By Dr. Marc Rayman
Asked by our friends at the W.A. Gayle Planetarium in Montgomery, Alabama.
Click play to hear me read this to you!
We think of a gas as something very . . . well, airy. After all, air is the gas we all know and love. We breathe it and fly planes right through it with no trouble. So it makes sense to think that a gas planet must be like a big, airy cloud floating out in space.
But take another look at Jupiter and Saturn—or pictures of them. Notice how round they are. You will never see a cloud on Earth so nearly spherical. Why are Jupiter and Saturn so round if they are just gas? For that matter, why are any planets round?
Well, the short answer is gravity. Gravity causes all matter to be pulled toward all other matter. Let's think about this in more detail. When the planets were first forming, the solar system was a big, swirling disk of gas and dust, with the newborn Sun at the center. Bits of dust and clouds of gas were attracted to each other because of gravity. As these bits and clouds clumped, they attracted still more matter in their neighborhood and grew larger and larger until there was no longer any stray material nearby for them to attract. The growing planets were like big solar system vacuum cleaners, sweeping up all the debris in their paths. And they became round because gravity pulls equally toward the center of large masses such as planets, so anything sticking out gets pulled back to make a ball.
The bigger a planet becomes, the heavier is the material weighing down on its center. Think of how it feels to dive under water. If you are wearing a face mask, you notice that as you dive deeper, the mask presses harder and harder on your face. Also, your ears start feeling the pressure even at 2 or 3 meters (5 or 10 feet) below the surface. The pressure you feel on your body is due to the weight of the water above you. The deeper you go, the heavier the water above you and so the greater the pressure on your body. Even on Earth's surface, each square inch of your body experiences 14.7 pounds of pressure due to the weight of the atmosphere above you. If you could dive down to the center of Earth, the pressure on your body would be about 3.5 million times as great! The center of Jupiter is more than 11 times deeper than Earth's center and the pressure may be 50 million to 100 million times that on Earth's surface!
The tremendous pressure at the center of planets causes the temperatures there to be surprisingly high. At their cores, Jupiter and Saturn are much hotter than the surface of the Sun!
Strange things happen to matter under these extraordinary temperatures and pressures. Hydrogen, along with helium, is the main ingredient of Jupiter's and Saturn's atmospheres. Deep in their atmospheres, the hydrogen turns into a liquid. Deeper still, the liquid hydrogen turns into a metal!
But what's at the very center of these planets? The material becomes stranger and stranger the deeper you go. Scientists do not understand the properties of matter under the extreme environments inside Jupiter and Saturn. Many different forces and laws of nature are at work, and the conditions inside these planets are very difficult to create in a laboratory here on Earth. But you can be sure that you wouldn't be able to fly through these bizarre materials! As we now know, the gas giants are much more than just gas.
Read "What do you get when you cross an Earthquake with a tidal wave?" to find out how Jupiter's huge gravitational forces affect the shape of its moons.