The Oort Cloud is believed to be the source of long-period comets in the Solar System . Although no confirmed direct observations of the Oort cloud have been made , astronomers believe that it is the source of all long-period and Halley-type comets entering the inner Solar System and many of the Centaurs and Jupiter-family comets as well . The outer Oort cloud is only loosely bound to the Solar System , and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way Galaxy itself .