to know what the word planet is for, and that takes us back to the ancient greeks who called

知道行星（planet）這個詞的意思。古希臘人稱

Mercury, Venus, Mars, Jupiter, Saturn, the Moon and sun planets. Basically if it moved

水星、金星、火星、木星、土星、月球和太陽作「行星」。 基本上，當一個天體在天上移動，

across the sky and was bright, it was a planet. This is a terrible start for the word because,

而且外表光亮，那就是一個行星。 這個定義非常模糊，因爲

1) it excludes Earth from the list and 2) it groups wildly different things together.

第一，行星此詞的定義不包括地球在內， 第二，這個字包含很多不同種類的天體。

But the greeks couldn't know how different the Moon was from Saturn, because the best

但古希臘人不知道月球和土星有什麼分別，因爲

technology they had to observe the Universe was sadly limited.

當時用作觀察宇宙的的科技非常有限。

It would take several thousand years until the industrious Dutch made the first telescopes

幾千年後的荷蘭人才製造了首批天文望遠鏡，

and astronomy became much more interesting. Astronomers could now confidently rearrange

天文學因而變得有趣。 現在的天文學家能有信心地重新整理

the solar system -- an elegant scientific advance that no one could possibly object

太陽系——一套沒有人能反駁的精美天體系統。

to -- and reclassify its parts, dropping the Sun and moon from the list of planets and

這套系統重新把天體分類， 把太陽和月球從行星的定義中除下，

adding Earth. Now, if it orbited the Sun, it was a planet.

並加上地球。現在的定義是， 當天體圍着太陽公轉，就是行星。

As time went on and telescopes got better and better each new century brought with it

隨着天文望遠鏡的發展，每個世紀都

the discovery of a new planet. Which brings us to this familiar solar system:

發現一個新行星，便形成了我們熟悉的太陽系：

nine planets orbiting one star. And looking at this model makes people wonder,

九個行星圍繞着一個恆星。 看到這個模型，有些人會想，

why do astronomers want to ditch Pluto? The problem is pictures like this in textbooks

爲什麼天文學家都想拋棄冥王星不讓它當行星？ 問題是，課本上像這樣的圖片

are lies. Well, not lies exactly, but unhelpful. They give the impression that the planets

都是謊言，也不完全是，只是不太有用。 這些圖片令人覺得那些行星的

are similar-ash in size and evenly-ish spaced, but the reality couldn't be more different.

大小和間距都差不多，這卻不是事實。

Here, dear Terrans, is our home planet Earth, and this is Jupiter next to it at the correct

人類們，這是我們的地球，這就是木星的正確大小，

scale -- rather bigger than you probably thought. If we take this diagram and adjust for the

也許比你想象的更大。 如果我們像這幅圖片調整至

correct sizes of the planets it looks like this. Unless you're watching the video in

所有行星都符合比例，就好像這樣。 如果你不是用全螢幕高清模式

fullscreen HD mode, you might not even be able to see Pluto.

觀賞這段影片，你可能連冥王星也看不到。

So size differences are vast, and Pluto is the smallest by far. But it's not just small

它們的大小相差很大，冥王星是暫時最小的。 冥王星也不只是小於

for a planet, it's also smaller than seven moons: Triton, Europa, our own Moon, Io, Callisto,

其他行星，甚至小於七個衛星： 海衛一、木衛二、我們的月球、木衛一、木衛四、

Titan, and Ganymede. Even if you show the correct relative sizes

土衛六和木衛三。 即使你顯示了正確的比例，

the distances are still a problem. Think about it, if Jupiter was this close to Earth it

距離也是問題。如果木星與地球這麼近，

wouldn't look like a dot in the night's sky but would be rather overwhelming -- so it

它不會是天空中的一小點，而是極之龐大，所以

must be really far away, which makes drawing it to scale rather a challenge. If you want

它一定離地球很遠，令符合比例變得更難。如果你把

the length of a piece of paper to represent the distance from Mercury to Pluto, then giant

一張紙的長度代表由水星都冥王星的距離，

Jupiter would be the size of a dust mite on that page, and Pluto a bacterium.

木星的大小就相當於一隻塵蟎， 而冥王星就是一個細菌。

But excluding Pluto from the plant club just for being tiny and far away isn't reason enough

但只因爲冥王星的細小和距離的長遠 似乎不至於把它逐出行星行列，

and quickly brings out the Pluto defenders. In order to understand what Pluto really is,

而又產生了很多冥王星保衛者。 如果要知道冥王星到底是什麼，

we need to first discuss a planet you've never heard of: Ceres.

我們要談一個你從來沒聽過的行星：穀神星。

Back in the 1801, astronomers found a new planet in the huge gap between Mars and Jupiter

1801年，天文學家在火星和木星的 大空隙中發現了一顆新行星。

-- it was a small planet, but they loved it anyway and named it Ceres.

