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Originally Posted by umop ap!sdn
Not to hijack the thread or anything
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Originally Posted by John Kierein
I've always wondered if there's a gravitational reason for this and if it is true for other planetary systems.
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(I wasn't paying attention and didn't notice at first that you were the OP. #-o )
But having completed a survey of binary stars with known orbit planes, I did find something interesting. I measured the apparent declination of the galactic center (GC) as seen from each star, referenced to the plane of its companion's orbit. I also took into account the obliquity of the orbit to the galactic plane.
If my math is correct (and please do correct me if I've made any errors) then in a perfectly random distribution one would expect a mean obliquity of 90° / sqrt(2), and a mean declination equal to the mean obliquity / sqrt(2).
In fact, I found a mean obliquity of 57.46° which is a little over 90% of the expected figure. So there's a slight tendency for stellar orbits to want to lay flat against the galactic plane. The mean declination of the GC is only 71% of the expected figure, meaning the orbits have a considerably greater tendency to line up to the GC as you describe. Furthermore, when each individual star system's apparent GC declination is divided by its obliquity, the resulting mean is higher - closer to 74%.
The difference between the 71% and 74% figures can be explained if stars whose orbit planes lie closer to the plane of the galaxy have less tendency to line up with the GC than stars whose planes have a high obliquity.
Here is a complete MS Excel spreadsheet of the stars I surveyed and resulting figures.