No. 50: Mar-Apr 1987
Some comets, such as Halley's, have periods of less than 200 years. Scientists have postulated that these comets, which orbit relatively close to the sun, originally came from the far-distant Oort Cloud on parabolic (non-returning) orbits around the sun. Perturbations by the planets, notably Jupiter, deflected them into the tighter orbits we see today. The problem is that the number of parabolic comets entering the inner solar system from the Oort Cloud of comets (located at the outermost fringes of the solar system) is 100 times too small to account for the existing population of short-period comets. M.E. Bailey believes this discrepancy can be removed if the Oort Cloud possesses a massive inner core of comets.
(Bailey, M.E.; "The Near-Parabolic Flux and the Origin of Short-Period Comets," Nature, 324:350, 1986.)
Reference. The Oort Cloud of comets is an entrenched part of astronomical dogma. For observations challenging its existence, see our catalog: The Sun and Solar System Debris. A description of this book may be found here.