TOEFL Listening: ETS-TOEFL听力机经 - GR591V5CUC949B11Q$

Lecture: Exoplanets Detecting: Narrator: Listen to part of a lecture in an astronomy class. Professor: Exoplanets are planets outside Earth solar system, planets in orbit around stars other than our Sun. The first confirmed discovery was in the late 1980s, and since then, astronomers have detected over 500 exoplanets. Now, detecting exoplanets directly, that is, trying to actually see them by way of a telescope has been extremely difficult. You see, exoplanets appear very small, very faint and extremely close to the star they orbit their parent star. And this means that exoplanet will often be hidden by the glare of the parent star. So trying to see an exoplanet is like, well, trying to see from far away the light of a flashlight shining out next to a big, bright spotlight, but more on direct methods in a minute. Let's start with how we know exoplanet even existed, even before we could directly see them with telescopes. Well, what happened was astronomer is used indirect methods to infer their presence. You see, they had observed that some stars appeared to wobble slightly, and they wondered what was causing these small, irregular movements. And well, after years of careful study, astronomers were able to determine that a given star's wobble was the result of an exoplanets gravitational force on that star, uh, tugging, which causes the star to move. Now, if we're looking at a binary star system, a system of two stars orbiting around a common point, both stars will appear to wobble. This means that when astronomers detect only one object that wobbled with no other wobbling object near it, they can be reasonably sure that what they're observing is the effect of an exoplanet orbiting a star. Another way to determine an exoplanet's existence, astronomers also try to detect the dimming of a star at regular intervals. This dimming will happen as a planet crosses in front of the parent star, temporarily blocking its rays. Ok, so indirect methods, they're fine for confirming that an exoplanet is there and providing us with other limited information about the planet itself. But the best way to determine its chemical composition, which would tell us things like what its environment is like, if it could support life. The only way for us to get extensive information of this sort would be to analyze the light coming directly from the exoplanet. Now until recently, we could only directly observe exoplanets under exceptional circumstances. Um, with the aid of the hubble space telescope and some very large ground based telescopes, we've been able to see, especially large exoplanets that are widely separated from their parents star. Well, fortunately, with the recent development of some imaging techniques, we've been able to directly detect and photograph smaller exoplanet using smaller telescopes. We've done this by combining two different newer technologies. Let's look at the first one, adaptive optics. Now, you all know that when we look at stars from Earth, they appear to twinkle, because when starlight passes through Earth's atmosphere, the light gets distorted. We can adjust for this distorting effect by equipping our telescopes with adaptive optics. Adaptive optics produce very sharp photographs of a star by rapidly adjusting the telescope lens to correct for this distorting effect. Telescopes equipped with adaptive optics can capture such sharp images of stars at the photographs look as if they were taken from space. However, adaptive optics won't solve a problem I mentioned earlier. That is, the bright light of the star will still obscure the orbiting exoplanet. So the trick is to suppress the starlight without blocking out the light from the exoplanet. In order to do this, we can attach a device called a chronograph to our telescope. There are now two types of chronograph. A traditional chronograph blocks out most of the light from the star by using a small black disc. However, there's a complication, even when we use this disk, some of the starlight leaks around the disc, and creates a series of alternating bright and dark rings around the star called a diffraction pattern. Now, this ring pattern will mask the image of the orbiting planet, and we still won't be able to capture an image of it, especially if the exoplanet orbits relatively close to the parent star. To solve this problem, a new type of chronograph called a vortex-chronograph was developed. A vortex-chronograph is a glass lens with a spiral pattern etched into it. The spiral pattern successfully blocks the star and nearly eliminates the surrounding diffraction patterns, allowing us to actually see exoplanets orbiting near their parent star.