太陽系’十二行星’

從小就學會太陽系有九大行星. 不過最近科學家要redefine星球的意思. 新的definition可能會導致很多本來小小的asteroid也會變成planet…. 太陽系除了現在的 Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto 之外, 可能會在火星木星中間多加 Ceres; Pluto的月亮Charon可能也分類為行星; Pluto後面再多加新行星Xena (2003 UB313) – 20060816planets.pdf

新的行星definition:
Planet: A round thing orbiting a star. More precisely, according to the draft definition: “A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.”

Related link: Planet debate: Proposed new definitions
右邊的連接是New Scientist相關報導的PDF.

Pic 1: If the new definition is adopted, the solar system will have 12 planets (Image: IAU/Martin Kornmesser)
Pic 2: A dozen candidate objects are being considered for possible planet status (Image: IAU/Martin Kornmesser)