WHY DO PLANETS ROTATE IN DIFFERENT SPEEDS.
We are all aware that planets and other celestial objects within our solar system revolve around the sun. The paths on which they traverse is generally called an orbit and the time taken to complete one such orbit is called one solar year. As you can imagine, solar years for different bodies are variable and depend on two most important factors, namely its distance from the sun and the shape of the orbit it takes while orbiting.
However, if you look only at the planets and satellites that exist within the solar system, you start to notice a peculiar pattern. In addition to revolving around the sun, they also have what is known as a rotation wherein they spin upon their own axes.
The time it takes for one celestial body to spin once on its axis is known as one solar day. The speed of spin, as was with revolution, is variable with each astronomical body and depends on the mass of the body, its axial tilt and angular momentum.
But how do these planets and satellites get their spin in the first place?
To understand that, we have to take a look at the formation of our solar system. When a star forms, it is essentially through a very violent chemical reaction wherein matter falls onto itself to create heavier ions, thus enabling stronger reactions. During this time, to attempt stabilization, matter starts bulging out from the spherical gaseous form of the sun.
When the bulge gets big enough, due to angular momentum, it is ejected out into the vicinity wherein the freshly ejected matter keeps spinning at the ejected angular momentum as per the laws of conservation of energy.
As time passes and the ejected matter cools down, it starts forming a colder crust on top while still spinning at the ejected angular momentum. Since space does not have friction as a medium as you’d have with air, in an ideal situation, the planet will keep rotating at the exact speed. But that is not always the case.
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The universe is packed with billions of star systems just like our own solar system. Also, not everything ejected out of a forming star will transform into a planet or satellite. Some of it, due to the lack of mass or gravitation, end up becoming stray space rocks that we call asteroids. They too revolve around the sun. Asteroids, owing to their weird shapes and light mass, often tend to have highly elliptical, elongated orbits around the sun and take thousands of years to complete one revolution.
The spin of the planet, as discussed earlier, is not fixed. It can be tampered with by external forces that can slow it down or even speed it up. It often is the case that planets when young collide with mammoth asteroids that directly influence its rate of spin. It is also directly responsible for the axial tilt a lot of planets exhibit, thus allowing seasons.
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It is believed that our moon too was a product of such a collision. In the early days of the solar system when the sun was still very reactive and was constantly ejecting out material into space, a gigantic asteroid either created from the sun or travelling from outer space, grazed past the earth thus giving it its tilt of 23.5 degrees and enabling seasons to flourish.
It is also believed that during this collision, a large chunk of matter was ejected from the earth that went on to form the moon. This hypothesis is backed by the fact that the moon is very close to earth and the surface of the moon is identical to that of the earth.
It is also believed that the basic building blocks of life like water and hydrocarbons were brought to earth by one such asteroid that then later flourished in the perfect temperature the earth provided and evolved into the human species. If we think about it, then we are the aliens we were looking for!
If you consider the moon, it is tidally locked with the earth which means that the moon only ever shows one face to the earth because of the earth’s gravitational field. The moon is essential to us because tides are a direct result of the moon’s gravitational pull on us. However, in due time, the gravitational pull of the earth will weaken enough for the moon to not tidally lock itself to the earth anymore and it will revolve around our planet while rotating in its own axis.
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Fast forward another five billion years and the sun will enter into its expansive red giant phase (after having exhausted fusion in the core and starting to fuse in the hydrogen shell surrounding the core leading to increased pressure, density, heat and size) and the gravitational pull of the sun will finally destroy the moon into bits of cosmic material that will start revolving around the earth thus forming a ring.
But back to our understanding of the spin, let us consider a practical example that demonstrates how spin in a planet really works. If you ever play pool, you will recollect that sometimes when you hit a ball from the side, instead of it going where it was intended to, it starts spinning along its own axis as it moves. That is due to angular momentum and that is exactly what happens with planets when they first form. The ball on the table, you’ll recollect, stops after a while.
But that is because of the felt cloth that the pool table is lined with which interacts with the ball creating friction making it stop. In space, there is no medium and therefore no such natural friction. If you imagine the ball now in space, it will keep spinning until acted upon by an external force as proved by Newton in his law of inertia. The planets are much alike. They keep spinning in the same speed until a collision either slows them down or speeds them up.
But why do planets rotate in different speeds?
Planets rotate in different speeds depending on the time they were formed and the interactions they’ve had with other astronomical bodies. But there is no pattern yet that has been discovered to predict the speed of an impending planet because planet formation is highly chaotic. Speeds of rotation of planets are generally calculated directly from telescopic observations.
Related questions.
How is the rotational speed of a planet calculated?
Rotational speeds of planets cannot be calculated/predicted because planet formation seems to be highly chaotic. The spin of planets (both rocky and gas) is determined by many factors, including:
The angular momentum of the material which was accreted on the planet,
Gravitational interactions with other planets,
The history of collisions as the planet formed.
Tidal interactions with the host star (if the planet is close in) and the gaseous and debris disks while the planet was forming.
In the solar system, for example, Mercury is in a 3:2 spin-orbit resonance — so it completes 3 rotations every 2 orbits. The spin periods of Earth and Mars, however are almost identical despite different masses and semi-major axes. Finally, Uranus has a shorter rotational period than earth — but is tilted almost 90degrees relative to the orbital plane.
What do you mean by angular momentum?
The angular momentum of an object is defined as the product of the moment of inertia and the angular velocity. It is analogous to linear momentum and is subject to the fundamental constraints of the conservation of angular momentum principle if there is no external torque on the object.
