Re: how do gas planets form

Let's start with how planets form. A striking fact about the orbits of the planets in our solar system is that they are nearly co-planar or they lie nearly in the same plane. If the solar system could be shrunk down so that it was the size of a sheet of paper, the planets' orbits would lie on that sheet of paper. There would be no orbits that would come out of the sheet. (Pluto's orbit is actually an exception. It would be tilted a small amount with respect to the sheet of paper, but even so, the rest of the planets' orbits would lie essentially on the sheet of paper.) A dramatic demonstration of this fact was provided by the Clementine spacecraft. The planets shown in this image almost "line up."

Astronomers think that the reason the planets' orbits are co-planar is because that reflects how the planets formed. When the Sun was forming, not all of the material made it into the Sun. Some of it was left over, and this left-over material "settled" into a disk, probably a donut-shaped structure orbiting the Sun. A key piece of evidence in favor of this theory has been the discovery of disks of material around other stars. One of the first discovered, and probably the most famous, is the disk around the star Beta Pictoris. (The link takes you to an image made by the Hubble Space Telescope, but the disk was discovered initially by the Infrared Astronomical Satellite in the 1980s.) More recently, the HST has shown many young stars surrounded by disks in a star-forming region in the constellation Orion.

What explains the difference in the planets' compositions? Why are the inner four planets small and rocky while the outer four planets are large and gaseous? There are two things to note about the disks from which planets form. First, there isn't a lot of room in the inner part of the disk. Take a pen, and make two circles of different diameters: o and O. Which circle took more ink to draw? Of course, the larger. In the same way, there was more material available in the outer parts of the planetary disk. The second important fact about the disk is that the inner parts were hotter because they were closer to the young Sun. The inner parts of the disk would have had temperatures of hundreds to thousands of degrees. The outer parts would have been cooler, with temperatures eventually dropping below the freezing point of water.

Planets form from smaller pieces. Dust grains in the planetary disk stick together to form small pebbles. Small pebbles stick together to form large pebbles, large pebbles form small rocks, small rocks form larger rocks, and so on, until finally all the material is used up, and the planet is formed.

When the inner, terrestrial planets (Mercury, Venus, Earth, and Mars) were forming they had two strikes against them. First, they were forming in the inner part of the disk where there wasn't a lot of material to start. Secondly, what kind of materials can survive temperatures of hundreds to thousands of degrees? Only rocks can. Materials like water would turn to steam and dissipate. Rocks are typically made from silicon, and there just isn't a lot of silicon in the Universe. That's why the inner planets ended up being rather small, rocky planets: Rocks were the only thing that could form that close to the Sun, and, that close to the Sun, there just wasn't a lot of room to form a big planet.

Farther out from the Sun, not only was there more room to form a planet, the temperatures were lower. Not only could dust grains start sticking together, but water droplets and snowflakes could form. These water droplets and snowflakes could then start freezing onto the rocks that were forming or start forming slushballs or slush boulders themselves. Compared to silicon, there's a lot more oxygen (remember that water is two hydrogen atoms and an oxygen atom) in the Universe.

What astronomers think is that the process essentially went wild. Rocks and slushballs started forming and getting bigger, so more stuff started sticking to them, so they got bigger, more stuff stuck to them, they got even bigger, and so forth. When essentially all of the material had finished sticking together, four large planets were left, with one of them being 90 times bigger than the Earth (Saturn) and another being 300(!) times bigger than the Earth (Jupiter). Because there's so much oxygen in the Universe, compared to the amount of silicon, the bulk of the gas giant planets is gaseous. (It's thought that at the very center of the gas giants there is a small core of rock, left over from when the planet started to form. In the case of Jupiter, this core is probably 10--15 times bigger than the Earth.)

So the summary is that planets form from disks, and gas giants form where the disk gets cool enough that water is not turned into steam, but can help form the planet.

(I should note that where I've written "water," it would be more accurate to write "water, methane, and ammonia." Methane is a product formed from carbon and hydrogen atoms, and ammonia is formed from nitrogen and hydrogen. Oxygen, carbon, and nitrogen are all far more abundant than is silicon so the gas giants are thought to be composed of lots of methane, ammonia, and water. Indeed, the blue color of Uranus and Neptune is an indication that they must have lots of methane in them. I wrote "water" because I assume that most people are more familiar with liquid and frozen water [ice] than they are with liquid and frozen methane and ammonia. If you'd like, though, you can re-read everything that I wrote substituting "water, methane, and ammonia" for "water.")