A planet (from Ancient Greek αστήρ πλανήτης (astēr planētēs), meaning "wandering star") is a celestial body orbiting a star or stellar remnant 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, 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 since it excludes many objects of planetary mass based on where or what they orbit. While eight of the planetary bodies discovered before 1950 remain "planets" under modern definition, some celestial bodies, such as Ceres, Pallas, Juno, Vesta (each an object in the Solar asteroid belt) and Pluto (the first-discovered trans-Neptunian object), that were once considered planets by the scientific community are no longer viewed as such.
The planets were thought by Ptolemy to orbit the Earth in deferent and epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. By careful analysis of the observation data, Johannes Kepler found the planets' orbits were not circular, but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology.
Planets are generally divided into two main types: large, low-density gas giants, and smaller, rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System. In order of increasing distance from the Sun, they are the four terrestrials, 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. Additionally, the IAU accepts five dwarf planets, with many others under consideration, and hundreds of thousands of small Solar System bodies.
Since 1992, hundreds of planets around other stars ("extrasolar planets" or "exoplanets") in the Milky Way Galaxy have been discovered. As of November 19, 2012, 851 known extrasolar planets (in 670 planetary systems and 126 multiple planetary systems) are listed in the Extrasolar Planets Encyclopaedia, ranging in size from that of terrestrial planets similar to Earth to that of gas giants larger than Jupiter. On December 20, 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.
Other articles related to "planet, planets":
... The outer planets are those planets in the Solar System beyond the asteroid belt, and hence refers to the gas giants, which are in order of their distance from the Sun Jupiter is ... Saturn is the second largest planet, with a large and bright ring system ... The outer planets all have ring systems, although all but Saturn's are faint ...
... Bhekar Ro is a fringe planet in Terran space with unpredictable weather ... colonised by a group of refugees fleeing from the Terran Confederacy, settling on the planet in hope of living independently and self-sustaining ... The only settlement on the planet is Free Haven, built by the first colonists who came to Bhekar Ro to escape from the Terran Confederacy ...
... for space traffic in orbit of the planet ... Unlike most planets in Terran space, Brontes IV has a more diverse environment, ranging from the typical wastelands of Terran colonies through highly volcanic areas to ... Several areas of the planet have large valleys rich in mineral deposits ...
... A lush jungle world, the Protoss are the only intelligent life to have evolved on the planet, though the process was enhanced and sped up by the Xel'Naga ... The planet suffered tremendous damage, though with the conflict's resolution the Protoss were able to rebuild and use the planet to expand outwards, colonizing ... Troops were trained and ships built on Aiur and sent to fight on distant planets by traveling through Warp portal buildings constructed on the battlefield ...
Famous quotes containing the word planet:
“The golden mean in ethics, as in physics, is the centre of the system and that about which all revolve, and though to a distant and plodding planet it be an uttermost extreme, yet one day, when that planets year is completed, it will be found to be central.”
—Henry David Thoreau (18171862)
“I have always believed that opera is a planet where the muses work together, join hands and celebrate all the arts.”
—Franco Zeffirelli (b. 1922)
“Spring is strong and virtuous,
Broad-sowing, cheerful, plenteous,
Quickening underneath the mould
Grains beyond the price of gold.
So deep and large her bounties are,
That one broad, long midsummer day
Shall to the planet overpay
The ravage of a year of war.”
—Ralph Waldo Emerson (18031882)