Narrator: Listen to part of a lecture in an astronomy class.

旁白：在天文学课堂上听讲座的部分内容。

Professor: Last week, we discussed the formation of Earth and the other rocky planets, of the planets in the inner solar system.

教授：上周，我们讨论了地球和其他岩石行星的形成，也就是太阳系内部行星的形成。

So what about the gas giants? Jupiter, Saturn, Uranus and Neptune.

那么气态巨行星呢？木星，土星，天王星和海王星。

Well, there's two theories, but first, let's recap.

嗯，有两种理论，但首先，让我们回顾一下。

We believe our solar system began as a huge spinning cloud of dust and gas,

我们相信我们的太阳系最初是由尘埃和气体组成的巨大的旋转云团，

which flattened and eventually collapsed in on itself.

最终它变扁并且自己瓦解了。

The matter at the centre condensed into a ball of hot gas and dust, eventually becoming our sun.

中心的物质凝聚成一团充满炽热的气体和尘埃的球，最终成为我们的太阳。

And what happened to the remaining cloud, to the disk encircling the sun when it was a young star?

剩下的云变成什么了？当它还是一颗新生恒星时，环绕着太阳的气体带又发生了什么？

Student: The rocky planets were born.

学生：岩态行星诞生了。

Um, dust, little grains of rock and metal within the disk, collided with each other and stuck together.

尘埃，气体带里的岩石和金属颗粒，相互碰撞，聚在一起。

And this process sort of snowballed over millions of years until the chunks grew into mini-planets, proto-planets.

这个过程类似于在数百万年的时间里一直在滚雪球，直到这些小块变成了迷你行星，即原行星。

Professor: Yes. This process is called accretion, and we call the disk an accretion disk.

教授：是的，这个过程叫做吸积，我们把这个环叫做吸积盘。

Now, think of it is two parts: an inner accretion disk, and an outer accretion disk.

我们把它想象成两个部分：一个内吸积盘和一个外吸积盘。

In the inner part, once an object gets large enough,

在内部，一旦一个物体变得足够大，

that objects gravitational field gets stronger, which speeds up the accretion process.

物体的引力场变得更强，加速了吸积过程。

You know, larger objects attract smaller ones and sort of gobble them up.

你知道，较大的物体会吸引较小的物体，然后把它们“吞掉”。

And eventually you get a full-sized planet in its own orbit.

最终我们就得到了一个拥有自己轨道的全尺寸的行星。

Ok, that's how the inner rocky planets probably formed by accretion.

好了，以上就是内部的岩石行星是如何通过冲积形成的。

But what about those gas planets in the outer solar system, in the outer accretion disk?

但是在外层的吸积盘里的那些太阳系外的气体行星呢？

Well, the first theory says the accretion process was similar to the one that formed the rocky planets, with some key differences.

第一种理论认为，吸积过程与形成岩石行星的过程相似，有一些关键的区别。

Remember, the gas giants are farther from the sun, where temperatures are much colder.

记住，气态巨行星离太阳更远，那里的温度要低得多。

So, in the outer accretion disk, compounds like water and ammonia exist in frozen form.

所以，在外层的吸积盘里，像水和氨这样的化合物以固态的形式存在。

Closer to the sun, they are more likely to be vaporized by solar radiation.

在靠近太阳的地方，它们更有可能被太阳辐射蒸发。

What this means is that in addition to rocky and metallic particles,

这意味着除了岩石和金属粒子外，

there'd be other solids like frozen water and frozen ammonia.

还会有其他固体，比如固态水和氨。

Student: So more solid substances are available to clump into proto-planets, right?

学生：所以有更多的固体物质可以聚集成原行星，对吧？

Professor: Precisely! So the solid cores of the gas giants could conceivably have formed by accretion.

教授：没错！所以可以想象，气态巨行星的固体核心可以通过吸积形成。

And once their mass reaches a certain point, around about 5 to 10 Earths, what would happen?

一旦它们的质量达到某个点，大约5到10个地球，会发生什么？

Student: 5 to 10 Earths...uh, with a mass that big, I guess gravity would start to pull in more and more material faster, right?

学生：5到10个地球。嗯……如此大的质量，我想重力会开始越来越快地将物质吸引进来，对吧？

Professor: Material, meaning gas.

教授：物质，其实就是指气体。

It would rapidly pull in more and more gas from the accretion disk.

它会迅速从吸积盘中吸入越来越多的气体。

So you end up with a solid core of rock, metal and ice surrounded by massive amounts of gas.

所以最终得到了一个核，由固体的岩石，金属和冰组成，被大量的气体包围着。

That's the core accretion theory.

这是核吸积理论。

Now the other theory is called the disk instability theory.

另一种理论叫做吸积盘不稳定理论。

The disk instability theory holds that gas begins the planet-making process, without a solid core.

吸积盘不稳定理论认为，气体在没有固态内核的情况下开始了行星制造的过程。

You see, most of the outer accretion disk would've been gas.

你看，大部分的外吸积盘构成都是气体。

We believe solid particles probably made up just one percent of the outer accretion disk.

我们相信固态粒子可能只占外部吸积盘的1%。

So this theory suggests that large planets, the gas giants, uh,

所以这个理论表明，大行星，即气态行星，

they develop from large clumps of mostly gas and some dust in an accretion disk.

它们是由大量的气体和吸积盘中的尘埃形成的。

Outer regions of an accretion disk can be unstable, gravitationally unstable,

吸积盘的外部区域可能不稳定，引力不稳定，

which is what causes these clumps to form, and in some cases, grow into proto-planets.

这就是导致这些气团形成的原因，并且在某些情况下，它们会成长为原行星。

Over time, dust particles within a gas clump coalesce, bond together and eventually fall toward the center, creating a core.

随着时间的推移，气体团中的尘埃颗粒凝聚，结合在一起，向中心跌落，形成一个核心。

Once this happens, the gas clump grows relatively quickly, as its gravity pulls in more and more gas and dust particles.

一旦这种情况发生，当它的重力吸引越来越多的气体和尘埃颗粒，气团就会相对迅速地积聚变大。

And this whole process can theoretically happen within one hundred thousand years.

这整个过程在理论上可以在十万年内发生。

Student: That's amazingly fast! So, which theory's correct?

学生：这简直太快了！那么，哪个理论是正确的呢？

Professor: That's the debate.

教授：这就是争论所在。

Most of my colleagues favor core accretion.

我的大多数同事都倾向于核心吸积理论。

Personally, I think the accretion theory works for the formation of rocky planets,

就我个人而言，我认为吸积理论适用于岩石行星的形成，

but not necessarily for gas planets.

但不一定适用于气体行星。

A major problem is that gas giants like Jupiter and Saturn would take too long to form through core accretion.

一个主要的问题是像木星和土星这样的气态巨行星需要很长时间才能形成核心的吸积。

Core accretion would take several million years.

核心的吸积需要几百万年的时间。

But observations of other star systems indicate that the disk's gas disappears more quickly than that.

但是对其他恒星系统的观测表明，吸积盘中的气体消失得比这更快。

Whatever is not drawn into planets ends up dissipated and evaporated by solar, wind and radiation from nearby stars.

任何没有被吸引到行星上的东西最终都会被附近的恒星产生的太阳能、风和辐射所蒸发掉。

So basically, a baby Jupiter would run out of gas before it grew up.

所以基本上，一个小木星在它成熟之前就会耗尽气体。

But the disk instability theory, well, the timing is right.

但吸积盘不稳定理论，好吧，时机是正确的。

That process is fast enough to finish before the gas runs out.

这个过程足够快，能够在气体耗尽之前完成。