TOEFL 듣기

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Listen to part of a lecture in an astronomy class. Professor: Now, when we talk about the formation of planets, one key idea is the nebular hypothesis. This theory suggests that our solar system began as a giant cloud of gas and dust, called a solar nebula. As this cloud collapsed under its own gravity, it began to spin faster and flatten into a disk. At the center, the material condensed to form the Sun. But what about the planets? Well, in the cooler regions of the disk, far from the young Sun, solid particles of ice and rock clumped together through collisions and gravitational attraction. These clumps eventually grew into planetesimals, and then into full-sized planets. However, this process didn't happen uniformly. In the inner part of the disk, where temperatures were higher, only metals and rocky materials could survive, which is why the inner planets are small and rocky. In the outer regions, abundant ice allowed the formation of much larger planets with thick atmospheres, the gas giants. So, the distance from the Sun played a critical role in determining not just the composition but also the size and structure of each planet.