Planet
Category:Planets\n:''For other uses see
Planet (disambiguation)
A
planet (from the Greek
planetes or "wanderers") is a body of considerable mass that orbits a
star and that doesn't produce energy through
nuclear fusion. Prior to the
1990s only nine were known (all of them in our own
solar system); as of
2004, 118 are known, with all of the new discoveries being
extrasolar planets, sometimes known as "exoplanets".
Planets are thought to form from the collapsing
nebula that a planet's star formed out of, aggregating from gas and dust that orbits the
protostar in a dense protostellar disk before the star's core ignites and its
solar wind blows the remaining material away.
Within the Solar System
Except for Earth, all of the accepted planets in the solar system are named after Roman godss. Moons are also named after gods and characters from classical mythology or from the plays of Shakespeare. 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 act of naming planets and their features is known as planetary nomenclature.
Accepted Planets
The commonly accepted list of major planets of our solar system (in increasing distance from the Sun) are\n* Mercury\n* Venus\n* Earth\n* Mars\n* Jupiter\n* Saturn\n* Uranus\n* Neptune\n* Pluto (Pluto's orbit lies partly within that of Neptune)\nEnglish-speaking schoolchildren often use a variety of mnemonics to remember the planets in this order:\n*Mary's Violet Eyes Make John Stay Up Nights, Period. (Or Poor John to include the Planetoids)\n*Mother Very Thoughtfully Made Jelly Sandwiches Under No Protest. (Or A Jelly Sandwich to include Asteroids)\n*My Very Energetic Mother Just Served Us Nine Pizzas. (Or "Pistachio Nuts" for those rare times when Pluto is within the orbit of Neptune)\n*My Very Educated Mother Just Showed Us Nine Planets
Other Objects
Recently an object, (2003 VB12) Sedna has been discovered orbiting the sun 13 billion kilometres away, three times farther than Pluto. Sedna, after the Inuit goddess of the sea, is the provisional name given to this 1180–2360 km (730-1470 miles) diameter object by NASA. Its official name for now is 2003 VB12. The diameter is still uncertain but believed to be between 1/2 and 3/4 of Pluto's. Several news sources have already reported Sedna as the tenth planet [1], but that is not generally accepted by astronomers. Another possible planet is 2004 DW, an object with an orbit and mass similar to Pluto's. Other candidates include 50000 Quaoar and 20000 Varuna.
Several hypothetical planets, like Planet X (supposedly beyond the orbit of Pluto) or Vulcan (thought to orbit inside the orbit of Mercury), were posited at various historical times, and were subjects of intense searches that found nothing.
Classification
\nAstronomers 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.\n* Terrestrial or rocky: Planets that are similar to Earth—with bodies largely composed of rock: Mercury, Venus, Earth, Mars\n* Jovian or gas giant: Those with a composition largely made up of gaseous material: Jupiter, Saturn, Uranus, Neptune. Uranian planets are a sub-class of gas giants, distinguished from true Jovians by their depletion in hydrogen and helium.\n* 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 moonss of the outer planets of our solar system (e.g. Titan).
The eight rocky and gaseous planets are universally recognized as major planets. For consistency's sake, some believe that due to size and composition Pluto (like Sedna) should be classified a minor planet, as the largest of the Trans-Neptunian objects in the Kuiper belt. For example, Mike Brown of Caltech defines a planet to be: any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit [1]\nUsing this definition, neither Pluto nor Sedna would be a major planet.
Many consider the Earth and its Moon to be a double planet, given that the Moon is rocky and almost the same size as Mercury.
Extrasolar Planets
Almost all extrasolar planets (those outside our solar system) discovered to date have masses which are about the same or larger than the gas giants within the solar system. (The only exception is three planets discovered orbiting a burned-out star, or supernova remnant, called a pulsar. These are comparable in size to the terrestrial planets) This is largely because the gravitational effect of massive planets is larger, making them easier to detect. However, it is far from clear if the newly discovered planets would resemble gas giants in our solar system or if they are of an entirely different type or types which are unknown in our solar system. In particular, some of the newly discovered planets orbit extremely close to their parent star, sometimes in highly elliptical 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.
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.
Interstellar planets are rogues in the interstellar space, not gravitationally linked to any given solar system. No interstellar planet is known to date, but they may exist.
See Also
\n* Prograde and retrograde motion\n* Planets in science fiction\n* Skies of other planets
External Links
\n*Planetary Motions\n*NASA Planet Quest\n*Working definition of "planet" from IAU WGESP -- the lower bound remained a matter of consensus in February 2003\n*Dan Green's page on planet classification\n*Gravity Rules: The Nature and Meaning of Planethood; S. Alan Stern; March 22, 2004
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