Not to mention that Pluto is hardly in neat alignment with anything, and it's not as if the other planets are particularly uniform either.
http://vathena.arc.nasa.gov/curric/space/planets/planorbi.html
Their inclinations and eccentricities are all different. Pluto is especially weird; I hear it's not even considered a planet anymore.
Outside the limits of Neptune's orbit lies a band of chunks of ice and rock called the Kuiper belt (named after it's discoverer). One of the biggest and closest of those chunks is Pluto, discovered in 1930. It's orbit is the most eccentric of all planets, so much so that for part of it's orbit, it actually crosses inside Neptune's. With the advent of extremely Earth- and space-based telescopes, more Kuiper belt objects have been found. One has recently been found that is actually somewhat larger than Pluto, and the whole definition of "planet" has been thrown open. Shall we consider the new object a planet? Shall we downgrade Pluto? Currently, the consensus is to keep Pluto as a planet simply due to historical reasons and to consider any other Kuiper objects as non-planets, but that's still being debated. Where the fecal material will really hit the rotary distributor will be if a very large Kuiper Belt object (KBO) is found, say one twice or 3 times the size of Pluto. It could happen.
Note that the Kuiper belt is in roughly the same plane as the planets and has some limits as to it's extent. There is also the Oort cloud, which is where most comets come from. This is a cloud of objects that forms a sphere around the Sun and extends half-way to Alpha and Proxima Centauri. Orbits there are in the order of 10,000, 100,000, or even 1,000,000 years. Occasionally, collisions out there (or even the gravitational influence of other stars or non-stellar bodies passing by) disrupt those and cause them to slowly start to plunge into the inner Solar system. Sometimes they fall right into the Sun; the ECHO probe that photographs the Sun sees them. Sometimes they miss the Sun, flying by it and giving us a spectacular show as comets. Those that have hyperbolic orbits leave the Solar system. Others may not come back for 100,000 years. Some are influenced by (usually) Jupiter and are captured to become periodic comets whose orbit can be known and whose reappearance can be predicted; the most famous is Halley's Comet (1/P) with an orbit of 76 years.