NARRATOR：

Listen to part of a lecture in an astronomy class.

MALE PROFESSOR：

Last week we discussed the formation of Earth and the other rocky planets-the planets in the inner solar system.

Uh, s-so what about the gas giants: Jupiter, Saturn, Uranus, and Neptune?

Well, there's two theories, but first let's recap.

We believe our solar system began as a huge spinning cloud of dust and gas, which flattened and eventually collapsed in on itself.

The matter at the center condensed into a ball of hot gas and dust, eventually becoming our Sun.

And... what happened to the remaining cloud, to the disk encircling the Sun when it was a young star?

FEMALE STUDENT：

The rocky planets were born. Um, dust- little grains of rock and metal within the disk collided with each other and stuck together.

And this process sorta snowballed over millions of years... until the chunks grew into mini-planets-protoplanets.

MALE PROFESSOR：

Yes. Uh, this process is called accretion, and we call the disk an accretion disk.

[Explaining] Now... think of it as two parts- an inner accretion disk and an outer accretion disk.

In the inner part, once an object gets large enough, that object's gravitational field gets stronger, which speeds up the accretion process.

You know, larger objects attract smaller ones and sorta gobble them up. And eventually you get a full-size planet in its own orbit.

OK... That's how the inner rocky planets probably formed-by accretion. But what about those gas planets in the outer solar system, in the outer accretion disk?

Well, the first theory says the accretion process was similar to the one that formed the rocky planets, with some key differences.

Remember the gas giants are farther from the Sun, where temperatures are much colder.

So, in the outer accretion disk, compounds like water and ammonia exist in frozen form.

Closer to the Sun, they're more likely to be vaporized by solar radiation.

What this means is that, in addition to rocky and metallic particles, there's be other solids, like frozen water and frozen ammonia.

FEMALE STUDENT：

[Following on]...so more solid substances are available to clump into protoplanets, right?

MALE PROFESSOR：

Precisely. So the solid cores of the gas giants could conceivably have formed by accretion.

And once their mass reaches a certain point- around about five to ten Earths- what would happen?

FEMALE STUDENT：

Five to ten Earths? With a mass that big, I guess gravity would start to pull in more and more material faster, right?

MALE PROFESSOR：

Material, meaning gas. It would rapidly pull in more and more gas from the accretion disk.

So, you end up with a solid core of rock, metal, and ice surrounded by massive amounts of gas. That's the core-accretion theory.

Now-the other theory's called the disk-instability theory.

The disk-instability theory holds that gas begins the planet-making process without a solid core.

You see, most of the outer accretion disk would've been gas; we believe solid particles probably made up just 1 percent of the outer accretion disk.

So, this theory suggests that the large planets-the gas giants- they develop from large clumps of mostly gas and some dust in an accretion disk.

Um, outer regions of an accretion disk can be unstable, uh, gravitationally unstable, which is what causes these clumps to form and, in some cases, grow into protoplanets.

Over time, dust particles within a gas clump coalesce-bond together-and eventually fall toward the center, creating a core.

Once this happens, the gas clump grows relatively quickly, as its gravity pulls in more and more gas and dust particles.

And this whole process can theoretically happen within 100,000 years.

FEMALE STUDENT：

That's amazingly fast! So, which theory's correct?

MALE PROFESSOR：

That's the debate. Most of my colleagues favor core accretion.

Personally, I think the accretion theory works for the formation of rocky planets, but not necessarily for gas planets.

A major problem is that gas giants like Jupiter and Saturn would take too long to form through core accretion.

Core accretion would take several million years. But observations of other star systems indicate that a disk's gas disappears more quickly than that.

Whatever's not drawn into planets ends up dissipated and evaporated by solar wind and radiation, from nearby stars.

So basically, a baby Jupiter would run out of gas before it grew up.

But the disk-instability theory... well, the timing's right. That process is fast enough to finish before the gas runs out.