太陽系的新秩序

Eight Planets and New Solar System Designations

圖示提供: International Astronomical Union

Explanation:
How many planets are in the Solar System?
This popular question now has a new formal answer according the International Astronomical Union (IAU): eight.
Last week, the IAU voted on a new definition for planet and Pluto did not make the cut.
Rather, Pluto was re-classified as a dwarf planet and is considered as a prototype for a new category of trans-Neptunian objects.
The eight planets now recognized by the IAU are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Solar System objects now classified as dwarf planets are: Ceres, Pluto, and the currently unnamed 2003 UB313.
Planets, by the new IAU definition, must be in orbit around the sun, be nearly spherical, and must have cleared the neighborhood around their orbits. The demotion of Pluto to dwarf planet status is a source of continuing dissent and controversy in the astronomical community.

說明:
太陽系有多少顆行星？
這個熱門的話題現在有了正式的答案，根據上星期國際天文聯盟(IAU)投票所定出的新行星定義，答案是八顆，冥王星被除名了！
冥王星被重新分類為矮行星，並成為海王星外天體的原型。
國際天文聯盟正式承認的八顆行星，分別是水星、金星、地球、火星、土星、木星、天王星、海王星。
太陽系天體被分類為矮行星者有穀神星、冥王星、和尚未正式命名的2003 UB313。
據新的國際天文聯盟定義，行星必需具有繞行太陽的軌道、外觀近乎球狀、和已清除軌道附近的物質。
把冥王星降級成矮行星，在天文學界將引發永不停歇的反對和爭論。

資料來源：Scientific American
　　　　　Department of Physics, NCKU