My very eager mother just served us nine pizzas.
Or was it pies? No matter.
This was the mnemonic taught to us so that we can remember the names of the nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
However, sometime in 2006, in light of new discoveries, the International Astronomical Union redefined the word 'planet' as: a celestial body that
a) is in orbit around the Sun,
b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
c) has cleared the neighbourhood around its orbit. - This last bit means it has a clear path around the Sun.
With this definition, Pluto was reclassified as a dwarf planet because of its size and its location within the area called Kuiper Belt.
For comparison, the definition of a 'dwarf planet' is: a celestial body that
a) & b) - same as in the definition of planet above,
c) has not cleared the neighbourhood around its orbit, - meaning it orbits in a zone that has many other objects in it, and
d) is not a satellite.
Currently, there are five dwarf planets namely: Ceres - which is the largest body in the asteroid belt, in the area between Mars and Jupiter; Pluto, Haumea, Makemake, and Eris - which are all in the Kuiper Belt, in the area beyond Neptune.
And if anyone asks, there are eight planets, not nine. And to remember them, think:
My very educated mother just served us nachos.
Hmm... I think I prefer nuggets, though...