OFFICIAL41 What do the speakers mainly discuss?

[00:00.00]MALE PROFESSOR: OK, we've been discussing the planets in our solar system, and how some of the ones farthest from the Sun were discovered. [00:07.97]Well, today I'd like to turn to what are called exoplanets, and how researchers detect them. [00:13.52]Maria?
[00:15.37]FEMALE STUDENT: Exoplanets are planets that orbit around a star other than our Sun, right? [00:20.36]They're not in our solar system…
[00:22.13]MALE PROFESSOR: Right. [00:23.70]They have different, what're called host stars. [00:25.89]The study of exoplanets has been getting more and more exciting; [00:29.13]hundreds of them have been discovered so far. [00:31.69]This is quite remarkable in view of the fact that the discovery of the first exoplanets was confirmed only in the mid-1990s. [00:39.23]Now we’re finding new ones every few weeks or so.
[00:42.17]FEMALE STUDENT: So, uh—exactly why are we interested in these exoplanets, anyway? [00:47.71]Is it to see if there's life on them? '[00:50.00]Cause it seems to me like the only exoplanets we ever hear about are gas giants, like Jupiter and Saturn, that couldn’t possibly support carbon-based life….
[00:59.03]MALE PROFESSOR: OK, well, let's talk about that.
[01:01.03]First, as for discovering life… [01:03.27]wellll, I think that sort of discovery is pretty far in the future, but it is an eventual goal.
[01:09.08]For now, the focus is on locating planets within a host star's so-called habitable zone, a zone that's a certain distance from its star. Because only planets within this zone could conceivably support carbon-based life.
[01:23.70]So what would such a planet need?
[01:26.11]FEMALE STUDENT: Water?
[01:26.72]MALE PROFESSOR: Yes, it'd need to be the right temperature to sustain liquid water.
[01:31.01]MALE STUDENT:# And it would need to be a rocky planet…. I mean, as opposed to a gas giant….
[01:36.91]MALE PROFESSOR: OK, good. An Earth-like planet.
[01:39.84]Now, as to that, there are some recently detected exoplanets that might actually be Earth-like.
[01:46.45]For example, there's a red dwarf star—that's what most stars are—uh, that's called Gliese 581.
[01:54.59]Gliese 581 is… well, it's a lot more interesting than that name makes it seem. [02:01.21]This host star is considered a near neighbor of our solar system because it's only about twenty light-years away. [02:08.00]That's pretty close, by astronomical standards. [02:11.03]And being a red dwarf star, it's small and relatively cool, at least compared with the Sun.
[02:17.06]And researchers have discovered planets orbiting Gliese 581. [02:22.00]These exoplanets have been named—ready?—[02:25.19]Gliese 581 b, c, d, e… in alphabetical order of their discovery.
[02:33.48]Gliese 581d and e are the planets I want to focus on now. [02:38.43]See, in 2009 a group of researchers made an announcement: [02:42.83]these two exoplanets, [02:49.57]Gliese 581d and e, do have some Earth-like qualities. Gliese 581d had actually been discovered a couple of years earlier, [02:54.30]and when its orbit was originally calculated, it was thought to be too far away from its host star to be warm enough to support a liquid ocean, let alone carbon-based life. [03:03.85]But then its orbit was recalculated, and now we see that Gliese 581d is within its host's habitable zone.
[03:11.78]MALE STUDENT:# So it might have an ocean?
[03:13.92]MALE PROFESSOR: Well, conceivably.
[03:15.52]See, Gliese 581d weighs seven times what Earth weighs, and it's unlikely that it's made entirely of rocks… because it's so massive. [03:24.75]The researchers studying it said that it could have a rocky core; an ice layer; a large, deep ocean; and an atmosphere.
[03:33.23]OK, and there was another announcement, along with the recalculated orbit of Gliese 581d. [03:39.23]That was the discovery of another planet in the system, Gliese 581e.[03:45.25] Compared with other exoplanets, its mass is quite small—only about twice that of Earth's.
[03:49.96]FEMALE STUDENT: So is Gliese 581e a more Earth-like planet?
[03:54.31]MALE PROFESSOR: Well, we have to consider its orbit. [03:57.09]Gliese 581e orbits its host star in a much shorter period of time than the other planets in the system, meaning it's very close to the star. And therefore too hot for water, for an ocean.
[04:08.74]However, the fact that it's relatively close to the size of Earth—small, in astronomical terms—that was pretty exciting. [04:15.58]It's impressive that we have the technology to detect it. And it bodes well for future research. [04:20.75]Who knows what we'll find the more we search?