Ecliptic orbits and unseen planets

I've been thinking of some things about our solar system. Eight out of the nine planets that we know of in our solar system orbit in the same plane...called ecliptic orbits. We'll call this plane the x plane for simplicity. Pluto has an inclined orbit. It is a little skewed toward the y plane but is closer to the x plane.

My questions are:

1. Could there be other planets that have an ecliptic orbit on the y plane that we haven't detected yet because they are on the y plane? (Probably not because we detect asteroids and comets that are not on the x plane)

2. Could there be a planet directly on our (Earth's) orbit only opposite the sun? (This would be very cool to have a real sister planet)

3. It just seems a little strange that planets orbit on the same plane. Is there something about gravity that aligns the planets to make them orbit in the same plane?

4. Are there solar systems in space that have planets orbiting on more of a spherical orbit? Like how electrons orbit a nucleous?

Just some questions to satisfy my curiousity.

[edit on 17-3-2006 by Griff]