There is strong circumstantial evidence for the existence of the Oort cloud. It is defined to be a spherical shell of cometlike bodies from about 50,000 AU to 80,000 AU radial distance from the sun. The Oort cloud is gravitationally bound to the sun and is my choice (at its outer limits) for the boundary of the solar system until some solar gravationally bound body at a further distance is discovered (or hypothesized).
Currently there seems to be a tendency to use the heliopause (at 90 to 100 AU) as the boundary.
Has anyone seen an "authorized" definition of the size of the solar system and do you know which organization has the appropriate credentials to so authorize? h34r: