According to Space.com, the order of planets is Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, starting with the one closest to the sun. After Pluto was discovered in 1930, it was introduced as the ninth planet in the solar system. It is the furthest from the Sun. However, astronomers struck Pluto off the list of planets, after classifying it as a dwarf planet.
About.com homework and study tips recommends the mnemonic device: "My very educated mother just served us nachos," as a way to memorize the order of planets. As NASA explains, eight planets alongside their natural satellites, the sun, tiny planets, asteroids and comets make up our solar system.
On Wikipedia, a list of planets by order of size shows Jupiter to be the biggest, but second to the Sun in the solar system. From the biggest to the smallest planet, the order is: Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars and Mercury. However, such an order can change depending on whether one is relying on radius or mass as the determining variable.
A planet can be denser than another, even if its radius is smaller, Wikipedia states. For instance, Uranus is bigger than Neptune, although it has less mass. Jupiter has a mean radius of 69, 911 kilometers while Mercury’s is 2,440 kilometers.