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''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.

Table of contents
1 Within the Solar System
2 Extrasolar Planets
3 See Also
4 External Links

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

English-speaking schoolchildren often use a variety of mnemonics to remember the planets in this order:

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.


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.

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] Using 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 interstellar space, not gravitationally linked to any given solar system. No interstellar planet is known to date, but their existence is considered a plausible hypothesis on the grounds that the results of computer simulations of the origin and evolution of planetary systems often include the formation and subsequent ejection of bodies of significant mass.

See Also

External Links