托福听力

🎧 听力原文

Narrator: Listen to part of a lecture in an astronomy class. Professor: Now, most of you know that planets orbit stars, but what about objects that don't quite fit that mold? Take brown dwarfs, for instance. They're often called 'failed stars' because they never achieved the mass needed to ignite hydrogen fusion in their cores. However, recent observations have challenged this simple picture. We've found brown dwarfs that are surprisingly cold and faint, much cooler than earlier models predicted. Interestingly, some of these cool brown dwarfs show signs of water clouds and even methane in their atmospheres, similar to giant planets like Jupiter. This blurring of the line between stars and planets raises a fundamental question: What exactly is a planet? Some astronomers argue that if an object forms like a star, from a collapsing cloud of gas and dust, it should be classified as a brown dwarf, regardless of its mass. But others suggest that the presence of atmospheric features, like clouds, might be more indicative of a planetary nature. So, as you can see, our definitions are still very much in flux.