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.

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    $\begingroup$ "Then why none of such planet is torn apart till now ?" My guess is that any planets and stars that would have rotated themselves into pieces have already done so $\endgroup$ – Steeven Oct 15 '20 at 21:27
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    $\begingroup$ ... or would never have formed in the first place. $\endgroup$ – Prof. Legolasov Oct 15 '20 at 21:28
  • $\begingroup$ Is an answer saying "the centrifugal force is not strong enough in the cases where things are not torn apart" good enough? $\endgroup$ – BioPhysicist Oct 15 '20 at 21:29
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    $\begingroup$ @RaoHammas that's not how forces work. The ground is acting on you with a normal force all the time, yet you're still intact. $\endgroup$ – Prof. Legolasov Oct 15 '20 at 21:37
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    $\begingroup$ @RaoHammas The centrifugal effect may have been acting over a long time. But gravity also acts over a long time. None of these two forces "wear out". They keep up and continue pulling in opposite directions, practically forever. Therefore it doesn't matter how long time you give it - the centrifugal effect will not suddenly "win" over gravity. $\endgroup$ – Steeven Oct 15 '20 at 21:38

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).

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    $\begingroup$ They wouldn't have formed in the first place unless their gravity was sufficient centripetal force. $\endgroup$ – Acccumulation Oct 15 '20 at 21:38
  • $\begingroup$ @Acccumulation True. Prof.Legolasov made the same comment above. I have added that detail to the answer. $\endgroup$ – Steeven Oct 15 '20 at 21:39
  • $\begingroup$ I'm not getting this point. "They would have never been formed in first place". Do you mean when big bang happened and pieces which managed to stay stable like earth are here today. And which didn't got perished ? $\endgroup$ – Rao Hammas Oct 15 '20 at 21:51
  • $\begingroup$ Also. I'm understanding what you are saying. I'm confused about this thing that. For equilibrium both forces should cancel out each other. If that's the case then why bend ? And if gravity is more again then why bend ? $\endgroup$ – Rao Hammas Oct 15 '20 at 21:53
  • $\begingroup$ @RaoHammas Celestial bodies where formed at the very beginning from material gathering together due to gravity. In the way it gathered together, a spin and thus a centrifugal effect was formed as well. If that centrifugal effect had been too large, the material would never have settled as a planet or star. $\endgroup$ – Steeven Oct 16 '20 at 6:32

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!

  • $\begingroup$ If i understand your point correctly that means it's not centrifugal force that's causing the bend ? But everywhere i read it's this force which is causing this an gravity keeping it together. ?? $\endgroup$ – Rao Hammas Oct 15 '20 at 21:55
  • $\begingroup$ @RaoHammas, the centrifugal force is indeed partially responsible of the bend. However, gravity is just as responsible since without it the planet will not be held in shape. It is the balance between the two that makes the shape bend in a particular way. $\endgroup$ – FriendlyLagrangian Oct 16 '20 at 8:45
  • $\begingroup$ exactly this is the point that's confusing. There isn't a balance (i think) if there is balance then it should not bend. Both forces should cancel out each other. Now Gravity could be greater but that's not the case because Fugal force has bend the planet. Means Fugal force is impacting more. i guess.. and if one force is impacting more than other then it should have some bad results.. $\endgroup$ – Rao Hammas Oct 16 '20 at 9:16
  • $\begingroup$ as @steeven said above, there must be some other structural forces (not sure if Science has defined that thing or not) that are keeping planet together. because what i'm understanding is, relationship between Gravity and Fugal isn't balance. Only they both are not responsible of this bend and still not being torn apart $\endgroup$ – Rao Hammas Oct 16 '20 at 9:19
  • $\begingroup$ The forces only balance at the bent shape. Would it be spherical and then the forces would be out of balance and will reach equilibrium at a bend shape. Again, think of a water balloon spinning, forces are at balance but not at a ball shape. $\endgroup$ – FriendlyLagrangian Oct 17 '20 at 10:34

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.

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    $\begingroup$ While this is all interesting, I'm not sure how it answers the question. $\endgroup$ – BioPhysicist Oct 16 '20 at 10:10
  • $\begingroup$ I noticed one thing in your answer. that is "YORP effects apply only to smallish bodies" and also said "They tore themselves apart as their rotation rate increased." By that you want to say, only small planets or whatever can torn apart but not a planet like earth ? Also, how we determine if earth is bigger or small. I mean as compared to some it's big and as compared to some it's small. Also, does this effect also applied to earth? $\endgroup$ – Rao Hammas Oct 16 '20 at 10:13
  • $\begingroup$ @RaoHammas I gave an explicit example of bodies that have been posited to have torn themselves apart. I ended with stars, where conservation of angular momentum indicates that stars cannot form unless they dump angular momentum. Stars obviously do form; we have a nearby star about one astronomical unit away. The angular momentum problem is unresolved. There is nothing wrong per se with unresolved problems in science. Pseudoscience has answers (wrong answers) to everything. Science does not claim to answer everything. Unresolved problems in science are how science advances. $\endgroup$ – David Hammen Oct 16 '20 at 10:29
  • $\begingroup$ i got you sir. So can i say, that answer to this question "why planets are not torn apart" is not 100% confirm ? $\endgroup$ – Rao Hammas Oct 16 '20 at 10:33

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