No. 69: May-Jun 1990
A key feature of our solar system is the Oort Cloud of comets that surrounds the sun and its family of planets. No one has yet seen the Oort Cloud directly, but the textbooks say that it must be there. In fact, all stars like our sun with planetary systems should have their own private Oort Clouds of comets - if the prevailing theory of planetary-system formation is correct. When we see a comet looping around the sun, it is because it has been jostled loose from the Oort Cloud by a passing star or molecular cloud. Further, some of these jostled comets should be kicked outwards and thus escape the solar system. Continuing with this reasoning, we on earth should sometimes see interstellar comets that have been shaken loose from other stellar systems. But we don't! T.A. McGlynn and R.D. Chapman worry about this.
"This lack of detections of extrasolar comets is becoming an embarrassment to the theories of solar system and comet formation."
McGlynn and Chapman calculate that we should have seen six interstellar comets in the past 150 years, but the actual number is zero. Such interstellar comets would be easy to spot because they would be moving much faster than our own comets. Two possible explanations for the missing interstellar comets are: (1) The Oort Cloud theory is wrong; and (2) Solar systems like ours are rarer than supposed.
(Anonymous; "Mystery of the Missing Comets," Sky and Telescope, 79:254, 1990.)
Comment. See SF#64 for musings about Halley's comet being an alien interloper.
Reference. More on missing short- period comets can be found in ACO6 in our catalog: The Sun and Solar System Debris. Ordering details here.