What do the speakers mainly discuss? [originaltext]Listen to part of a lecture i

Listen to part of a lecture in an astronomy class. (P =Professor, S = Student)
P: We already studied the planets in our solar system, and how they were detected. Okay, today, I’d like to turn our attention to what are called exoplanets and how researchers discovered them. Well, exoplanets are basically planets that are found outside the solar system. They orbit a star, what’s called host star, in a solar system other than that of Earth. Due to improved observational technologies, the rate of detection has increased rapidly. Over the last 15 years, astronomers have detected nearly 500 exoplanets, and the study of exoplanets has been getting exciting. What makes it so astonishing is the fact that the discovery of the first exoplanet was confirmed only in the mid-1990s. Now, we’re finding new ones every few weeks or so. Yes?
S: I wonder if any of these exoplanets harbor lives on them. Is that why astronomers are so interested in them? I mean, the only exoplanets we know like Jupiter and Saturn, are gas giants and couldn’t support carbon-based life.
P: Okay, let’s talk about that first. As for the habitability, it depends on many factors and well… I think that sort of discovery is pretty far in the future, but it is an eventual goal. We’re now going to consider locating planets around the host star in an area called the habitable zone. It’s the region around a star where conditions are best for life to form. Theoretically, it is possible for a planet to support carbon-based life. So, what conditions are required?
S: Water!
P: Yes, and also it needs to have sufficient atmospheric pressure to maintain liquid water on its surface. What else?
S: Um… it would need to be a rocky planet. I mean, as opposed to a gas giant.
P: An earth-like planet. Okay, good! Actually, there are some recently discovered exoplanets that are earthlike. Um… for instance, it’s a star, a red dwarf star, that’s what most stars are. It’s called Gliese 581. Gliese 581 is… a red dwarf, but it’s more interesting than that name makes it seem. This host star is located 20 light years away, which is pretty close by astronomical standards. So it’s considered a near neighbor of our solar system. And its estimated mass is about a third of that of the Sun. That is, it’s relatively small and cool at least compared with the sun. And researchers have discovered other planets orbiting Gliese 581. These are Gliese 581b, c, d, and e, in alphabetical order of their discovery. What I’d like to focus on now is Gliese 581d, and e. In 2009, a group of astronomers reported that these two exoplanets, orbiting not far from the inner and outer edge of its habitable zone, do have some earth-like qualities. Gliese 581d, had actually been discovered a couple of years earlier, and was initially judged to be too far from its host star, making it too cold for life. However, later its orbit was reexamined and now we see that it is potentially habitable.
S: So, does it have a liquid ocean like Earth?
P: Well, conceivably. See, Gliese 581d, with a mass of roughly seven Earths, or half Uranus, it is too massive to be entirely made of rocks. The researchers studying it said that it could have a rocky core, an ice layer, a large deep ocean, and an atmosphere. Okay, another announcement made by researchers was the discovery of Gliese 58le. This planet is relatively small compared with other exoplanets. Its mass is only about twice that of Earth.
S: So, is Gliese 58le a more earth-like exoplanet?
P: Well, Gliese 58 le orbits its host star in a much shorter period of time than the others in this system. I mean, if you consider its orbit, you’ll find out that it’s very close to the star. Therefore, too hot for water to form liquid. However, the fact that it’s relatively close to the size of earth, small, in astronomical terms, that was pretty exciting. And it’s very impressive that we have more advanced technology to discover it. There’s good reason to believe that the more we search, the more we’ll find.