Opik-Ooort cloud questions

[Paddy]

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.

[citivolus]

My understanding is as follows. Due to the distance involved and the fact that they are not all disturbed in exactly the same way due to having separate initial velocities, there would be considerable difference in the impact on any two bodies by a given force. It would take a very long time for an object at the distance of the Oort cloud to reach the Sun (up to and including millions of years), giving these slight variations in motion time to make huge differences in orbit and arrival time.

As for the shape being a sphere rather than a plane, that would be due to the diminishing impact of the Sun's gravity on the objects as distance is increased. The closer objects in the cloud would like closer to the plane of the solar system, while further objects would be more spread out due to the higher ratio of gravity from extra-solar objects impacting their orbits. It is postulated that all the objects originated in the solar plane, but that the outer ones have migrated in their orbits over time due to those extra-solar forces. The overall shape of the positional probability of the objects would probably look more like two balloons pressed together inside another balloon than a sphere.

As for detecting a companion star at a distance of 25,000-50,000 AU, the extremely long orbital period would make the wobble imperceptible to an observer sitting inside the system riding around on a planet which is itself spinning and experiencing precession.

Regards,
Eric

[bojan]

The disc-like shape of the cloud closer to the Sun from which the planets are formed is the consequence of the preservation of orbital momentum of the primordial cloud as a whole.. After collapse due to gravity, the material has a tendency to concentrate in the disk simply because the momentum of the whole cloud is preserved this way..
The Oort cloud is not collapsed so it has its original shape, that is very roughly spherical. The perturbation of the nearby stars as well as tidal forces of the Milky way spreads it out additionally.

[Paddy]

Thanks gentlemen for your responses - very interesting. Your answers on the lack of detectable solar wobble if the sun has a brown dwarf partner makes sense, as does the long time frame for preventing showers of comets. But there seem to be 2 opposite views on the shape of the cloud here. One is that the solar system starts as a plane and other gravitational influences turn the Oort cloud into something else, the other that we start with a sphere and it becomes a disc due to gravity which leaves the Oort cloud alone. I like both! I'm still curious about people's thoughts on passing stars and gravitational tides.

I find this quite a fascinating area, contemplating such far flung regions of the solar system and I also enjoy the uncertainty of it all. I hope that there might be some more discussion.

[bojan]

First it is a (roughly) spherical or irregular-shaped cloud which collapses and turns into a more or less flat disc.. which then collapses further into a central star and planets orbiting in a plane..
So there is no contradiction here :-)

[sheeny]

A spherical (or indeed random) shaped cloud with an initial angular velocity will always collapse to a disc. Gravity tries to collapse the cloud to a point, but the angular momentum must be preserved. So gravity builds the star, but the angular momentum prevents some material falling into the star, forming a disc where the centripetal force from the angular momentum of the material matches the gravitation field strength. So the disc always forms perpendicular to the axis of the initial spin.

Al.

[Paddy]

Would this be a correct understanding of the process? Things in a rotating spherical distribution can move towards the centre (or central disc) in the direction parallel to the axis of spin as there is no force opposing the mutual gravitational attraction, but the orbital momentum of some objects will be enough to stop them collapsing towards the axis of rotation of the sphere - these end up forming the disc. For some it will not and these will become part of the star. Others are so far out that the mutual gravitational attraction is not strong enough to pull them into the disc or may compete with gravity from objects outside the solar system and these form the Oort cloud.

[bojan]

This is roughly how I understand this process of star & planetary system formation.. and existence of Oort cloud.
Except that initial collapse is not parallel to the axis of spin, but perpendicular to it.. until the central star is formed with spin corresponding to the overall rot momentum of the material that collapsed.
Planets are formed around lumps in the disk, and again those "sub-disks" are formed for the same reasons, in roughly the same plane (ecliptic)

[Paddy]

Thanks for the replies! I think I understand a bit more. I am still quite intrigued by the ideas of passing stars, hidden brown dwarfs and gravitational tides.