HEADSCRATCHERS

Hi, I'm Pete, and I love science and the natural world. I work with the Royal Institution (Ri) in London, where you can find exciting, hands-on science events for young people. We've teamed up with The Week Junior Science+Nature to answer your burning science questions.

- Peter Gallivan

How do gas planets stay the shape they are if they’re made of gas? Harriet & Edwin

The four outer planets of the solar system, (Jupiter, Saturn, Uranus and Neptune) are known as the “gas giants”. This is because they are mostly made up of the light elements helium and hydrogen, which are gases. These planets aren’t all gas though, because they also have small solid cores.

Jupiter is the largest of these gas giants and the largest of all the planets in the solar system, being eleven times wider than Earth and more than twice the mass of all the other planets put together. Saturn is the solar system’s second-largest planet.

The force that holds these gassy planets together is the same that holds rock-solid Earth together – gravity. This force pulls objects toward the centre of a planet. It’s why when you jump, you soon land back on the ground. Because gravity pulls equally on all sides, the planets keep their spherical (ball) shape.