The Outer Planets
There are four outer planets in our Solar System. These are also sometimes called gas giants since they are much bigger than any of the inner planets and they are not mainly made up of rock or other solids.
All the gas giants have a rocky or solid core but the bulk of the planet is made up liquids and gases such as hydrogen and helium.
The diagram shows the orbit of Pluto as well - but Pluto is no longer classed as a planet since it is so small. One interesting fact about the orbit of Pluto, though, is that for most of the time it is further from the Sun than all of the planets - although, at certain points in its orbit, Pluto is closer to the Sun than Neptune.
Jupiter is the largest planet in the Solar System. In fact its mass is 2.5 times greater than the mass of all the other planets in the Solar System added together!
Although it is composed mainly of hydrogen and helium its mass is so great that it causes long-lived 'weather systems' in its atmosphere. For example, many astronomers have been able to see a red spot on the surface of the planet. This is, in fact, a huge storm that has been observed for nearly 200 years and may well be much older than that.
Jupiter is so big that you could fit 1321 Earths into the volume taken up by the planet.
The most distinctive feature of Saturn is its ring system. It is the second largest planet in the Solar System - you could fit 763 Earths into the volume taken up by Saturn.
Saturn has at least 60 moons (or satellites). Titan is the biggest satellite and it has a dense atmosphere - similar to Earth's in some ways. It is much smaller than Earth, and much more cold (-179°C), however some scientists have suggested that life could exist on Titan in certain conditions.
Uranus is different to all the other planets in the Solar System in that it spins on its side when it orbits the Sun. Uranus has at least 27 satellites.
You could fit 63 Earths into the volume taken up by Uranus.
Neptune is the planet most distant from the Sun. It has at least 13 satellites.
You could fit 57 Earths into the volume taken up by the planet Neptune.
In 1989 the Voyager 2 spacecraft observed a great dark spot on Neptune's surface which many people consider to be a giant storm system - similar to Jupiter's Red Spot.