Planet -

Japanese: 惑星 - わくせい

Also known as planets. Eight relatively large celestial bodies in the solar system. In order of closestness to the sun, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. There are also asteroids between Mars and Jupiter. Unlike stars, they shine by reflecting the light of the sun and do not twinkle, and those closest to Earth (Mercury, Venus, and Mars) go through phases. Planets exhibit apparent motions along the ecliptic, such as direct motion, retrograde motion, conjunction, opposition, and stationary motion. They revolve in elliptical orbits according to Kepler's laws, with their orbital planes being almost on the same plane, and the directions of revolution and rotation (except for Venus) are all the same, and the semi-major axes of their orbits follow Bode's law. The planets are divided into inner and outer planets by the orbit of the Earth, and are distinguished by calling Mercury to Mars terrestrial planets (relatively small, made of rock, with a specific gravity of around 5 and a rotation period of more than one day), and Jupiter to Neptune gas-like planets (relatively large, composed mainly of hydrogen and helium, with a specific gravity of 1 to 2 and a rotation period of around half a day). At the General Assembly of the International Astronomical Union in August 2006, the definition of a planet in the solar system was adopted as follows: (1) it orbits the Sun, (2) it has a sufficiently large mass that it is almost spherical in shape due to its own gravity, and (3) there are no other prominent celestial bodies in its orbit. Due to this definition, Pluto, which had long been considered a planet, was classified as a dwarf planet. In 1995, a research group at the Geneva Observatory announced the discovery of a planet at 51 Pegasus, and many planets have been discovered around stars other than the solar system in recent years, including this one, but most of them are based on indirect evidence.

Source : Heibonsha Encyclopedia About MyPedia Information

Japanese:

遊星とも。太陽系に属する比較的大きい８個の天体。太陽に近い順に水星，金星，地球，火星，木星，土星，天王星，海王星がある。また火星と木星間には小惑星がある。恒星と違って太陽の光を反射して輝き，またたきをせず，地球に近いもの（水星，金星，火星）は満ち欠けをする。惑星は黄道に沿って順行，逆行，合，衝，留などの見かけの運動を示す。ケプラーの法則に従い楕円軌道を公転し，軌道面はほぼ同一平面上にあり，公転と自転（金星を除く）の方向はすべて同じで，軌道半長径はボーデの法則に従う。地球の軌道を境に内惑星と外惑星に分け，また水星〜火星を地球型惑星（比較的小さく，岩石からなり，比重５前後，自転周期は１日以上），木星〜海王星の４個を木星型惑星（比較的大きく，水素・ヘリウムを主成分とし，比重１〜２，自転周期は半日前後）と呼んで区別する。2006年８月の国際天文学連合総会において，太陽系の惑星とは，（１）太陽の周りを回り，（２）十分大きな質量をもつもので自己の重力によりほとんど球状の形で，（３）その軌道付近に他に目立つ天体がない，との定義を採択した。この定義により，長らく惑星とされてきた冥王星は準惑星（dwarf planet）とされることになった。なお，1995年にはジュネーブの天文台の研究グループがペガスス座51番星の惑星の発見を発表，これを含めて太陽系以外の恒星の惑星は近年数多く発見されているが，ほとんどは間接的な証拠によるものである。

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