Planet



The word planet comes from the Greek word πλανήτης (planetes) which is derived from the word πλάνης (planes) meaning "wanderer."

Originally the term was applied to any object that moved in the sky including stars. However, as a more complete picture of our universe emerged from the work of astronomers, the classification of a planet became more specific. In our solar system there are eight definite planets; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. In 2006 the International Astronomical Union (IAU) demoted Pluto from its classification as a planet to dwarf planet because it did not conform to recently established criteria.

Terrestrial

 * Main Article: Terrestrial planet

The inner four planets consist chiefly of iron and rock, Mercury, Venus, Earth and Mars. They are called the terrestrial planets because they have solid rocky surfaces, and are somewhat similar in size and composition to the Earth.

Gas Giant

 * Main Article: Gas giant

Beyond the orbit of Mars lie Jupiter, Saturn, Uranus and Neptune, the gas giants of our solar system. The four outer planets are giant worlds with thick, gaseous outer layers. Almost all their mass consists of hydrogen and helium, giving them compositions more like that of the sun than that of Earth. Beneath their outer layers, the giant planets have no solid surfaces. The pressure of their thick atmospheres turns their insides liquid, though they may have rocky cores.

Dwarf

 * Main Article: Dwarf planet

The furthest and smallest once considered planet Pluto is solid as ice when compared to the terrestrial planets. On August 24, 2006, the International Astronomical Union (IAU) formally downgraded Pluto from an official planet of our solar system to a mere dwarf planet. This occurred following the discovery of Eris, a scatter-disk object heavier even than Pluto but which, like Pluto, had not cleared its neighborhood of debris. There are now eight official planets of our solar system according to the IAU. The dwarf planet class includes Eris, Pluto, and the former asteroid Ceres, which is at least large enough to collapse under its own weight into a spheroid.

Extrasolar Planets
An extrasolar planet or exoplanet is any planet outside our solar system. Speculation on the existence of exoplanets probably dates to September of 1916, with the first article describing the unusually large proper motion of Barnard's Star, a red dwarf star about 1.82 parsecs distant from Earth. As of March 23, 2008, 277 exoplanets in orbit around 238 stars are now known. Most such planets are gas giants, but speculation about the finding of an Earth-like exoplanet continues. The lightest-mass exoplanet found thus far is a 7.5-Earth-mass planet in orbit around the star Gliese 876.

Some evolutionists assert that planetary and galaxy formation take millions of years. However, in 2004, the Spitzer Space Telescope detected a clearing of dust around a star that is "only" a million years old. They theorize that the object that cleared the dust is an exoplanet at least as large as Jupiter. This would be (by evolutionary standards) the youngest planet ever observed. According to Alan Boss, an astronomer for the Carnegie Institution of Washington, the find "has profound implications for the prevalence of planetary systems similar to our own. That means you can make gas giant planets - a major component of our own solar system - in a short time scale, in even the shortest-lived disc." The discovery suggested scientists would have to rethink their models about planetary formation.

Planet Re-defined
On August 24, 2006, the International Astronomical Union (IAU) formally downgraded Pluto from an official planet of our solar system to a mere dwarf planet. There are now eight official planets of our solar system and according to The Final IAU Resolution on the definition of "planet" ready for voting statement on the IAU website,

RESOLUTION 5A The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

(1) A "planet"1 is a celestial body that (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.

(2) A "dwarf planet" is a celestial body that (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) shape2, (c) has not cleared the neighbourhood around its orbit, and (d)is not a satellite.

(3) All other objects3, except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

1The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. 2An IAU process will be established to assign borderline objects into either dwarf planet and other categories. 3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

While parameters (b) and (c) appeal to assumptions of unobservable processes in planetary origin, the overall assumption is the naturalistic philosophy of the formation of the solar system. From a creationist perspective the planets were made mature and with planetary laws to govern them on day four of creation.

Historically there may have been difficulty in defining what exactly constitutes a planet, there appears to be several characteristics traditional planets share:


 * In orbit around the sun.
 * Basically spherical.
 * Dominant of their orbit, having no other significant objects in their path around the sun.

Related Links

 * IAU International Astronomical Union homepage
 * Planet by Wikipedia

Planète 행성 Планета