Our solar system has four inner rocky worlds, four gaseous
outer worlds, plus icy Pluto. All are in the same orbital
plane, except Pluto. Pluto’s small size and odd characteristics
lead some astronomers to believe that it’s not a
planet but a comet-like object captured from the outer
solar system. The similar orbits of the planets in the
solar system result from conditions that prevailed inside
the disk of gas and dust from which they formed.