Memorize the order of the planets, divide them into rocky and gaseous ones, and understand why Pluto is no longer a planet.
Who lives in the solar system
Eight planets orbit the Sun. In order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
It's handy to split them into two groups.
— Rocky (terrestrial): Mercury, Venus, Earth, Mars. They are small, solid, and lie closer to the Sun.
— Gas and ice giants: Jupiter, Saturn, Uranus, Neptune. They are huge, have no solid surface you could stand on, and have many moons.
The largest planet is Jupiter: all the other planets put together would fit inside it, with room to spare. The smallest is Mercury, only a little larger than our Moon.
And what about Pluto? In 2006, astronomers agreed to call it a dwarf planet. The reason: many other similar icy bodies were found near Pluto's orbit. A true planet must “clear the neighborhood” around its orbit of such neighbors, and Pluto hasn't done that. It didn't get worse or smaller — scientists simply refined the definition.
Lesson notes
Eight planets orbit the Sun. In order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
It's handy to split them into two groups.
— Rocky (terrestrial): Mercury, Venus, Earth, Mars. They are small, solid, and lie closer to the Sun.
— Gas and ice giants: Jupiter, Saturn, Uranus, Neptune. They are huge, have no solid surface you could stand on, and have many moons.
The largest planet is Jupiter: all the other planets put together would fit inside it, with room to spare. The smallest is Mercury, only a little larger than our Moon.
And what about Pluto? In 2006, astronomers agreed to call it a dwarf planet. The reason: many other similar icy bodies were found near Pluto's orbit. A true planet must “clear the neighborhood” around its orbit of such neighbors, and Pluto hasn't done that. It didn't get worse or smaller — scientists simply refined the definition.