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Question about obliquity and orbital planes





Ankhanu
I'm not really into astronomy or astrophysics, and I'm sure some Google work would answer the question, but I thought I'd try to get some discussion happening on Frih… things have been quiet here the past while.

I'm wondering if there is a relationship between planetary (or moon) axial tilt and orbital path? As far as I know, off the top of my head, most planets and moons rotate about axial planes that are relatively close to perpendicular (Within about 25deg) to their orbital planes, while there are some notable strong exceptions (i.e. Uranus, Pluto). Is obliquity related to orbital plane, or is the seeming tendency towards a right angle incidental?
kelseymh
Ankhanu wrote:
I'm not really into astronomy or astrophysics, and I'm sure some Google work would answer the question, but I thought I'd try to get some discussion happening on Frih… things have been quiet here the past while.

I'm wondering if there is a relationship between planetary (or moon) axial tilt and orbital path? As far as I know, off the top of my head, most planets and moons rotate about axial planes that are relatively close to perpendicular (Within about 25deg) to their orbital planes, while there are some notable strong exceptions (i.e. Uranus, Pluto). Is obliquity related to orbital plane, or is the seeming tendency towards a right angle incidental?


My understanding (and I'm a particle physicist, not an astrophysicist) is that the approximate zero obliquity is a simple consequence of conservation of angular momentum. Planets coalesce out of a debris disk which is orbiting around a protostar.

Since the disk is planar (duh Smile ), its angular momentum vector is normal to that plane. The planets which form necessarily also have their angular momentum vectors essentially normal to that plane. Later collisions, or tidal interactions, between the resulting large bodies can apply torques which cause the rotation axes to tilt.
Ankhanu
Sounds pretty reasonable to me, and fits with my limited understanding of physics at that sort of scale Smile
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