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Planetary Definition 行星的定義

根據2006年8月16日，國際天文聯合會IAU

共同開會提出的”新行星定義草案”

According to the new draft definition, two conditions must be satisfied for an object to be called a "planet."

有以下幾點 ( 中文釋義轉自”台北天文館新聞” )
IAU將在8/24決定行星的定義，可能改變太陽系家族成員的定位

(1) First, the object must be in orbit around a star, while not being itself a star.

第一，必須環繞一顆恆星運行，而且其自身既不能是恆星，也不能是某個行星的衛星。也就是說，他們挑選了「重力」作為最基本的行星科學定義物理因子，讓自然界自行決定何者為行星。

(2) Second, the object must be large enough (or more technically correct, massive enough) for its own gravity to pull it into a nearly spherical shape.

第二，行星的質量應該大到足以使其因自身的重力可和剛體的力維持流體靜力平衡（hydrostatic equilibrium）的狀態，簡單地說，就是可以維持「圓球形」

The shape of objects with mass above 5 x 1020 kg and diameter greater than 800 km would normally be determined by self-gravity, but all borderline cases would have to be established by observation.

符合條件的行星可能同時也應具有

(1) 超過 5 x 1020 公斤的星球質量

(2) 直徑起碼 800 公里

真實的定義範圍還待更進一步的觀測確認！

The 12 Planets 十二新定義行星

If the proposed Resolution is passed, the 12 planets in our Solar System will be Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Charon and 2003 UB313. The name 2003 UB313 is provisional, as a "real" name has not
 yet been assigned to this object. A decision and announcement of a new name are likely not to be made during the IAU General Assembly in Prague, but at a later time. The naming procedures depend on the outcome of the Resolution vote.

如果前述的草案決議通過，我們的太陽系的12大行星將會是水星、金星、地球、火星、榖神星、木星、土星、天王星、海王星、冥王星、夏隆、2003 UB313。其中2003 UB313的真實名稱尚未決定。一個新的名稱與發布無法同時在國際天文聯合會上進行，但是最後命名的結果�
N會取決於投票。

※事後將會由命名委員給予一個新的名稱！

※2003 UB313 的發現者Mike Brown團隊，給予2003 UB313 ”Xena”的暱稱。

There will most likely be more planets announced by the IAU in the future. Currently a dozen "candidate planets" are listed on IAU's "watchlist" which keeps changing as new objects are found and the physics of the existing candidates becomes
 better known.

The draft "Planet Definition" Resolution will be discussed and refined during the General Assembly and then it (plus four other Resolutions) will be presented for voting at the 2nd session of the GA 24 August between 14:00 and 17:30 CEST.

在未來，國際天文聯合會會宣布更多的行星，目前還有十數顆行星的候選者列在觀察員名單上。

行星新定義的草案決議與其他四項議案將在中歐夏令時間8月24日的14:00～17:30經由大會會員第二次開會投票！

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The Plutons 類冥王星星體與行星分類

The IAU draft Resolution also defines a new category of planet for official use: "pluton". Plutons are distinguished from classical planets in that they reside in orbits around the Sun that take longer than 200 years to complete (i.e. they o
rbit beyond Neptune).

這次新定義行星草案中使用了新的名詞”類冥王星行星”( 冥王星型行星 )。以類冥行星與古典行星區分，是因為它們所在的軌道 ( 繞行軌道於海王星之外 ) 公轉太陽週期大多超過200年以上，

Plutons typically have orbits that are highly tilted with respect to the classical planets (technically referred to as a large orbital inclination). Plutons also typically have orbits that are far from being perfectly circular (technically r
eferred to as having a large orbital eccentricity). All of these distinguishing characteristics for plutons are scientifically interesting in that they suggest a different origin from the classical planets.

類冥行星軌道通常較古典行星傾角較高，而且軌道接近於橢圓 ( 軌道偏心率大 ) ，所以類冥行星的一些特別性質與來源與古典行星不同。故有此新的分類！




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