托福听力

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Listen to part of a lecture in an astronomy class. The professor is discussing the discovery of exoplanets. Professor: So, before the 1990s, we had no direct evidence of planets outside our solar system. But in 1995, astronomers detected the first exoplanet orbiting a sun-like star. How? They used a technique called the radial velocity method. This method relies on the gravitational pull between a planet and its star. As a planet orbits, it causes the star to wobble slightly. This wobble affects the star's light spectrum—it shifts toward the red when the star moves away from us and toward the blue when it moves toward us. By measuring these periodic shifts, astronomers can infer the planet's mass and orbit. However, this method works best for massive planets that are close to their stars, because they create a larger wobble. That's why early discoveries were mostly 'hot Jupiters'—gas giants in tight orbits. Later, the transit method, which observes a star's dimming when a planet passes in front of it, allowed us to find smaller planets. So, each technique has its strengths and limitations, and together they give us a more complete picture of distant planetary systems.