原文已被隐藏，你可用 快捷键 - 或点击 显示原文 按钮来查看原文
第1段
1 .<-NARRATOR:->Listen to part of a lecture in an astronomy class.
旁白：请听一段天文学讲座的节选片段。
第2段
1 .<-MALE PROFESSOR:->Last week we discussed the formation of Earth and the other rocky planets-the planets in the inner solar system.
教授：上一周我们讨论了地球以及其他岩态行星的形成，这些行星位于太阳系内部。
2 .Uh, s-so what about the gas giants: Jupiter, Saturn, Uranus, and Neptune?
哦，那么，气态行星是怎么形成的呢？比如说木星、土星、天王星和海王星。
3 .Well, there's two theories, but first let's recap.
现在有两种理论。但是首先，我们先简要回顾一下上节课的内容。
4 .We believe our solar system began as a huge spinning cloud of dust and gas, which flattened and eventually collapsed in on itself.
我们相信太阳系始于一大团旋转的尘土和气体的云团，这一团云团最终变平并且自己瓦解。
5 .The matter at the center condensed into a ball of hot gas and dust, eventually becoming our Sun.
它的物质浓缩成了一个热气和尘土构成的球，最终变成了太阳。
6 .And... what happened to the remaining cloud, to the disk encircling the Sun when it was a young star?
那么剩下的云层又变成了什么呢？在太阳还是一颗新生恒星的时候，作为环绕它的气体带？
第3段
1 .<-FEMALE STUDENT:->The rocky planets were born. Um, dust- little grains of rock and metal within the disk collided with each other and stuck together.
学生：岩态行星就形成了。嗯，灰尘、小的岩石碎片和气体带里的金属物质互相碰撞，彼此聚合，
2 .And this process sorta snowballed over millions of years... until the chunks grew into mini-planets-protoplanets.
这个过程就类似于一场百万年的滚雪球，直到这些碎片变成了小行星和原行星。
第4段
1 .<-MALE PROFESSOR:->Yes. Uh, this process is called accretion, and we call the disk an accretion disk.
教授：是的。这个过程被称为吸积。我们把这个环叫做吸积盘。
第5段
1 .[Explaining] Now... think of it as two parts- an inner accretion disk and an outer accretion disk.
我们把它分成两个部分来考虑：内部的吸积盘和外部的吸积盘。
2 .In the inner part, once an object gets large enough, that object's gravitational field gets stronger, which speeds up the accretion process.
在内部的吸积盘里，一旦一个物体变得足够大，它的重力就会变得更大，这就加速了吸积的过程。
3 .You know, larger objects attract smaller ones and sorta gobble them up. And eventually you get a full-size planet in its own orbit.
你知道，大的物体会吸引小的物体，把它们都“吞吃”掉。最终，我们就得到了一个有自己轨道的足尺的行星。
4 .OK... That's how the inner rocky planets probably formed-by accretion. But what about those gas planets in the outer solar system, in the outer accretion disk?
好的。这就是内部的岩态行星怎么通过吸积作用形成。但是那些位于太阳系外部、处于外部吸积盘内的气态行星，是怎么形成的呢？
第6段
1 .Well, the first theory says the accretion process was similar to the one that formed the rocky planets, with some key differences.
第一个理论认为，形成气态行星的吸积过程与形成岩态行星的吸积过程相似，但是有几个关键的不同。
2 .Remember the gas giants are farther from the Sun, where temperatures are much colder.
记得吗，气态行星距离太阳更远，在它们所处的位置，温度是更低的。
3 .So, in the outer accretion disk, compounds like water and ammonia exist in frozen form.
所以，在外部的吸积盘里，水和氨气的混合物是以冰冻状态存在的。
4 .Closer to the Sun, they're more likely to be vaporized by solar radiation.
离太阳越近，这种化合物就更有可能在太阳辐射下蒸发成气体。
5 .What this means is that, in addition to rocky and metallic particles, there's be other solids, like frozen water and frozen ammonia.
这就意味着除了岩石和金属颗粒，还有其他的固体比如说冰和固态氨气。
第7段
1 .<-FEMALE STUDENT:->[Following on]...so more solid substances are available to clump into protoplanets, right?
学生：所以越来越多的固体物质可以聚成原行星，对吗？
第8段
1 .<-MALE PROFESSOR:->Precisely. So the solid cores of the gas giants could conceivably have formed by accretion.
