enter image description here

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?


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.

  • $\begingroup$ 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 '19 at 0:39

You could also ask:

Why is each planet a different size.


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.

  • $\begingroup$ is the tiltation not governed by a specific law? $\endgroup$ Jun 8 '18 at 21:22
  • $\begingroup$ "the axial tilt of each planet is essentially a random number" $\endgroup$
    – James K
    Jun 8 '18 at 21:27
  • 2
    $\begingroup$ 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. $\endgroup$
    – userLTK
    Jun 8 '18 at 22:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.