NARRATOR：

Listen to part of a lecture in an astronomy class.

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.

Well, today I'd like to turn to what are called exoplanets, and how researchers detect them. Maria?

FEMALE STUDENT：

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

MALE PROFESSOR：

Right. They have different, what're called host stars.

[excitedly] 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're finding new ones every few weeks or so.

FEMALE STUDENT：

[hesitantly] So, uh-exactly why are we interested in these exoplanets, anyway? Is it to see if there's life on them? '

Because 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....

MALE PROFESSOR：

OK, well, let's talk about that.

First, as for discovering life... wellll, 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 [slowly] habitable zone, a zone that's a certain distance from its star.

Because only planets within this zone could conceivably support carbon-based life.

[leading] So what would such a planet need?

FEMALE STUDENT：

Water?

FEMALE STUDENT：

Yes, it'd need 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....

MALE PROFESSOR：

OK, good. An Earth-like planet.

Now, as to that, there are some recently detected exoplanets that might actually be Earth-like.

For example, there's a red dwarf star-that's what most stars are-uh, 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 twenty 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 Gliese 581.

These exoplanets have been named-ready?-Gliese 581 b, c, d, e... in alphabetical order of their discovery.

Gliese 581d and e are the planets I want to focus on now.

See, in 2009 a group of researchers made an announcement: these two exoplanets, Gliese 581d 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 581d is within its host's habitable zone.

MALE STUDENT：

So it might have an ocean?

MALE PROFESSOR：

Well, conceivably.

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.

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 581d.

That was the discovery of another planet in the system, Gliese 581e.

Compared with other exoplanets, its mass is quite small-only about twice that of Earth's.

FEMALE STUDENT：

So is Gliese 581e a more Earth-like planet?

MALE PROFESSOR：

Well, we have to consider its orbit.

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.

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'll find the more we search?