What is the cause of planetary rotation. Not orbital rotation

Question

I am a senior in high school, sixteen years old. I think differently than most in my class. Recently, I have been thinking about alternative causes for the rotation of our planets. My current thought on the matter has to do with the shape of the planets as well as their apparent orbits. All planets are ellipsoids, meaning that it is not perfectly circular, all planets, to my knowledge, travel in elliptical orbits. Is it possible that the rotation of a planet is caused by an uneven force caused by the differences in distance from opposite points on the planet and the sun? Now from this, there is a problem, yet it has an easy solution. The force pulling the one side of the planet would cause a constant acceleration, however this would only be the case if the planets were perfectly spherical. Because the planets are ellipsoids, it would continue to accelerate until the planet reaches its perihelion, at this time, the closest point on the planet would be on the opposite side, the resulting acceleration would be negative, and would slow down the planetary rotation, keeping it somewhat constant. The factors that may affect this would be the planetary mass, eccentricity of the planet, as well as distance from the sun. Some professional help would be appreciated, I would like to hear the feedback on this along with some constructive argument.

Answer 1

It is great that you "think differently" about problems - that is at the heart of all innovation. When it comes to the rotation of planets, you have to go back to the origins of the solar system:

Planets are formed by accretion: a large cloud of debris starts to experience some gravitational pull, and as one "lump" becomes bigger than the others, it starts to pull the other lumps towards it. However, all those lumps were initially part of a large dust cloud that itself was encircling the (proto)sun - so they have some angular momentum. In general, for particles to be in a stable circular orbit at a distance $r$, where they experience a force of gravity proportional to $1/r^2$, they need a velocity proportional to $1/\sqrt{r}$ so the angular momentum will scale with $\sqrt{r}$. That means that particles that are attracted to the "lump" from a larger orbit will tend to speed up as they approach, and hit the lump on the leading edge; while the ones that come from a lower orbit will hit the trailing edge. Either way, they end up transferring their angular momentum to the lump, since angular momentum must be conserved in the system.

So planets that are formed from a debris cloud that was circling the sun must acquire rotation during their formation - and because the energy associated with that angular momentum is HUGE, it takes a long time for things like tidal forces (which you are describing in a roundabout way in your question) to slow that rotation down.

The fact that this slowing down can happen is seen in the moon - it's relatively close to the earth and so experiences a very strong tidal drag, and this has resulted in the moon slowing its rotation until the same side is always facing us. It hasn't stopped rotating - but it revolves about its axis at the same rate as it orbits the earth.

Answer 2

Recently, I have been thinking about alternative causes for the rotation of our planets.

You're sixteen. I've noticed that while people of your age can understand Newton's first law of motion, they don't understand the rotational analog of this law. Just as an external force is needed to change an object's momentum, an external torque is needed to change an object's angular momentum. Without that external torque, a rotating object will continue rotating forever.

The Earth's current rotation is a combination of it's primordial rotation, modified to some extent by external torques exerted on the Earth by the Moon, the Sun, and the planets. The primordial rotation of a terrestrial planet is mostly a consequence of the last big thing that whacked the protoplanet while it was forming. In the case of the Earth, that "last big whack" was most likely a Mar-sized object that collided obliquely with the proto-Earth. Some of that Mars-sized object stuck to the Earth, but other parts didn't. Most of those other parts eventually collided and formed the Earth's Moon. The parts that stuck? A lot of that sank to the center of the Earth. The Earth has an oversized core thanks to that collision, while the Moon has an undersized core.