托福听力

🎧 听力原文

Listen to part of a lecture in an astronomy class. The professor is discussing the formation of planets. Professor: So, we've talked about how stars form from collapsing clouds of gas and dust. But what about the planets that orbit them? Well, these planets don't just appear out of nowhere. They form from a rotating disk of material that surrounds the young star, called a protoplanetary disk. In this disk, small particles of dust and ice collide and stick together, gradually building up larger and larger bodies. These are called planetesimals. Over millions of years, these planetesimals can grow into full-sized planets through a process we call accretion. Now, an interesting question is why the inner planets, like Earth, are rocky, while the outer planets, like Jupiter, are mostly gas and ice. One key factor is temperature. In the inner part of the disk, it's too hot for volatile compounds like water and methane to condense into solid ice. So only rocky and metallic materials can solidify. In the colder outer regions, these volatiles can condense, allowing planetesimals to accrete much more material, which explains why the outer planets are so massive.