0
$\begingroup$

I was wondering, how big could a planet get, with gravity staying the same even with the added mass, before collapsing DUE TO its gravity?(without becoming a black hole)

Specifically, I have a world with 75% Earth Gravity,(0.75) and it's roughly 15 Earth diameters across. It has a hollow core, and only generates gravity due to the rotating, cold-metal sphere at its center, which rotates roughly at 15% of the Earth's rotation speed.

EDIT The world itself is a terrestrial world, and it orbits a small, metal orb, which brought the material together during the early days of its system. The orb emits a faint yet strong repulsive force, keeping the planet's material away from the orb itself, yet still orbiting it.

$\endgroup$
  • 1
    $\begingroup$ Or, do you actually mean a dyson sphere instead of 'planet' .. sorry, but as is your question doesn't appear to make any sense? $\endgroup$ – Pelinore Dec 3 '19 at 20:54
  • 1
    $\begingroup$ OK now the edit you've made contradicts the text that comes b4 it, is your planet orbiting this sphere or is the planet hollow with the sphere inside it or are there two sphere's? $\endgroup$ – Pelinore Dec 3 '19 at 21:05
  • 1
    $\begingroup$ Largest known terrestrial planet is Kepler-177-C, it radius is approximately 0.65 Jupiter Radii. Beyond that it’s gas giants. Some of the brown dwarfs are 2.2x bigger than Jupiter. $\endgroup$ – Darius Arcturus Dec 3 '19 at 21:19
  • 1
    $\begingroup$ A rigid sphere cannot really "orbit" something it encloses, not stably. I'd remove that word from your description, if I were you. The hollow core paragraph before it made slightly more sense, to be honest. I'm also not certain that a cold solid core will make a good geomagnetic dynamo even if it is spinning. $\endgroup$ – Starfish Prime Dec 3 '19 at 21:26
  • 2
    $\begingroup$ rotation does not create gravity, mass does. $\endgroup$ – Nosajimiki Dec 3 '19 at 22:10
1
$\begingroup$

There's a bunch of questions that commenters have already raised but I'm going to give it my best shot. I'm assuming that the metal object is like the 'sun' for this planet, so this planet orbits it. No idea if this is feasible, but I'm going to assume it is (i.e this metal thing has enough mass to attract this planet of yours).

First of all: planets don't exactly collapse. The entire point of the formation of a planet is that a bunch of heavier-than-air matter collides and combines into a denser mass. This can be said of all rocky bodies in our solar system.

But you are describing a hollow planet. Not sure if this is scientifically possible but assuming massive hollow spheres work the same way as smaller ones, we know that the inside of this sphere will have a net gravity of 0 (assuming all mass of hollow sphere is distributed evenly across the surface). Outside the sphere, this planet's gravity pulls towards its center. How would this planet collapse? With some big moves, that's how:

  1. Large impacts: an impact big enough to put a significant hole in this planet, making this hollow sphere a hollow sphere with a whole in it. This planet is in fact planet sized, so this won't necessarily collapse the planet unless the whole was big enough, but it's bound to cause issues. This would violate the net-zero gravity on the inside of the planet, and little by little the planet would likely disintegrate and turn into a ball of mass (like planet Earth) or turn into a ring around that metal object that this planet is apparently orbiting. Fun fact: the asteroid belt is thought of as a failed planet. Maybe this is what went down...? Probably not but cool thought. This planet was originally, however, created well (all weight distribution across surface was uniform enough). Keep in mind that this uniformity doesn't have to be perfect (so small hills and stuff would still be ok on the surface) but it can't be wildly inconsistent.
  2. Uneven weight distribution: If one part of the sphere is significantly denser than another, we won't have the uniform net-zero gravity thing happening in the hollow part of this sphere. Again, the problem here is not with the spherical nature of this planet, but with inconsistencies in its mass-distribution. But upon creation this planet must have been stable - this weight redistribution must have taken place after the planet was created.
  3. Terrible construction: If the internal surface of the planet is made of denser, but more brittle material, and is inconsistent-enough distribution of mass across the inner-surface of this planet (violating the net-zero gravity inside the sphere), we could have problems. Let's say this material is large chunks of iron - eventually these chunks of iron will find their way into the center of the hollow planet and take other bits along with it, possibly leading to a slow but exponentially growing rate of disintegration of the planet.

I don't have much math to back this up but if I find something I'll edit my answer. I hope this helps.

EDIT1: I just realized I didn't technically answer the question. My answer is that mass doesn't matter, mass distribution matters and anything that messes with mass distribution matters. That causes destruction/collapse. So, if this planet had the mass of Jupiter, was hollow and only 1 km thick at all parts of its surface (mass perfectly evenly distributed and perfectly constructed so problem 3 doesn't become relevant), with no holes or anything, there's a good chance it would be perfectly stable with no chance of disintegration.

EDIT2: Use proper materials when building it. The only way any such planet could realistically be built and not fall apart is if you used really strong materials (stronger than diamonds and carbon nanotubes, maybe make something up) to hold it together and maintain its structure.

| improve this answer | |
$\endgroup$
  • $\begingroup$ Btw you can combine the 3 problems I listed for a proper mess. Idk how you're writing your story but sometimes you can just make problems worse and worse - I just gave you 3 ways how. $\endgroup$ – cyber101 Dec 3 '19 at 22:40
  • 1
    $\begingroup$ The lack of gravity within the center of a hollow planet does not mean the shell is not under the influence of its own gravity. Each layer will be burdened by the weight of all higher layers and subject to the gravity of all lower layers, and a planet-sized shell composed of any mundane material will immediately collapse to a solid sphere. $\endgroup$ – Christopher James Huff Dec 3 '19 at 22:57
  • 2
    $\begingroup$ However, you are right that planets don't collapse, they are already as collapsed as they can get, apart from minor surface features like mountains. That's what the "hydrostatic equilibrium" part of the definition of planets refers to. $\endgroup$ – Christopher James Huff Dec 3 '19 at 22:59
  • $\begingroup$ You're right - I should have added that you'd need to build the planet out of some very sturdy material (literally diamonds or carbon nanotubes) to prevent an automatic collapse. $\endgroup$ – cyber101 Dec 3 '19 at 23:23
  • 2
    $\begingroup$ @cyber101 : No, neither diamond nor carbon nanotubes will do this job .. in this case when Christopher James Huff says in comments any 'mundane material' he means any material currently known to (or even one merely postulated as being a vague, if highly unlikely to the nth power, possibility by) science, basically his turn of phrase is just a polite way of saying no such material exists, especially in the quantity required. $\endgroup$ – Pelinore Dec 3 '19 at 23:49

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