Why do planets have an axial tilt? From the above image we can see that each planet's axial tilt angle varies and differs from the others.
What was the cause of this, was this from the beginning of our solar systems birth? Or did it happen later?
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1$\begingroup$ I think it would be weirder if they weren’t all different; just like microscopic dipoles align in the presence of an electric field, if the planets all aligned with their axises, I would think something weird was going on. This subject is an extremely active one and theories about solar system formation change practically on the monthly, so take any answer with a grain of salt given that context. $\endgroup$– Justin TOct 6, 2021 at 4:18
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3 Answers
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In the early stages of the formation of the solar system, planetesimals start condensing and everything rotates with angular momentum inherited from the collapsing cloud of gas and dust, so the planetesimals all have their orbit and spin axes closely aligned with that of the proto-Sun. And while a planetesimal continues to grow by attracting nearby material its spin orientation isn't affected very much.
But eventually the planetesimals get large enough that they start attracting one another, and collisions occur. These collisions can disturb the spin axis of a planetesimal, especially if the collision is off-centre, and if the two bodies have roughly similar momentum.
Big planetesimals grow faster than small ones, and are less likely to have their spin axis disturbed. So proto-Jupiter manages to capture most of the matter that doesn't end up in the proto-Sun, and suffers little disruption to its spin axis. The smaller planets aren't so lucky.
That's the rough picture, and it misses a lot of details. Some of the details are still unclear, and more work is needed to clarify them.
The biggest mystery is Uranus. We don't know why Uranus is rolling around on its side, but we presume that was caused by some interaction fairly late in its history because its major moons orbit in its equatorial plane, pretty much. One theory is that it suffered collateral damage in whatever event upset Saturn's spin axis. Note that Saturn's rings and large moons orbit in Saturn's equatorial plane.
Mercury has a short orbital period and hence a high orbital speed. It's tidally locked to the Sun, so that it spins 3 times on its axis in 2 revolutions around the Sun; that tidal locking also locks it's spin axis fairly closely to its revolution axis.
Venus is also a bit of a mystery. It was formerly believed to be tidally locked, with its year equal to its day, but actually its rotational period is about 243 (Earth) days, about 20 days longer than its year, and it's spinning backwards! We don't know why, but suspect it may be due to a strong coupling between it's surface and very dense atmosphere. Also note that Venus experienced planet-wide severe tectonic activity not so long ago (on an astronomical timescale), so its surface is relatively new, and a lot of the planet's angular momentum may have been transferred to its atmosphere.
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$\begingroup$ Angles don't seem to be terribly quantized, so there's a lot of possibilities. Next 10km rock that slams into mars will likely change its axial tilt a bit. $\endgroup$ Jul 21, 2019 at 0:39
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$\begingroup$ @apk You could say that. The modern convention is to say that it has prograde rotation with an axial tilt of 97.77°. But that's equivalent to retrograde rotation with a tilt of 82.23°. The main thing is that its axis is tilted at almost a right angle to its orbital plane (which is inclined ~1° to the solar system's invariable plane) $\endgroup$– PM 2RingAug 25, 2022 at 7:54
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You could also ask:
Why is each planet a different size.
or
Why is each planet a different colour.
or even
Why are the apples in my fruit bowl pointing in different directions.
The reason is that, unless I make the effort to point the apples in the same direction they will point in random directions.
The planets were formed independently, (though at about the same time) and the environment of each planet, was slightly different. After their initial formation, they have been subject to different histories of perturbation and collision.
The overall effect is that the axial tilt of each planet is essentially a random number (though not uniformly distributed). If you choose 8 random numbers you would be surprised if two came out exactly the same.
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$\begingroup$ is the tiltation not governed by a specific law? $\endgroup$– ParanJun 8, 2018 at 21:22
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$\begingroup$ "the axial tilt of each planet is essentially a random number" $\endgroup$– James KJun 8, 2018 at 21:27
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3$\begingroup$ I voted you up for the apples in my fruit bowl example. @ParanBharali It's not precisely random. The protoplanetary disk tends to be reasonably flat and the initial rotation tends to be, somewhat, aligned with that. However, large bodys form, they can interact or collide and that can change the axis significantly. Gravitational interactions may be a factor too. It's not precisely random, it's more often somewhat close to perpendicular, but there is significant randomness to it. $\endgroup$– userLTKJun 8, 2018 at 22:24
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My hypothesis is that the reason the planets have axial tilt is the topology of the spacetime around a sun. Have you ever tried pushing a heavy shopping troll while traversing a steep slope? You have to rotate the trolly off 0° from the direction to want to travel, to an angle that is directly proportional to the weight of the trolley and the angle of the slope. In order to continue to traverse the slope with the trolley in a straight line.
I think that is what’s going on with planetary axial tilt.
Our Earth seems to oscillate it’s angular tilt from 21° to 24° over some period of time. This isolation could be caused by the sun‘s slight wobble or spiral in its forward moving path.
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1$\begingroup$ General relativity is good for a lot scenarios, but this one is purely classically explained; the Earth precesses because of the combined interactions of the sun and the moon on the Earth’s bulge, not because of relativistic effects $\endgroup$– Justin TOct 6, 2021 at 4:13
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$\begingroup$ Are you sure that’s it? What you say seems to be the general consensus on Wikipedia. I mean if you take the oceans off earth it looks pretty asymmetrical. Also the moon orbiting will have some effect. in my shopping trolly analogy, the bulge and the moon would just be the way the shopping trolly was weighted. The axial tilt would still be essentially caused by curvature of space or at least the suns dragging effect on the earth as the sun moves along its path around the galaxy. I could definitely be wrong. I’m just exploring this idea to see if it holds up. $\endgroup$ Oct 7, 2021 at 5:28
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$\begingroup$ I guess you could say objects in motion tend to stay in motion until another force is applied to them. All non vector motion tends to spiral, spin and or rotate in some way. $\endgroup$ Oct 7, 2021 at 5:29