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Planet

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The word planet comes from the Greek word πλανήτης (planetes) which is derived from the word πλάνης (planes) meaning "wanderer."[1]

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.

Contents

Types

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.[2]

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.[2]

Dwarf

Main Article: Dwarf planet

The furthest and smallest once considered planet Pluto is solid as ice when compared to the terrestrial planets.[3] 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.[4] 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.

Table of Planets

List of all the planets, from the innermost to the outermost:
Name Perihelion Aphelion Eccentricity Sidereal year Inclination Mass Sidereal day
Mercury (planet) 460000000000.307 AU46,000,000 km
28,583,074.843 mi		 698200000000.467 AU69,820,000 km
43,384,136.642 mi		 0.20560.206 87.9690.241 a87.969 da 0.12217304763967 °0.122 rad
7.778 grad		 3.302E+230.0553 M⊕3.302e+23 kg
1.73908e-4 M♃		 50673601,407.6 h58.65 da
Venus 1074800000000.718 AU107,480,000 km
66,784,975.742 mi		 1089400000000.728 AU108,940,000 km
67,692,177.682 mi		 0.00670.0067 224.7010.615 a224.701 da 0.0591666616426083.39 °0.0592 rad
3.767 grad		 4.8685E+240.815 M⊕4.8685e+24 kg
0.00256 M♃		 -20997000-5,832.5 h-243.021 da
Earth 1470900000000.983 AU147,090,000 km
91,397,488.666 mi		 1521000000001.017 AU152,100,000 km
94,510,558.339 mi		 0.016710220.0167 365.2563661 a365.256 da 8.7266462599717E-075.0e-5 °8.72665e-7 rad
5.55556e-5 grad		 5.9736E+241 M⊕5.9736e+24 kg
0.00315 M♃		 86164.223.935 h0.997 da
Mars 2066200000001.381 AU206,620,000 km
128,387,715.74 mi		 2492300000001.666 AU249,230,000 km
154,864,342.241 mi		 0.09350.0935 686.981.881 a686.98 da 0.0322885911618951.85 °0.0323 rad
2.056 grad		 6.4185E+230.107 M⊕6.4185e+23 kg
3.38047e-4 M♃		 88642.4424.623 h1.026 da
Jupiter 7405200000004.95 AU740,520,000 km
460,137,795.276 mi		 8166200000005.459 AU816,620,000 km
507,424,143.005 mi		 0.048390.0484 4330.58662511.857 a4,330.587 da 0.0227765467385261.305 °0.0228 rad
1.45 grad		 1.8987E+27317.721 M⊕1.8987e+27 kg
1 M♃		 357309.925 h0.414 da
Saturn 13525500000009.041 AU1.35255e+9 km
840,435,606.061 mi		 151450000000010.124 AU1.5145e+9 km
941,066,670.643 mi		 0.05650.0565 10759.2229.457 a10,759.22 da 0.0433714319120592.485 °0.0434 rad
2.761 grad		 5.6846E+2695.124 M⊕5.6846e+26 kg
0.299 M♃		 38361.610.656 h0.444 da
Uranus 274130000000018.324 AU2.7413e+9 km
1.70336e+9 mi		 300362000000020.078 AU3.00362e+9 km
1.86636e+9 mi		 0.04570.0457 30681.6153001584.002 a30,681.615 da 0.0129852496348380.744 °0.013 rad
0.827 grad		 8.6832E+2514.53 M⊕8.6832e+25 kg
0.0457 M♃		 -62064-17.24 h-0.718 da
Neptune 444445000000029.709 AU4.44445e+9 km
2.76165e+9 mi		 454567000000030.386 AU4.54567e+9 km
2.82455e+9 mi		 0.01130.0113 60189.5475164.79 a60,189.548 da 0.030874874467781.769 °0.0309 rad
1.966 grad		 1.0243E+2617.14 M⊕1.0243e+26 kg
0.0539 M♃		 5799616.11 h0.671 da


Extrasolar Planets

In this artist's conception, a possible newfound planet spins through a clearing in a nearby star's dusty, planet-forming disc. This clearing was detected around the star CoKu Tau 4 by NASA's Spitzer Space Telescope. Astronomers believe that an orbiting massive body, like a planet, may have swept away the star's disc material, leaving a central hole.
In this artist's conception, a possible newfound planet spins through a clearing in a nearby star's dusty, planet-forming disc. This clearing was detected around the star CoKu Tau 4 by NASA's Spitzer Space Telescope. Astronomers believe that an orbiting massive body, like a planet, may have swept away the star's disc material, leaving a central hole.
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.[5] As of March 23, 2008, 277 exoplanets in orbit around 238 stars are now known.[6] 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.[7]

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.[8]

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. [9]

”

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.

References

  1. ↑ "Entry for 'planet'," Wiktionary, January 9, 2008. Accessed January 14, 2008.
  2. ↑ 2.0 2.1 Worldbook at NASA: Solar System by the U.S. National Aeronautic and Space Administration
  3. ↑ "Our Solar System." NASA. Accessed March 10, 2008.
  4. ↑ "The Final IAU Resolution on the definition of "planet" ready for voting." International Astronomical Union, August 24, 2006. Accessed March 10, 2008.
  5. ↑ Bell, George H. "The Search for the Extrasolar Planets: A Brief History of the Search, the Findings and the Future Implications." Arizona State University, April 5, 2001. Accessed March 23, 2008.
  6. ↑ Jackson, Randall, curator. "PlanetQuest: Exoplanet Exploration." JPL, NASA. Accessed March 23, 2008.
  7. ↑ <http://exoplanets.org/> Center for Integrative Planetary Science, University of California, Berkeley, California. Accessed March 23, 2008.
  8. ↑ Mullen, Leslie. "Young Planet Challenges Old Theories." Astrobiology Magazine online, May 28, 2004. Accessed March 7, 2008.
  9. ↑ "IAU0602: the Final IAU Resolution on the Definition of 'Planet' Ready for Voting," International Astronomical Union, 2005. Accessed January 14, 2008.

Related Links


See Also


v • d • e
Astronomy / Cosmology
Feature articles
Categories · Portal
The Solar System
Star Sol
Factitious Planet Vulcan
Terrestrial Planets Mercury • Venus • Earth • Mars
Gas giants Jupiter • Saturn • Uranus • Neptune
Dwarf Planets Ceres • Pluto • Eris
Asteroid Belt Major asteroids • C-type asteroids • S-type asteroids • M-type asteroids
Trans-Neptunian Objects Kuiper belt • Scattered disk • Oort cloud • Nemesis
Moons Terrestrial • Martian • Jovian • Saturnine • Uranian • Neptunian • Plutonian • Eridian
Featured moons Moon • Phobos • Deimos • Io • Europa • Ganymede • Callisto • Mimas • Enceladus • Tethys • Dione • Rhea • Titan • Hyperion • Iapetus • Miranda • Ariel • Umbriel • Titania • Oberon • Triton • Nereid • Charon • Nix • Hydra • Dysnomia
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