Why are the orbits of the Sun’s planets arranged in a more-or-less flat plane?

            The planets are believed by many authorities to have coalesced from a disk-shaped cloud of gas early in the evolution of our solar system, beginning roughly 5 billion years ago, said Joe Rao, Lecturer at the Hayden Planetarium of the American Museum of Natural History in New York.

            “We believe that the sun and planets evolved from a huge, swirling cloud of gas and dust that  condensed over a time span of hundreds of millions of years”, Rao said. “This gas cloud was very likely in the shape of a disk, and when the planets came to be within this cloud, they pretty much all formed basically along the same plane.”

There is a notable exception the tiny planet Pluto, which has an orbit that is tipped as much as 17 degrees to the plane of the solar system, Rao pointed out. It has been hypothesized that Pluto might be everything from an escaped moon of Neptune to a special type of minor planet or asteroid. The anomalous orbit of Pluto was one of the reasons for the great debate earlier this year concerning Pluto’s exact status as a member of the solar system, Rao said.