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 the end of 2002 over 100 are known, with all of the new discoveries being extrasolar planets.
Astronomers often call asteroids minor planets, and call the larger planetary bodies (those which are commonly called planets) major planets. Planets within the solar system can be divided into categories according to composition. Those that are similar to Earth - with bodies largely composed of rock - are called terrestrial or rocky planets. Those with a composition largely made up of gaseous material, as with Jupiter, are called Jovian or gas giant planets. 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 planets of our solar system (in increasing distance from the Sun) are
- Earth -- with its Moon, sometimes considered a "double planet."
- Pluto -- with its moon Charon sometimes considered a double planet, and also sometimes not considered a planet at all, but simply the largest of the Trans-Neptunian objects in the Kuiper belt.
Several hypothetical planets, like Planet X (supposedly beyond the orbit of Pluto) or Vulcan (thought to orbit inside the orbit of Mercury), were proposed, and were subjects of intense searches that found nothing.
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 closely 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 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 possibility of extraterrestrial intelligence.
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.
See also: retrograde