教授：很准确！所以可以想象的是气态行星的固态核心可以通过吸积作用形成。
2 .And once their mass reaches a certain point- around about five to ten Earths- what would happen?
一旦它们的质量达到了地球的5到10倍，什么事情会发生呢？
第9段
1 .<-FEMALE STUDENT:->Five to ten Earths? With a mass that big, I guess gravity would start to pull in more and more material faster, right?
学生：地球质量的5到10倍的话……嗯，如果质量如此之大的话，我猜重力会越来越快地把其他物质吸引过来，对吗？
2 .<-MALE PROFESSOR:->Material, meaning gas. It would rapidly pull in more and more gas from the accretion disk.
教授：物质，也就是气体。这个巨大的行星会迅速地从吸积盘里吸引越来越多的气体。
3 .So, you end up with a solid core of rock, metal, and ice surrounded by massive amounts of gas. That's the core-accretion theory.
所以我们就得到了一个由岩石、金属和冰组成的核，这个核由大量的气体包围着。这就是核吸积理论。
4 .Now-the other theory's called the disk-instability theory.
第二个理论被称为吸积盘无用论理论。
第10段
1 .The disk-instability theory holds that gas begins the planet-making process without a solid core.
吸积盘无用论理论认为，气体开始了行星形成的过程，这时候并没有一个固态的核心。
2 .You see, most of the outer accretion disk would've been gas; we believe solid particles probably made up just 1 percent of the outer accretion disk.
你看，大多数的外部吸积盘的构成都是气体。我们相信，固态粒子可能只占外部吸积盘的百分之一。
3 .So, this theory suggests that the large planets-the gas giants- they develop from large clumps of mostly gas and some dust in an accretion disk.
所以这个理论认为，大的行星，气态大行星，嗯，它们是形成于大团的吸积盘的气体和灰尘。
4 .Um, outer regions of an accretion disk can be unstable, uh, gravitationally unstable, which is what causes these clumps to form and, in some cases, grow into protoplanets.
吸积盘的外部区域是不稳定的，在重力上是不稳定的，这就导致了大团气体的形成，在某些情况下，这就导致了原行星的形成。
5 .Over time, dust particles within a gas clump coalesce-bond together-and eventually fall toward the center, creating a core.
经过较长时间，大团气体里的尘埃粒子聚合在了一起，最终向中心积聚，形成了一个核。
6 .Once this happens, the gas clump grows relatively quickly, as its gravity pulls in more and more gas and dust particles.
这个过程一旦发生，大团气体就变大地非常快，因为它的重力会吸引来越来越多的气体和尘土粒子。
7 .And this whole process can theoretically happen within 100,000 years.
这整个过程在理论上可以在十万年之间发生。
第11段
1 .<-FEMALE STUDENT:->That's amazingly fast! So, which theory's correct?
学生：真是令人吃惊地快啊！那么哪个理论是正确的呢？
第12段
1 .<-MALE PROFESSOR:->That's the debate. Most of my colleagues favor core accretion.
教授：这就是争论所在。我的同事们多数支持核吸积理论。
2 .Personally, I think the accretion theory works for the formation of rocky planets, but not necessarily for gas planets.
我个人认为核吸积理论适用于岩石星球的形成，但是对气体星球不一定成立。
3 .A major problem is that gas giants like Jupiter and Saturn would take too long to form through core accretion.
一个大问题在于巨大的气体星球，像木星和土星，如果通过核吸积的话耗费的时间就太长了。
4 .Core accretion would take several million years. But observations of other star systems indicate that a disk's gas disappears more quickly than that.
核吸积可能会耗费数百年的时间。但是对其他行星系统的观测证明盘状气体会更快消失。
5 .Whatever's not drawn into planets ends up dissipated and evaporated by solar wind and radiation, from nearby stars.
对于任何没有被吸引到星球上的东西都会被周围星系的太阳风、辐射分散和蒸发。
6 .So basically, a baby Jupiter would run out of gas before it grew up.
因此，基本上一个在木星形成之前就会耗尽所有的气体。
7 .But the disk-instability theory... well, the timing's right. That process is fast enough to finish before the gas runs out.
但是盘吸积理论，如果时间是对的话，这个过程会在气体耗尽之前就完成。