Oort's model tells us comets come from a giant cloud surrounding the solar system, but how were the comets originally formed, and how did the Oort cloud evolve? There are two main schools of thought, and both arise from the current belief that the solar system was created 4.5 billion years ago from a huge, spinning cloud of gas and dust called the . As the cloud flattened into a rotating disk, gravity would have caused concentrations of gas and dust to condense into kilometer-size objects called "planetesimals." Planetesimals would then collect to form the much larger planets.
In the more likely view of comet formation, kilometer-size planetesimals in the outer system were "slung" off by Uranus and Neptune when they came near one of the newly-formed outer planets, eventually forming the Oort cloud. Most of the comets would have been ejected along fairly "flat" paths from the already flattened planetary disk, like stones being skipped on a pond. In fact, more recent study suggests that an "inner" Oort Cloud just beyond the orbit of Neptune, contains 10 to 100 times more comets than the outer cloud.
Oort's theory suggests that we see only a few of the comets in the cloud; the vast majority are moving in huge, nearly-circular orbits far outside the solar system, and are not visible from the inner planets. What makes the "inner" comets different? Why do a select few from the Oort cloud periodically descend towards the sun and become visible to earthly observers? Read more in the section on comet orbits to find out why.