A planet is an object in orbit around a star that is not a star in its own right. Much like "continent," "planet" is a word without a precise definition, with history and culture playing as much of a role as geology and astrophysics. The IAU lists nine planets in our solar system, although many astronomers contest that figure, with some saying it should be lowered and others demanding it be raised. With the discovery since 1995 of over 150 planet-sized objects in orbit around other stars, the problem of defining a planet has become rather more acute.

Planets are believed to form from a collapsing nebula that a star formed from, aggregating from gas and dust that orbited the protostar in a dense protostellar disk before the star's core ignited and its solar wind blew remaining material away.


Within the solar system

Main article: Solar system.

All of the accepted planets in the solar system are named after Roman gods, except for Earth which was not seen as a planet by the ancients, and Uranus, which is named for a Greek god. It should be noted that some non-European languages, such as Chinese, use their own planetary names. Moons are also named after gods and characters from classical mythology or (in the case of Uranus) 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 process of naming planets and their features is known as planetary nomenclature.

Planets in the Solar System

Planets in approx. scale of size, but not distance. Note a portion of the solar disc shown at the top
Planets in approx. scale of size, but not distance. Note a portion of the solar disc shown at the top

According to the authority of the International Astronomical Union, there are nine planets in our solar system (in increasing distance from the Sun):

  1. Mercury (astronomical symbol ☿)
  2. Venus (♀)
  3. Earth (♁) - with the Moon
  4. Mars (♂) - 2 satellites (Deimos, Phobos)
  5. Jupiter (♃) - 63 confirmed natural satellites
  6. Saturn (♄) - 46 confirmed satellites
  7. Uranus (♅) - 27 moons
  8. Neptune (♆) - 13 moons
  9. Pluto (♇) (many astronomers contend it should be classified as a Kuiper belt object and not also a planet) - 1 satellite Charon


Astronomers distinguish between minor planets, such as asteroids, comets, and trans-Neptunian objects; and major (or true) planets. Isaac Asimov suggested the term mesoplanet be used for planetary objects intermediate in size between Mercury and Ceres, which would include the five objects mentioned above (Pluto, Sedna, Orcus, Quaoar, and Varuna).

Planets within Earth's solar system can be divided into categories according to composition.

  • Terrestrial or rocky: Planets that are similar to Earth — with bodies largely composed of rock: Mercury, Venus, Earth, Mars
  • Jovian or gas giant: Those with a composition largely made up of gaseous material: Jupiter, Saturn, Uranus, Neptune. Uranian planets, or ice giants, are a sub-class of gas giants, distinguished from true Jovians by their depletion in hydrogen and helium and a significant composition of rock and ice.
  • 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 moons of the outer planets of our solar system (e.g. Triton).

The eight rocky and gaseous planets are universally recognized as major planets. Ceres was called a planet when first discovered, but was reclassified as an asteroid when many similar objects were found. Given recent discoveries of many trans-Neptunian objects which are very similar to Pluto in orbit, size and composition, many people think it should be similarly redefined as a minor planet. 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, for several reasons:

  • The Moon, as measured by its diameter, is 1.5 times larger than Pluto.
  • The gravitational force of the Sun on the Moon is larger than the gravitational force of the Earth on the Moon (by about a factor of 2.2)

The latter fact is not unique in the solar system, but is unusual for such a large satellite. Other satellites for which the Sun's gravity is actually stronger than the primary's:

Extrasolar planets

Main article: Extrasolar planet.

Most extrasolar planets (those outside our solar system) discovered to date have masses which are about the same or larger than Jupiter's.

Exceptions include at least planets discovered orbiting PSR B1257+12 a burned-out star, or supernova remnant, called a pulsar, comparable in size to the terrestrial planets; planets orbiting the stars Mu Arae, 55 Cancri and GJ 436 which are approximately Neptune-sized [2] (; and a planet orbiting Gliese 876 that is estimated to be about 6 to 8 times as massive as the earth and is probably rocky in origin.

It is far from clear if the newly discovered large 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, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent star, in nearly circular 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. There is also a class of hot Jupiters that orbit so close to their star that their atmospheres are slowly blown away in a comet-like tail: the Chthonian planets.

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.

Current technology is not sensitive enough to detect planets of relatively small mass and orbiting far because such planets cause very small "wobble effect" on their star. Discoveries of smaller planets will require radical improvements in telescopes.


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