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The International Astronomical Union recently voted on a proposal that would give strict definitions as to what a planet is in our Solar System. The resolution that passed gives the following definitions:

A planet is 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.
A dwarf planet is 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, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

This means that Pluto is no longer a planet, as its orbit is partly shared with Neptune.

Bill Ferris, on the Minor Planet Mailing List, raises an excellent point:

Playing devil’s advocate; I’d like to know how Jupiter, Saturn, Uranus and Neptune meet the newly adopted criteria to be defined as planets.

All have proximate ring systems; Saturn’s being the most extensive and visually dramatic. Jupiter has more than 60 known satellites; Saturn more than 30; Uranus more than 20; and Neptune a baker’s dozen. The giants share their neighbourhoods with material that, in each case, has a total combined mass greater than that of all the known asteroids combined. Additionally, Jupiter shares its orbit with hundreds of asteroidal objects (small solar system bodies) known as trojans.

If the distinguishing characteristic that separates a planet from a dwarf planet is having “cleared the neighbourhood around its orbit,” how do the gas giants meet that criteria? Their neighbourhoods are nothing if not well-populated.

He’s right. The neighbourhoods around the gas giants are quite crowded. Because of this, all four fail the “it has cleared its orbit of other objects” test, and therefore all four gas giants are, in fact, dwarf planets.

So to remember the planets in the Solar System, just remember this simple mnemonic:

My Very Educated Mother.