planet inclinations and moons

[jnb]

I've just been reading a book on neurobiology which makes the comment that over long time scales without the influence of the moon the inclination if the Earth' s axis would alter chaotically making the development of higher life forms less probable.

Nothing wrong with that except that Earth is the only planet with a relatively large natural satellite so if that is the case we should expect to see a random range of inclinations but what we see is that with the exception of Uranus and Venus all the planet's have inclinations of about 25° (earth, mars, Saturn, Neptune) or 0° (mercury, Jupiter). Venus has an inclination of either 0° with a retrograde motion or 180° depending on how you look at it.

If the inclinations should be unstable why are they not more randomly distributed?er

[jnb]

Edit - that should be astrobiology not neurobiology (blame the autocorrect on my tablet)

[ronin]

Part of the question is What is chaotically?

It could mean a range of +- 20 degrees, in which case the planets satisfy it. We have a range of that.

I doubt that it means they will flip to a different orientation every few hundred years. Astronomical times are usually in millions of years and to change there would have to be a cause, quantum mechanics don't work on things the size of planets.

We do have a fair selection anyway, Venus is going the other direction as you say.

We have 4 at about 25, 2 at 0, Uranus at a plain weird 90ish and Venus going the wrong way.

In effect we have 0 -90 and one going the wrong way.

How much chaotic do you want?

[acey]

Chaotic changes in planetary obliquity (axial tilt) can occur due to orbital resonances, while tidal effects can enhance stability. To quote from Wikipedia:

All four of the innermost, rocky planets of the Solar System may have had large variations of their obliquity in the past. Like Earth, all of the rocky planets have a small precessional rotation of their spin axis. This rate varies due to, among other things, tidal dissipation and core-mantle interaction. When each planet reaches certain values of precession, orbital resonances may cause very large, chaotic changes in obliquity. Mercury and Venus have most likely been stabilized by the tidal dissipation of the Sun. The Earth was stabilized by the Moon, as above, but before its capture, the Earth, too, could have passed through times of instability. Mars' obliquity is currently in a chaotic state; it varies as much as 0° to 60° over some millions of years, depending on perturbations of the planets.^[16][26] The obliquities of the outer planets are considered relatively stable. Some authors dispute that Mars' obliquity is chaotic, and show that tidal dissipation and viscous core-mantle coupling are adequate for it to have reached a fully damped state, similar to Mercury and Venus.^[2][27]

http://en.wikipedia.org/wiki/Axial_tilt

[Tiki]

If the inclinations should be unstable why are they not more randomly distributed?er

They could indeed be randomly distributed and the 'apparent grouping' could just be down to chance. It could also be along the lines of an 'Order from chaos' type of process. Eg. If you drop grains of sand one at a time onto the same spot the trajectories of  each individual grain will be random; the uniform conical heap formed by the many will have quite a precise form though.

[jnb]

Part of the question is What is chaotically?

It could mean a range of +- 20 degrees, in which case the planets satisfy it. We have a range of that.

I doubt that it means they will flip to a different orientation every few hundred years. Astronomical times are usually in millions of years and to change there would have to be a cause, quantum mechanics don't work on things the size of planets.

We do have a fair selection anyway, Venus is going the other direction as you say.

We have 4 at about 25, 2 at 0, Uranus at a plain weird 90ish and Venus going the wrong way.

In effect we have 0 -90 and one going the wrong way.

How much chaotic do you want?

If chaotically means +/- 20 degrees then different mechanisms are required to explain the inclination of Venus and Uranus so we require two different mechanisms to explain inclinations and a third mechanism to restrict the range of inclinations. If Venus and Neptune are included then there are no limits on the range which brings it back to being improbable that chance alone would have 6 out of 8 planets so closely aligned. However having read further on it I've found a number of references that suggest that the sun's influence alone is sufficient to make inclinations of the inner planets stable in the long term. The moon stabilises the Earth's inclination but that does not mean there is no stabilising influence on other planets.