As the solar nebula spun, gases at the center collapsed to form the Sun. The remaining cloud flattened into a rotating disk, and gravity caused denser areas of gas and dust to shrink and condense into objects called planetesimals, about a kilometer in size. Some planetesimals collided and stuck together as they whirled around, forming blobs which grew into the planets.
Planets are a few thousands of kilometers across.
But some of the planetesimals in the solar system did not become part of a planet. Scientists think they know how planetesimals may have been ejected from the inner solar system into the Oort Cloud . The cartoon at right shows what would have happened when a planetesimal drifted close to a giant planet , like Jupiter. The planet's gravity would pull on the smaller object, like a ball swung on a rope. After Jupiter lets go, the planetesimal shoots off in a new direction on a new path.