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For these reasons, astronomers realized that the supply of comets would require replenishment over the supposed great age of the solar system. Comets cannot be interstellar, else they would have hyperbolic orbits. Some astronomers have proposed cryovulcanism as the source of comets, but comets are not sufficiently dissimilar in composition to support this.<ref name=Faulkner1/>

For these reasons, astronomers realized that the supply of comets would require replenishment over the supposed great age of the solar system. Comets cannot be interstellar, else they would have hyperbolic orbits. Some astronomers have proposed cryovulcanism as the source of comets, but comets are not sufficiently dissimilar in composition to support this.<ref name=Faulkner1/>

In 1950, astronomer [[Jan Oort]] first proposed the existence of a "cloud of comets" in the outer fringes of the solar system.<ref name=Sonoma>Tenn, Joseph S. "[http://www.phys-astro.sonoma.edu/BruceMedalists/Oort/ The Bruce Medalists: Jan Hendrik Oort]." Department of Physics and Astronomy, Sonoma State University, October 11, 2006. Accessed June 2, 2008.</ref><ref name=Oort>Oort, J.H., 1950. The structure of the cometary cloud surrounding the solar system and a hypothesis concerning its origin. Bulletin of Astronomy of the Netherlands, 11:91–110.</ref> Oort calculated that this cloud might contain a trillion comets.<ref name=Arnett/> Short-period comets are supposed to come from the [[Kuiper belt]].<ref name=Weisstein/><ref name=Faulkner1/><ref name=Arnett/><ref name=Harvey>Harvey, Samantha. "[http://solarsystem.nasa.gov/planets/profile.cfm?Object=OortCloud&Display=OverviewLong Solar System Exploration: Oort Cloud]." [[National Aeronautics and Space Administration|NASA]], March 29, 2006. Accessed June 2, 2008.</ref> Oort originally speculated that the total mass of the Oort cloud would be as much as the mass of [[Jupiter]]. More recent studies have suggested that the total mass might be forty times the mass of the earth.<ref name=Faulkner2/>

In 1950, astronomer [[Jan Oort]] first proposed the existence of a "cloud of comets" in the outer fringes of the solar system.<ref name=Sonoma>Tenn, Joseph S. "[http://www.phys-astro.sonoma.edu/BruceMedalists/Oort/ The Bruce Medalists: Jan Hendrik Oort]." Department of Physics and Astronomy, Sonoma State University, October 11, 2006. Accessed June 2, 2008.</ref><ref name=Oort>Oort, J.H., 1950. The structure of the cometary cloud surrounding the solar system and a hypothesis concerning its origin. Bulletin of Astronomy of the Netherlands, 11:91–110.</ref> Oort calculated that this cloud might contain a trillion comets.<ref name=Arnett/> Short-period comets are supposed to come from the [[Kuiper Belt]].<ref name=Weisstein/><ref name=Faulkner1/><ref name=Arnett/><ref name=Harvey>Harvey, Samantha. "[http://solarsystem.nasa.gov/planets/profile.cfm?Object=OortCloud&Display=OverviewLong Solar System Exploration: Oort Cloud]." [[National Aeronautics and Space Administration|NASA]], March 29, 2006. Accessed June 2, 2008.</ref> Oort originally speculated that the total mass of the Oort cloud would be as much as the mass of [[Jupiter]]. More recent studies have suggested that the total mass might be forty times the mass of the earth.<ref name=Faulkner2/>

== Problems with the Oort Cloud Hypothesis ==

== Problems with the Oort Cloud Hypothesis ==