Any sensible way of averaging the planes would be dominated by the Sun anyway.
It would be reasonable to think that is you have a single reference point but looking at the rotations of the planets within our solar system (including the retrograde Venus and the 90degree flip of Uranus) it’s looks to be a case of luck and circumstance being the determinants.
It’s pretty wonderful how many things in our solar system (with some exceptions) are spinning in the same direction, basically. I know for sure:
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The planets all circle the sun in approximately the same direction. If we use the plane of the orbit of the earth around the sun as a reference (it’s called the ecliptic), then most other planets are pretty close to that plane, off by only a few degrees. You could imagine the solar system as a model and put your right hand out, thumb up and fingers curving in the direction the planets circle the sun, and define a “north” for the solar system. It’s the direction your thumb points.
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The earth itself rotates in the same direction (approximately) about its own axis as its orbit of the sun, but that axis is tilted by 23.5 degrees, which gives rise to seasons on earth.
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Meanwhile, yep, the moon goes around the earth which way? Yes, following the right-hand rule, it orbits around the earth once again the same direction as earth rotates about its own axis and about the sun (again, approximately, not exactly).
I believe the reason why all these rotations, quantified by “angular momentum” have the same axis and direction (the same direction your thumb is pointing) is because of what Darren said above, because it arises from the initial conditions of formation of the solar system (the gas cloud collapse). It stands to reason that the sun would also rotate in the same direction.
Mostly by Jupiter, actually. The most sensible way of averaging the rotation would be to take the total angular momentum, which depends both on mass and on how far that mass is from the center. The Sun has significantly more mass than Jupiter, but Jupiter is far enough out to more than overwhelm that mass advantage.
Yeah. I was thinking of the overwhelming mass and forgot to take into consideration how wast the solar system is.
Which still leaves the question of how the axis of the total angular momentum of the planets compares to the axis of rotation of the Sun. And if they are significantly different, why?
Late add after some reading …
Here’s wiki on point:
I think there’s a problem with their table in that it displays all the relative inclinations as positive numbers. Surely some should be negative; after all every planet’s orbit cannot be a positive offset from the average of all the planets’ orbits. Although I’m not aware of any well-defined convention to define positive versus negative inclinations.
Nevertheless, this info doesn’t give any good explanation for why the Sun’s equator is offset from the invariable plane by very roughly 5 degrees.
The fact that the Sun is not aligned with the plane of the solar system is not particularly unusual. Now that there is a lot of data on exoplanets this can be tested. Many stars have rotations which are not aligned with the orbits of their planets; this happens particularly often with hot-Jupiter type planets, or if the system is binary. Our system is a particularly well-behaved system as far as spin-orbit alignments and eccentricity are concerned, and this might make it more hospitable to life.
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Inclination cannot be negative: it is just an angle between two planes.
Here is a file with some data. I haven’t double-checked it, but you can read off the longitude of the ascending node of various planets:
Table 2a.
Keplerian elements and their rates, with respect to the mean ecliptic and equinox of J2000,
valid for the time-interval 3000 BC -- 3000 AD. NOTE: the computation of M for Jupiter through
Pluto *must* be augmented by the additional terms given in Table 2b (below).
a e I L long.peri. long.node.
AU, AU/Cy rad, rad/Cy deg, deg/Cy deg, deg/Cy deg, deg/Cy deg, deg/Cy
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Mercury 0.38709843 0.20563661 7.00559432 252.25166724 77.45771895 48.33961819
0.00000000 0.00002123 -0.00590158 149472.67486623 0.15940013 -0.12214182
Venus 0.72332102 0.00676399 3.39777545 181.97970850 131.76755713 76.67261496
-0.00000026 -0.00005107 0.00043494 58517.81560260 0.05679648 -0.27274174
EM Bary 1.00000018 0.01673163 -0.00054346 100.46691572 102.93005885 -5.11260389
-0.00000003 -0.00003661 -0.01337178 35999.37306329 0.31795260 -0.24123856
Mars 1.52371243 0.09336511 1.85181869 -4.56813164 -23.91744784 49.71320984
0.00000097 0.00009149 -0.00724757 19140.29934243 0.45223625 -0.26852431
Jupiter 5.20248019 0.04853590 1.29861416 34.33479152 14.27495244 100.29282654
-0.00002864 0.00018026 -0.00322699 3034.90371757 0.18199196 0.13024619
Saturn 9.54149883 0.05550825 2.49424102 50.07571329 92.86136063 113.63998702
-0.00003065 -0.00032044 0.00451969 1222.11494724 0.54179478 -0.25015002
Uranus 19.18797948 0.04685740 0.77298127 314.20276625 172.43404441 73.96250215
-0.00020455 -0.00001550 -0.00180155 428.49512595 0.09266985 0.05739699
Neptune 30.06952752 0.00895439 1.77005520 304.22289287 46.68158724 131.78635853
0.00006447 0.00000818 0.00022400 218.46515314 0.01009938 -0.00606302
Pluto 39.48686035 0.24885238 17.14104260 238.96535011 224.09702598 110.30167986
0.00449751 0.00006016 0.00000501 145.18042903 -0.00968827 -0.00809981
NB those elements are with respect to the mean ecliptic plane
Thank you. Longitude of ascending node would be a logical way to define positive and negative inclination versus a mean. e.g. ascending node in eastern hemicircle is positive, ascending node in western hemicircle is negative. Again this is arbitrary, but distinguishes between the case where two orbits are coplanar and offset from the reference plane versus being in opposition by the same magnitude versus the reference plane. Assuming appropriate planet sizes and orbital radii, the former case would be mostly additive and the later would be mostly offsetting.
At least right now before the nodes precess however they will.
With the Grand Tack hypothesis having our Jupiter carom around the inner Solar system like a billiard ball, I could see this happening.
And then you’d have two planets with ascending nodes one just barely on the east side of your arbitrary dividing line, and one just barely to the west of the line, and so they’d get assigned inclinations of, say, ±5º, and it’d look like they were 10º apart instead of being nearly coplanar. Much simpler to just always call inclination positive, and use another number to indicate where the ascending node is.
Yeah. The more I chew on it the less sensible my earlier thinking appears.
Ultimately the calculation of the invariant plane takes into account both the absolute amount of inclination of each planets’ orbit and the orientation of that orbit. The output is another plane with an inclination and an orientation.
We forgot to address this part of the original question.
No, the Sun does not spin along the same plane as it’s own revolution around the galaxy. The Sun spins about 60 degrees away from that angle, and every star in the sky makes a different angle with the galactic plane. As far as I am aware there are no significant alignments between stellar spin and the galactic plane.
Thanks. I was about to bring that up.