[Paddy (Patrick)]

I've got a few queries about the Opik-Oort cloud hypothesis for long period comets.

The first is about how the orbit of these comets gets disrupted. A number of bits I've read suggest that "passing stars" may perturb the orbits of objects in the cloud. I might be a bit naive, but I don't imagine stars passing by very often or very close. I know the cloud is thought to be 1-2 light years out, but how often will another star come within gravitational range? Another suggestion is that the sun has a brown dwarf companion star that we don't know about. Fair enough that we might not see this, but if we can detect the wobble induced in other stars by planets, wouldn't we have detected a wobble in the sun produced by a dark companion star? Another hypothesis is that galactic tides may do the job. I assume that this means the change in gravity as we orbit the galaxy, as tides from passing galaxies would be pretty rare. This seems a reasonable thought, but wouldn't such a tide affect lots of these comets at once and since there are billions comets in the hypothesised cloud, wouldn't we get showers of them?

Also, since all of the other objects in the solar system are roughly in a plane, why is the Opik Oort cloud like a sphere?

Any thoughts appreciated.