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Lecture 4 - The Formation Of Gas Planets

Narrator: Listen to part of a lecture in an astronomy class.

Professor: Last week we discussed the formation of Earth and the other rocky planets, of planets in the inner solar system. Uh, 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 it’s centered 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?

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 sort of snowballed over millions of years until the chunks grew into mini-planets, protoplanets.

Professor: Yeah. This process is called accretion. And we call the disk an accretion disk.

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 sort of gobble them up. And eventually, you get a fullsized planet in its own orbit.

Okay. 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 would be other solids like frozen water and frozen ammonia.

Student: So more solid substances are available to clump into protoplanets, right?

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 5 to 10 Earths, what would happen?

Student: 5 to 10 Earths…uh, with a mass that big, I guess gravity would start to pull in more and more material faster, right?

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 is 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 have been gas. We believe solid particles probably made up just one percent of the outer accretion disk.

So this theory suggests that large planets, the gas giants, uh, they develop from large clumps of mostly gas and some dust in an accretion disk. Outer regions of an accretion disk can be unstable, gravitationally unstable, which is what causes these clumps to form, and in some cases, grow in to proto- planets.

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 one hundred thousand years.