A planet (from Ancient Greek ἀστὴρ πλανήτης (astēr planētēs), meaning "wandering star") is an astronomical object orbiting around a Star or Stellar remenent that is:
- massive enough to be rounded by its own gravity
- is not massive enough to cause thermonuclear fusion, and
- has cleared its neighbouring region of planetesimals.
The term "planet" is ancient, with ties to history, science, mythology, and religion. The planets were originally seen by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, however, human perception of the planets changed, incorporating a number of disparate objects.
In 2006, the International Astronomical Union (IAU) officially adopted a resolution defining planets within the Solar System. This definition has been both praised and criticized and remains disputed by some scientists because it excludes many objects of planetary mass based on where or what they orbit.
Planets are generally divided into two main types; large, low-density gas giants and rocky, smaller terrestrial planets. Under IAU definitions, there are eight planets in the Solar System. In order of increasing distance from the Sun, there are four terrestrial planets, Mercury, Venus, Earth, and Mars, then the four gas giants: Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites.
Although not planets, the IAU accepts five dwarf planets, with many others under consideration, and hundreds of thousands of small Solar System bodies.
Since 1992, thousands of planets have been found orbiting other stars in the Milky Way (called 'extrasolar planets). As of March 8, 2018, 3,743 known extrasolar planets in 2,796 planetary systems (inclunding 625 multiple planetary systems), ranging from small, Moon-sized plants to planets that are twice the size of Jupiter. More than 100 of these planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth from the Sun, i.e. in the habitable zone.