冥王星不再是行星，太陽系變為八大行星

昨日
也就是24號晚上十點（台灣時間）的時候
國際天文協會（International Astronomical Union , IAU）進行表決
決定了太陽系內只有八大行星，而冥王星則變為第一批矮行星(dwarf planet)的一個
在這邊大概提一下事情的始末好了
畢竟冥王星的行星地位也陪伴我二十多年了
在14號時，IAU於捷克開會，討論是否要重新定義行星
後來決定於24號表決兩個草案5A與5B
http://www.iau2006.org/mirror/www.iau.org/iau0602/index.html
其內容大致如下：

5A
The IAU therefore resolves that planets and other bodies in our Solar System
be defined into three distinct categories in the following way:

(1) A planet is a celestial body that ( a ) is in orbit around the Sun, ( b )
has sufficient mass for its self-gravity to overcome rigid body forces so
that it assumes a hydrostatic equilibrium (nearly round) shape, and ( c ) has
cleared the neighbourhood around its orbit.

行星：
( a )要繞著太陽旋轉
( b )要有足夠的質量使自身的重力可和剛體的力維持流體靜力平衡（也就是要接近圓形）
( c )其軌道附近無其他物體的天體

(2) A dwarf planet is a celestial body that ( a ) is in orbit around the Sun,
( b ) has sufficient mass for its self-gravity to overcome rigid body forces so
that it assumes a hydrostatic equilibrium (nearly round) shape2, ( c ) has not
cleared the neighbourhood around its orbit, and ( d ) is not a satellite.

矮行星：
( a )要繞著太陽旋轉
( b )要有足夠的質量使自身的重力可和剛體的力維持流體靜力平衡（也就是要接近圓形）
( c )其軌道附近有其他物體的天體
( d )不是個衛星

(3) All other objects orbiting the Sun shall be referred to collectively as
"Small Solar System Bodies".

其他繞太陽旋轉的則稱為Small Solar System Bodies（太陽系小天體吧）


5B

Insert the word "classical" before the word "planet" in Resolution 5A,
Section (1), and footnote . Thus reading:

(1) A classical planet is a celestial body . . .

在第一草案的行星前加入classical一字（應該是翻做經典）

草案內容大概是如此
如果5A通過的話，那麼太陽系的行星則只會剩下八個
而於十一小時半前，表決結束了
http://www.iau2006.org/mirror/www.iau.org/iau0603/index.html
其結果如下：

The IAU members gathered at the 2006 General Assembly agreed that a "planet"
is defined as a celestial body that ( a )is in orbit around the Sun, ( b ) has
sufficient mass for its self-gravity to overcome rigid body forces so that it
assumes a hydrostatic equilibrium (nearly round) shape, and ( c ) has cleared
the neighbourhood around its orbit.

This means that the Solar System consists of eight "planets" Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of
objects called "dwarf planets" was also decided. It was agreed that "planets"
and "dwarf planets" are two distinct classes of objects. The first members of
the "dwarf planet" category are Ceres, Pluto and 2003 UB313 (temporary name).
More "dwarf planets" are expected to be announced by the IAU in the coming
months and years. Currently a dozen candidate "dwarf planets" are listed on
IAU's "dwarf planet" watchlist, which keeps changing as new objects are found
and the physics of the existing candidates becomes better known.

The "dwarf planet" Pluto is recognised as an important proto-type of a new
class of trans-Neptunian objects. The IAU will set up a process to name these
objects.

稍微講一下內容
5A通過，但是5B則沒有通過
於是太陽系只剩下八大行星：
水星、金星、地球、火星、木星、土星、天王星、海王星
而依照此定義的第一批矮行星則有了三個：
穀神星、冥王星、2003 UB313（也就是Xena）
而IAU將會繼續定出更多的矮行星出來，並且設定一個命名程序

照這樣來看
當年被降格為小行星的穀神星現在反而是升格了
冥王星不再屬於行星之列，變成以後會出現非常多的矮行星的一員
教科書看來是一定要重編了，又多出另一種天體的類別
冥王星自發現到現在都還沒繞完一就已經脫離行星之列
雖然他的爭議實在很大，但也是小時候看天文書籍已經根植在記憶之中
畢竟也是人類行星探索歷史的一部份
突然被降格也是有點失落
至於夏隆的問題IAU這次似乎是沒有提到
不過可能就和冥王星一樣，變成雙星系的矮行星了吧
不管怎樣
八大行星應該不會再變了
矮行星應該會愈變愈多
也許以後大家會忘記冥王星
只當他是眾多矮行星的其中一個吧