Eight worlds round one star
The planets are the eight large worlds that circle the Sun. The four nearest it, Mercury, Venus, Earth and Mars, are small, with solid, rocky surfaces. The four beyond them are giants. Jupiter and Saturn are gas giants, Uranus and Neptune ice giants, and none of them has a hard surface to stand on.
They are spread over a huge distance. Earth circles about 150 million kilometres from the Sun, and astronomers use that distance as a measuring stick, the astronomical unit. Neptune is about 30 times as far out. Even sunlight needs 4.2 hours to get there.
Smaller still are the dwarf planets: Pluto, out beyond Neptune, and Ceres, in the asteroid belt between Mars and Jupiter. Both are round and circle the Sun. But in 2006 the International Astronomical Union decided that a planet must also have cleared the neighbourhood around its orbit. These two share theirs with many other bodies.