它非常細小，但天文學家們非常愛它， 於是起了「穀神星」這個名字。

The next year astronomers found another small planet in the same area and named it Pallas.

翌年，天文學家在同一個區域中 發現了令一個行星，叫「智神星」。

A few years later they found a third one, Juno, and then, funnily enough, a fourth one,

幾年後，他們發現了第三個——「婚神星」， 然後又發現了第四個——

Vesta. And for a several decades children learned the 11 planets of the solar system.

「灶神星」。接着的幾十年， 小孩們都在學太陽系的11個行星。

But, astronomers kept finding more and more of these objects and became increasingly uncomfortable

但天文學家不斷發現到更多這些天體，而且他們不太想

calling them planets because they were much more like each other than planets the on either

叫他們作行星，因爲這些 不太像行星的天體都非常相似，

side, so a new category was born: asteroids in the asteroid belt -- and the tiny planets

所以新的天體分類由此誕生：小行星。 他們在小行星帶中，而這些「行星」

were relabeled which is why you've never heard of them. And it was a good decision too, as

被天文學家重新分類，所以你便從來沒聽過 這些「行星」。這是一個好的決定，因爲

astronomers have now found hundreds of thousands of asteroids, which would be a lot for a kid

天文學家現已發現成千上萬小行星，如果它們還是行星，

to memorize if they were all still planets. Back to Pluto. It was discovered in 1930 making

可憐的小孩還要背熟它們的名字。 說回冥王星，冥王星在1930年被發現，

it the 9th planet. First estimates put Pluto about the size of Neptune, but with more observations

成爲第九個行星。最初天文學家估計冥王星的 體積於海王星差不多，但經過更多觀察，

that was revised down, and down and down. While Pluto shrank astronomers started to

冥王星的體積逐漸縮小，縮小，再縮小。 冥王星「正在縮小」之際，天文學家開始

find other, similar objects orbiting in the same zone.

找到其他想似的物體在同一個區域公轉。

Sound familiar? While school kids kept memorizing the nine

好像似曾相識吧。當學生們在背這九個

planets, some astronomers grew uneasy about including Pluto because the size estimates

行星的時候，某些天文學家反對 把冥王星稱爲行星，因爲冥王星的估計體積

continued to shrink, they learned that Pluto is made mostly ice, and they continued to

一直下滑，他們知道冥王星主要由冰組成，而他們

find lots and lots of icy objects at the edge of the solar system just like Pluto.

繼續發現大量這種像冥王星的 冰球在太陽系的邊緣漂浮。

This problem could be ignored as long as no one found an ice ball bigger than Pluto, which

如果沒有人能發現比冥王星大的冰球， 可以擱置這個問題。

is exactly what happened in 2006 with the discovery of Eris. Once again, astronomers

但2006年，隨著鬩神星的發現，天文學家

recategorized the solar system and grouped these distant objects, including Pluto, into

把太陽系重新分類，並將這些 包括冥王星的遙遠星體組合成

a new area called Kuiper belt. And that's the story of Pluto -- a miscategorized

一個新的區域，「柯伊伯帶」。 這就是冥王星的故事——一個被編錯分類的天體，

planted that finally found its home -- just like Ceres. But this story is really less

終於找回自己的家，像穀神星。 而這個故事的重點似乎不是關於

about Pluto than it is about realizing the word 'planet' isn't very helpful.

冥王星，而是「行星」這個字不太有用。

The first four planets are nothing at all like the next four, so it's even a little

首四個行星於末四個行星分別很大， 所以把他們組成一組非常奇怪，

weird to group these eight together which is why they often aren't and are separated

所以他們有時被分成

into terrestrial planets and gas giants. And now that we have telescopes that can see

類地行星和氣體巨行星。 現在我們有這些能看到

planets around stars not our own, and we've found rogue planets drifting in empty space

其他星系的望遠鏡，也發現到無繞恆星公轉的流浪行星，

and brown dwarfs -- objects that blur the very line between planet and star -- the word

以及棕矮星——介乎行星和恆星之間的天體，

planet becomes even less clear. So as we increase our knowledge of the Universe

「行星」這個詞的定義亦開始模糊。 隨着我們對宇宙的認識增加，

the category of 'planet' will probably continue to evolve, or possibly, fall out of favor

「行星」這個類別會不斷演化，甚至完全

entirely. But, for the time being the best way to categorize

消失。而現在把太陽系分類的

the stuff in our solar system is into one star, eight planets, four terrestrial, four

最佳方法是：一顆恆星、八顆行星、 四顆類地行星、四顆

gas giants, the asteroid belt, and the distant Kuiper belt, home to Pluto�

氣體巨行星、小行星帶，以及冥王星的家——遙遠柯伊伯帶。