Comet Reserviors

Think about the orbits of comets: they live on highly eccentric orbits in the solar system -- that's why they are comets! (What would they be if they didn't live on highly eccentric orbits?)

The orbits (and thus periods) of comets often change significantly. Why?

They come in two types:

The orbits of comets suggest two reservoirs for comets: the Oort Cloud and the Kuiper Belt.

The Oort Cloud

Based on the orbits of long period comets, in 1950 Jan Oort proposed that a distance, spherical distribution of cometary nuclei surrounding the solar system. Distance: 3,000 - 100,000 AU (Pluto=30 AU, nearest star=275,000 AU)
Number of Comets: > 1012
Mass: ~ 10 -25 Mearth (or bigger!)
  The Oort Cloud has never been seen -- objects are a million times fainter than the faintest things we can see even with big telescopes.

The Kuiper Belt

The Oort cloud cannot explain the origin of the short-period comets. There must also be a more nearby, flattened distribution of comet nuclei -- the Kuiper Belt.

(A good Kuiper Belt information page by Dave Jewitt)

Hypothesized in 1951, first object found by Jewitt and Luu in 1992

Now there are many Kuiper Belt objects known. The biggest one (to date) is the recently discovered 2003 UB313 (aka Xena and Gabrielle), slightly bigger than Pluto.

Are these planets?

Unlike objects in the Oort Cloud, Kuiper Belt objects most like formed in place. Their history is one of simple formation conditions rather than dynamical evolution.

Interactions with the outer planets shape the orbits of short-period comets.