A planet (from the Greek πλανήτης, planetes or "wanderers") is a body of considerable mass that orbits a star and that produces no energy through nuclear fusion. Prior to the 1990s only nine were known (all of them in our own solar system); as of 3 November, 2004, 133 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
- Main article: Solar system.
Except for Earth (which was not perceived as being a planet by the ancients), all of the accepted planets in the solar system are named after Greek or Roman gods (however, some non-European languages such as Chinese use different 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 act of naming planets and their features is known as planetary nomenclature.
According to the authority of the International Astronomical Union, there are nine planets in our solar system (in increasing distance from the Sun):
Mercury (astronomical symbol ☿)
Earth (♁) - with the Moon
Mars (♂) - 2 satellites (Deimos, Phobos)
Jupiter (♃) - 63 confirmed natural satellites
Saturn (♄) - 30 confirmed satellites
Uranus (♅) - 27 moons
Neptune (♆) - 13 moons
Pluto (♇) (many astronomers contend it should be classified as a Kuiper belt object and not also a planet) - 1 satellite Charon
Recently an object, 90377 Sedna, has been discovered orbiting the Sun 90 AU (13 billion kilometres) away, three times farther than Pluto. Sedna, after the Inuit goddess of the sea, is a 1180–2360 km (730-1470 miles) diameter object. Several news sources have already reported Sedna as the tenth planet , but that is not generally accepted by astronomers. Another possible planet is 90482 Orcus, 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. 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 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  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:
- 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 three planets discovered orbiting PSR B1257+12 a burned-out star, or supernova remnant, called a pulsar, comparable in size to the terrestrial planets; and planets orbiting the stars Mu Arae, 55 Cancri and GJ 436 which are approximately neptune sized .
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.