Why are planets not torn apart?

Question

There are two forces we know, Centripetal force (or Gravity) and Centrifugal force. Both are applied to all the planets including earth. Planets are kind of spherical due to gravitational force but also not perfectly round due to this centrifugal force (or any other force idk) applied.

I read a lil before asking this question here and found, centrifugal force (or whatever) is not really great on earth but obviously it's enough to cause bend to it. But, I found this about Altair

A significant percentage of stars in the sky rotate much faster and bulge noticeably at their equators. Like Altair is notable for spinning very, very rapidly and completes a full rotation on its axis every 10.4 earth hours. Accordingly, astronomers estimate that Altair is at least 14 percent wider at the equator than it is from pole to pole.

I'm sure there must be many stars and planets with greater centrifugal force on them like Altair. But why then none of the planet or star is torn apart till now ? I mean less or more a force is applied and that force is making an impact and huge impact maybe in many cases. Then why none of such planet is torn apart till now ?

If you say Gravity or centripetal force keeping it together and maybe canceling out other forces then why even this bend ? that means other forces have an impact (if i understood it correctly).

I'm not a physicist but have interest in physics. Would be thankful if someone can explain in simple words.

Answer 1

Then why none of such planet is torn apart till now ?

Any planets and stars that would have or could have rotated themselves into pieces have presumably already done so (which practically means that they would never have been formed in the first place.) What we see around us today is from a human time scale a more or less stable state of the universe.

Chances are that everything that inevitably happens to celestial bodies, already has happened a looong time ago or will happen a looong time from now. Our human lifetimes are simply too short, too negligible, for us to experience celestial changes. We are the blink of an eye in the astronomical thunder storm.

All the planets and stars that exist today with all the various rotations are thus presumably those that have managed since their creation to hold on to themselves. The centrifugal effects on them may have deformed them, but they have ended out with a new and strong enough shape to not be torn apart (a structure strong enough to overcome the centrifugal effect).

Since then, the centrifugal effect and gravity have counteracted each other for billions of years and will continue doing so into eternity, at least from a human perspective.

Those celestial objects that are very large while rotating very fast, may be closer to the limit of what their material structures can bear. Nevertheless they are just below the limit, because anything above the limit has already fallen apart a long time ago (or was never formed in the first place).

Answer 2

If I am understanding your questions correctly, just because the force of gravity and the centrifugal force are at equilibrium, it doesn't mean the shape has to be spherical. Think for instance of a water balloon flying through the vacuum that is also spinning. The internal force of the water trying to scape and the force of the rubber are at equilibrium but the shape of the balloon is definitely not spherical!

The point is, provided both forces balance, there is no reason to expect the body to burst into pieces. Of course if you were to add more mass without increasing the angular momentum of the planet then it would become more spherical as the force of gravity just became greater. On the contrary, if you make the planet spin faster without adding any additional mass you are solely increasing the centrifugal force making it look more like a disk.

A word of caution is in order. Your notion of bursting into pieces has to do with the fact that you correctly perceive a planet such as earth as a somewhat compact dirt sphere while the picture we were discussing doesn't take any of this into account. All we have talked so far would only apply for a rotating "ball of dust", which is of course unable to burst into anymore pieces!

Answer 3

The Yarkovsky–O'Keefe–Radzievskii–Paddack effect, or YORP effect for short, can make the rotation rate of small astronomical bodies increase over time. This increasing or decreasing rotation rate is a side effect of the Yarkovsky effect, in which solar radiation on the small body and thermal radiation by the small body can make the orbit of the small body very slowly spiral toward or away from Sun.

The asteroid 54509 YORP has been named after this effect. Radar observations show that the rotation rate of 54509 YORP is slowly increasing over time. Simulations show that the increasing rotation rate cannot be attributed to interactions with the Earth and with other planets. The increasing rotation rate is consistent with the YORP effect.

A good number of binary asteroids (pairs of asteroids that orbit one another) have been discovered. While inelastic collisions can account for some of these binary asteroids, the YORP effect is posited to be responsible for at least some of these binary asteroids: They tore themselves apart as their rotation rate increased.

Radiation pressure is proportional to cross sectional area, which is proportional to the square of an object's radius, while thermal radiation is proportional to mass, which is proportional to the cube of an object's radius. The square-cube law means that the Yarkovsky and YORP effects apply only to smallish bodies. What about larger bodies, such as stars and planets?

One of the key unresolved problems in star formation is the so-called angular momentum problem. As a protostar collapses its rotation rate increases due to conservation of angular momentum. The collapsing protostar will eventually reach a state where it can no longer gain mass unless there is a mechanism by which the collapsing protostar can shed angular momentum without losing mass. While a number of mechanisms have been proposed, the angular momentum problem remains unsolved.