They're also sometimes called Gas Giants, because they are the largest planets and have thick atmospheres. They're not gas all the way through however, they have layers of liquid hydrogen and "small" rocky cores. These planets are so big, however, that by "small" cores, you should understand that they are still more massive than Earth. You just shouldn't think of a planet like Jupiter as being a rocky ball with a thick atmosphere and ocean--no, it's just different. The "ocean" is actually metallic hydrogen, hydrogen pressed so tight that the electrons are shared like in a metal.

Let's go through the Jovian planets one by one first.

Jupiter:

Jupiter's diameter is 11 times that of the Earth, and it is about 320 times more massive than Earth.

Saturn:

Saturn's diameter is 9 times that of the Earth. It's the least dense planet, with a density less than that of water! (Warning: stupid joke!) Yes, if you put Saturn in a bath-tub, it would float... (...but it would leave a ring!) sorry

Uranus and Neptune are pretty similar. That's Uranus shown above. Careful how you pronounce it in mixed company. The blue color comes from methane in the atmosphere. Both Uranus and Neptune have diameters about 4 times that of Earth. Neptune is a little more massive, but Uranus is a little bigger.

That's Neptune above.

1. Jovian planets, unlike the Earth, give off their own heat. The Earth is hot inside, but the heat coming out the surface from the hot interior is a miniscule fraction of the heat that Earth gets from the Sun. Jupiter gives out about twice as much energy as it gets from the Sun.

   If Jupiter were something like 80 times more massive it could have been a star! A star not only gives out light of its own, but gives it out because of nuclear fusion reactions. In order to start those reactions you have to be pretty hot in the first place.

   So why are the Jovian planets hot, and why do they give off so much energy? We're not sure for Jupiter. It could be that Jupiter still has the heat it was born with. Or it could still be contracting a little bit, pulled together by its gravity. That would heat it up. For Saturn, it's thought that a Helium rain falls down from the upper layers, and warms the layers below by pelting them.

2. Making Jupiter more massive wouldn't have made it much bigger. The textbook describes this as being like stacking pillows. If you have a small stack, adding a pillow will make it taller. But adding a pillow to a large stack will make each of the many pillows beneath shrink a bit--and that adds up. So a planet more massive than Jupiter would mostly be denser, instead of actually bigger in size.

3. Jovian planets spin real fast. Jupiter's day is less than 10 hours. Remember that Jupiter is 11 times the size of the Earth. So at the equator, Jupiter's going through
4. Jovian planets, being gaseous and liquid, do not rotate as solids rotate. They undergo differential rotation. Stars like the Sun do this too. At the equator, Jupiter spins faster than at the poles. At the equator, a Jupiter day is 9 hours 50 minutes. At the poles, it's 9 hours 55 minutes!

5. Jovian planets are not very spherical! Look closer at the Hubble image of Saturn above. Because of the fast rotation and the liquid and gas makeup of the planet, the planet can easily be deformed by the centrifugal force of rotation into an oblate shape with an equatorial bulge.

There's also an amazing storm called The Great Red Spot. It's big enough to fit 2 or 3 Earths! It's at least a few hundred years old, as it's been seen as long as people have been able to see it through a telescope.

So what causes this red spot and what causes all the cloud patterns in Jupiter?

Two main things: the rapid rotation, and the heat coming out from the inside.

The rapid rotation causes a killer coriolis effect. On the Earth, the coriolis effect causes the equator-to-pole winds to break up into 3 zones for each hemisphere. Jupiter, going much faster, has more zones. You can see here an animation from the Cassini spacecraft view of Jupiter, showing that the atmosphere is going in opposite directions at different latitudes.

The Great Red Spot is also affected by the Coriolis effect... And because there's no solid ground beneath it to absorb its energy, it stays there for centuries.

What causes all the colors in the clouds? We don't know about all of them. We're not exactly sure why the Great Red Spot is red. But the whites and the browns, we think we know.

Here on Earth there's only one kind of cloud: water clouds. They form where the vaporized water can make small crystals--in other words, where it's cold enough. Jupiter also, we think, has water clouds. (The Galileo spacecraft that dropped a probe into Jupiter's atmosphere didn't see much water, but it may have been in an unusual place.)

On Jupiter, you can go higher up in the atmosphere than the water clouds and you'd see other clouds: ammonium hydrosulfide (NH₃HS), which cause the brown color, and then even further up, ammonia clouds (NH₃), which cause the white colors. Each of these freezes at a different temperature, and the further up you go in Jupiter, the colder you get--because Jupiter is heated from within.

Why is the magnetic field so strong? Because Jupiter's inside contains a thick layer of metalic hydrogen, spinning very fast.

What are the consequences of having such a strong magnetic field? For one, Jupiter has very strong auroras. An aurora happens when solar wind particles come down on a planet, directed towards the poles by the magnetic field.

Also, Jupiter's moon Io has volcanoes that spew Sulfur gas into space. This Sulfur gas gets pulled around by Jupiter's magnetic field into something called the Io plasma torus.