The names of the nine planets in order are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Many individuals use a mnemonic, or memory device, to remember these names. Pluto used to be considered a planet but is no longer classified as one.
The International Astronomical Union changed the status of the Pluto to a dwarf planet as it is unable to fulfill the criteria used by the IAU in defining a full-sized planet. Pluto meets up with two of the three criteria; however it “has not cleared its neighboring region of other objects.”
As of August 2006, the IAU renamed Pluto as a dwarf planet. This now means only the gas giants (Uranus, Neptune, Saturn and Jupiter), which are part of the outer system, and rocky worlds, part of the inner solar system, are known as planets. The inner system is a region in space that contains terrestrial planets, including Mars, Earth, Venus and Mercury, and an asteroid belt.
According to the IAU the criteria used for defining full-sized planets include three factors: first, the planet must orbit the sun; second, the planet must have enough mass to assume hydrostatic equilibrium, which means a it's a near-round shape; and third, the planet has “cleared the neighborhood around its orbit,” meaning it can attract nearby objects gravitationally and force them into orbit or onto the planet.
