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Pluto was first found nearly a century ago in 1930.
冥王星首次被发现是在近一个世纪前的1930年。
And the uproar at the discovery of a new planet was tremendous.
一颗新行星的发现引起了巨大骚动。
In 2006, there was another uproar, as the beloved ninth planet in our solar system was given a highly publicized downgrade.
2006年，又有一场骚动，人们喜爱的太阳系第九大行星被高调降级。
Pluto, scientists told us, should no longer be called a planet, but instead a dwarf planet.
科学家告诉我们，冥王星不应该再被称为行星，而应该被称为矮行星。
Well, what does that mean?
好吧，那是什么意思？
Why should we change our minds?
我们为什么要改变主意呢？
Is Pluto a planet or not?
冥王星是不是行星？
In 2006, the International Astronomical Union considered a controversial question.
2006年，国际天文联合会考虑了一个有争议的问题。
What is a planet?
什么是行星？
Up until then, planet was defined pretty loosely.
在此之前，行星的定义相当宽松。
It just referred to big objects in the solar system.
它只是指太阳系中的大型物体。
But which objects exactly?
但究竟是哪些物品呢？
Some moons, like Jupiter's moon, Ganymede, are larger than the planet Mercury.
有些卫星，比如木星的卫星，木卫三，比水星还大。
And some objects, like Eris have never been known as planets, despite being very nearly as large as Pluto.
有些天体，比如阋神星，虽然和冥王星差不多大，却从未被称为行星。
So what makes a planet a planet?
那么，是什么让一颗行星成为行星呢？
The IAU came to a decision that would annoy Pluto's fans even a decade later.
国际天文学联合会(IAU)做出的决定即使在十年后也会让冥王星的粉丝们感到恼火。
They agreed on updated criteria for planethood.
他们一致同意更新行星身份的标准。
To classify as a planet, an object must orbit the sun and not another planet.
要被归类为行星，该物体必须绕太阳运行，而不是绕另一颗行星运行。
And it must be massive enough for its gravity to have pulled it into a roughly spherical shape -- no problem for Pluto there.
而且它的质量必须足够大，它的引力才能把它拉成一个大致为球形的形状——这对冥王星来说没问题。
Unfortunately, there is another qualification for planet status.
不幸的是，行星地位还有另一个条件。
To be a planet, an object must be large enough to have cleared the path of its orbit of other debris orbiting the sun.
要成为行星，该物体必须足够大，才能清除其轨道上围绕太阳运行的其他碎片。
Unlike the eight planets the IAU now recognizes, Pluto hasn't managed that.
与国际天文学联合会现在承认的八颗行星不同，冥王星没有做到这一点。
Checking every box except one means that Pluto can't be considered a true planet.
每一个都符合只有这一条不符合，这意味着冥王星不能被认为是真正的行星。
Instead, it's classified as a dwarf planet, along with Eris and other large objects, like Ceres, Haumea, and Makemake.
相反，它被归类为矮行星，与阋神星和其他大型天体一起，如谷神星、妊神星和鸟神星。
The ranks of the dwarf planets may grow over time, too.
矮行星的数量也可能随着时间的推移而增加。
Since they are relatively small and usually very distant, scientists believe we haven't found all of them yet.
由于它们相对较小，而且通常非常遥远，科学家们认为我们还没有发现所有的。
But don't feel too bad for Pluto.
但不要为冥王星感到太难过。
Being a dwarf planet isn't really a downgrade.
作为一颗矮行星并不是真正的降级。
It's just a change in how we talk about it.
只是改变了我们谈论它的方式。
In 2015, the New Horizons Spacecraft sent us the most detailed pictures and data about Pluto we've received yet.
2015年，新视野号飞船向我们发送了迄今为止关于冥王星的最详细的图片和数据。
We found that the dwarf planet has geological features we hadn't imagined -- towering mountains, ice ridges, vast, smooth areas of carbon monoxide ice.
我们发现，这颗矮行星具有我们想象不到的地质特征--高耸的山脉、冰脊、广阔而光滑的一氧化碳冰层。
What we call Pluto can't change the spectacular reality of it.
我们所说的冥王星改变不了它的壮观现实。
So maybe your very elegant mother has to serve you noodles instead of nine pizzas.
所以也许你优雅的妈妈给你吃的是面条而不是九个披萨。
But sometimes change is good.
但有时改变是好的。
Science can't advance without changes in how we understand the universe.
如果我们不改变对宇宙的理解，科学就不可能进步。
Pluto is still out there.
冥王星仍然在那里。
And as we learn more about it, we understand just how spectacular it really is.
随着我们对它了解得更多，我们就明白了它到底有多壮观。
