What the planets are like

The four inner planets, nearest the Sun (Mercury, Venus, Earth, Mars), are smaller, with solid, rocky surfaces. The four outer planets (Jupiter, Saturn, Uranus, Neptune) are much larger than the Earth and made mostly of gas and ice. These giants have large, flat rings around them; Saturn's are the easiest to see.

Some planets have nicknames:

• Venus shines brightly at dawn and dusk, so it is called the Morning Star or Evening Star, though it is not a star.
• Mars is the Red Planet, because its soil is reddish.
• The Earth is the Blue Planet: so much of it is covered with water that it looks blue from space.

Planets get most of their energy from the Sun, so in general, the farther from the Sun, the colder the planet. But an atmosphere can trap heat. That is why Venus is hotter than Mercury, even though Venus is farther from the Sun.

Pluto lies beyond Neptune and also goes around the Sun, but it is smaller than our Moon. It was called a planet when it was found. Later, many more small objects like it were found, and in 2006 the IAU set new rules for what counts as a planet. Now Pluto and objects like it are called dwarf planets.