TPO 41 Lecture 4(文本在APP页面简介处)

学托福联系V: bravoyouth

Listen to part of a lecture in an astronomy class.

Professor

OK.We've been discussing the planets in our solar system and how some of the ones farthest from the sun were discovered. Well, today I'd like to turn to what are called the exoplanets and how researchers detect them. Maria?

Female Student

Exoplanets are planets that orbit around a star other than our sun, right? They are not in our solar system.

Professor

Right. They have different, what are called host stars. And the study of exoplanets has been getting more and more exciting, hundreds of them have been discovered so far. This is quite remarkable in view of the fact that the discovery of the first exoplanets was confirmed only in the mid-1990s. Now we are finding new ones every few weeks or so.

Female Student

So, exactly, why are we interested in these exoplanets anyway? Is it to see if there is life on them? Cause it seems to me like the only exoplanet we ever hear about are gas giants, like Jupiter and Saturn. That couldn't possibly support carbon-based life.

Professor

OK, well, let's talk about that. First, as for discovering life, well, I think that sort of discovery is pretty far in the future. But it is an eventual goal. 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. So, what would such a planet need?

Female Student

Water?

Professor

Yes, it needs to be the right temperature to sustain liquid water.

Male Student

And it would need to be a rocky planet. I mean as opposed to a gas giant.

Professor

OK, good. And earth-like planet. Now, as to that, there are some recently detected exoplanets that might actually be earth-like. For example, there is a red dwarf star that’s what most stars are. That's called Gliese 581. Gliese 581 is, well, it’s a lot more interesting than that name makes it seem. This host star is considered a near neighbor of our solar system because it's only about 20 light years away. That's pretty close by astronomical standards. And being a red dwarf star, it's small and relatively cool at least compared with the sun. And researchers have discovered planets orbiting Gliese581, these exoplanets have been named, ready? Gliese 581 B, C, D, E, in alphabetical order of their discovery. Gliese 581 D and E are the planets I wanna focus on now.

See, in 2009, a group of researchers made an announcement. These two exoplanets, Gliese 581 D and E do have some earth-like qualities. Gliese 581D had actually been discovered a couple of years earlier 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. But then its orbit was recalculated and now we see that Gliese 581 D is within its host's habitable zone.

Male Student

So it might have an ocean?

Professor

Well, conceivably. See, Gliese 581 D weighs 7 times what earth weighs, and it's unlikely that it's made entirely of rocks because it's so massive. The researchers studying it said that it could have a rocky core, an ice layer, a large deep ocean and an atmosphere. OK, and there was another announcement along with the recalculated orbit of Gliese 581 D. That was the discovery of another planet in the system— Gliese 581 E, compared with other exoplanets, its mass is quite small, only about twice of that of Earth.

Female Student

So is Gliese 581 E a more earth-like planet?

Professor

Well, we have to consider its orbit. Gliese 581 E 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. However, the fact that it’s relatively close to the size of earth—small, in astronomical terms. That was pretty exciting. It's impressive that we have the technology to detect it and it bodes well for future research. Who knows what we will find the more we search.