objects of planetary mass based on where or what they orbit. While eight of the planetary bodies discovered before 1950 remain "planets" under modern definition, some celestial bodies, such as Ceres, Pallas, Juno, Vesta (each an object in the Solar asteroid belt) and Pluto (the first-discovered trans-Neptunian object), that were once considered planets by the scientific community are no longer viewed as such.

The planets were thought by Ptolemy to orbit the Earth in deferent and epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. By careful analysis of the observation data, Johannes Kepler found the planets' orbits to be not circular, but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some 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.

Planets are generally divided into two main types: large, low-density gas giants, and smaller, rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System. In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four gas giants, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites. Additionally, the Solar System also contains at least five dwarf planets^[3] and hundreds of thousands of small Solar System bodies.

Since 1992, hundreds of planets around other stars ("extrasolar planets" or "exoplanets") in the Milky Way Galaxy have been discovered. As of August 24, 2012, 778 known extrasolar planets (in 624 planetary systems and 105 multiple planetary systems) are listed in the Extrasolar Planets Encyclopaedia, ranging in size from that of terrestrial planets similar to Earth to that of gas giants larger than Jupiter.^[4] On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e^[5] and Kepler-20f,^[6] orbiting a Sun-like star, Kepler-20.^[7][8][9] A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.^[10]

[edit] History

Further information: History of astronomy and Definition of planet

See also: Timeline of Solar System astronomy

Jupiter, approaching that of stellar objects known as "brown dwarfs".^[43] Brown dwarfs are generally considered stars due to their ability to fuse deuterium, a heavier isotope of hydrogen. While stars more massive than 75 times that of Jupiter fuse hydrogen, stars of only 13 Jupiter masses can fuse deuterium. However, deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.^[44]

[edit] 21st century

With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There 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.

A growing number of astronomers argued for Pluto to be declassified as a planet, since many similar objects approaching its size had been found in the same region of the Solar System (the Kuiper belt) during the 1990s and early 2000s. Pluto was found to be just one small body in a population of thousands.

Some of them including Quaoar, Sedna, and Eris were heralded in the popular press as the tenth planet, failing however to receive widespread scientific recognition. The announcement of Eris in 2005, an object 27% more massive than Pluto, created the necessity and public desire for an official definition of a planet.

Acknowledging the problem, the IAU set about creating the definition of planet, and produced one in August 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).^[45]

[edit] Extrasolar planet definition

In 2003, The International Astronomical Union (IAU) Working Group on Extrasolar Planets made a position statement on the definition of a planet that incorporated the following working definition, mostly focused upon the boundary between planets and brown dwarfs:^[2]

Artistic comparison of Eris, Pluto, Makemake, Haumea, Sedna, 2007 OR₁₀, Quaoar, Orcus, and Earth. These eight trans-Neptunian objects have the brightest absolute magnitudes, although several other TNOs have been found to be physically larger than Orcus, and several more may yet be found.

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^[46]) 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 the 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 or 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 of exoplanets in academic journals.^[47] Although temporary, it remains an effective working definition until a more permanent one is formally adopted. However, it does not address the dispute over the lower mass limit,^[48] and so it steered clear of the controversy regarding objects within the Solar System. This definition also makes no comment on the planetary status of objects orbiting brown dwarfs, such as 2M1207b.

One definition of a sub-brown dwarf is a planet-mass object that formed through cloud-collapse rather than accretion. This formation distinction between a sub-brown dwarf and a planet is not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.^[49] One reason for the dissent is that oftentimes it may not be possible to determine the formation process: for example an accretion-formed planet around a star may get ejected from the system to become free-floating, and likewise a cloud-collapse-formed sub-brown dwarf formed on its own in a star cluster may get captured into orbit around a star.

Dwarf planets 2006–present
Ceres	Pluto	Makemake	Haumea	Eris

The 13 Jupiter-mass cutoff is a rule of thumb rather than something of precise physical significance. The question arises: what is meant by deuterium burning? This question arises because large objects will burn most of their deuterium and smaller ones will burn only a little, and the 13 MJ value is somewhere in between. The amount of deuterium burnt depends not only on mass but also on the composition of the planet, on the amount of helium and deuterium present.^[50]

Another criterion for separating planets and brown dwarfs, rather than deuterium burning, formation process or location is whether the core pressure is dominated by coulomb pressure or electron degeneracy.^[51][52]

Sumber: http://en.wikipedia.org/wiki/Planet