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A planet (from the Greek πλανήτης, planetes or "wanderers") is a body of considerable mass that orbits a star and that produces very little or 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.
1 Within the 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 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 nomenclaturePlanetary nomenclature like terrestrial nomenclature, is used to uniquely identify a feature on the surface of a planet or natural satellite so that the feature can be easily located, described, and discussed. How names are approved When images are first.
1.1 Accepted planets
The commonly accepted list of major planets of our solar system (in increasing distance from the SunThe Sun (also called Sol is the star in our solar system. Planet Earth orbits the Sun. Other bodies that orbit the Sun include other planets, asteroids, meteoroids, comets and dust. Not all objects passing through the solar system have been orbitally capt) are
- MercuryMercury is the closest planet to the Sun, and the second-smallest planet in the solar system. Mercury ranges from 0. 5 in apparent magnitude; Mercury is sufficiently "close" to the Sun that telescopes rarely examine it (the greatest elongation is 28. (☿)
- VenusVenus is the second planet from the Sun, named after the Roman goddess Venus. It is a terrestrial planet, very similar in size and bulk composition to Earth; it is sometimes called Earth's "sister planet" as a result of this similarity. Although all plane (♀)
- EarthEarth also known as the Earth or Terra is the planet on which we live, the third planet outward from the Sun. It is the largest of the solar system's terrestrial planets, and the only planetary body that modern science confirms as harbouring life. The pla (♁)
- MarsMars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. Mars has two small moons, Phobos and Deimos, both small and odd (♂)
- Jupiter (♃)
- SaturnSaturn is the sixth planet from the Sun. It is a gas giant, the second-largest planet in the solar system after Jupiter. It was named after the Roman god Saturn. Its symbol is a stylized representation of the god's sickle ( Unicode: ♄). Physical ch (♄)
- Uranus (♅)
- Neptune (♆)
- Pluto (♇) (Pluto's orbit lies partly within that of Neptune; some astronomers consider Pluto to be merely a large asteroid, not a true planet)
English-speaking schoolchildren often use a variety of mnemonics to remember the planets in this order:
- Mary's Violet Eyes Make John Stay Up Nights, Period. (Or Poor John to include the Planetoids)
- Mother Very Thoughtfully Made Jelly Sandwiches Under No Protest. (Or A Jelly Sandwich to include Asteroids; The "T" is for Terra.)
- My Very Energetic Mother Just Served Us Nine Pizzas. (Or " Pistachio Nuts" for those rare times when Pluto is within the orbit of Neptune)
- My Very Educated Mother Just Showed Us Nine Planets
- Mary Very Early Made Jane Shake Up Nellie's Pillow
- Mr Versuvius Easily Made Jumping Seals Undergo Needless Pain
- My Very Easy Memory Jingle Seems Useful Naming Planets"
- Most Very Energetic Martians Just Swim Using No Pool
Similar mnemonics exist in many other languages.
