Why is the solar system flat? Do the planets revolve around the sun on a 2D plane?
The solar system is almost all in a plane because it was formed from a relatively flat disk of material.
The solar system is however nor 2 dimensional. All of the planets have orbits which are inclined at slightly different angles.
The Earth's orbit is the reference plane. The planet with the orbit closest to that of Earth is Uranus which is inclined at an angle of
Some bodies have orbits well out of the plane of the ecliptic. The dwarf planet Eris has an orbit inclined at