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What is the maximum diameter and mass can a rock-based planet be before it collapsed to degenerate matter?

If "rocky" isn't defined enough, assume basalt.

"Degenerate" means the planet (or at least its core) has been gravitationally crushed to neutronium.

Assume no dense iron core, just rocky stuff.

I'm pretty sure maximum diameter will be at a mass less than the maximum.

Bohemian
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  • Hi Bohemian. Yes, you're building a world, but you might consider asking this on [astronomy.se] or [physics.se]. Do you realize that the gravity of such a planet would be so great that the denizens of Flatland would have trouble living there? – JBH Apr 22 '19 at 02:26
  • You know it would become a star waaay before it would collapse into neutronium, right? Not sure what you're asking. – Escaped dental patient. Apr 22 '19 at 02:27
  • Just to clarify my comment, a given mass, sufficient to collapse into a neutron star (ie. a ball of neutronium) would first need to have sufficient mass to be a giant star (several times the mass of the sun). It would take a long time for the -> sworling matter -> star -> collapse into a neutron star to happen, much longer than for our sun - the general rule is the bigger, the slower. What you ask seems to have glossed over the gas giant stage of planets that have a rocky core and gone for something several orders of magnitude larger. I suggest you do some research in this area. – Escaped dental patient. Apr 22 '19 at 02:52
  • Are you looking for the maximum size of a naturally-occurring silicate (rocky) planet, or the maximum size that some sufficiently advanced alien could build one to without running into physical problems? – Cadence Apr 22 '19 at 02:57
  • @Cadence I was thinking naturally occurring. Actually, I was wondering what the maximum diameter and its surface g would be for a naturally occurring non-gas planet. – Bohemian Apr 22 '19 at 02:59
  • What's a non-gas planet, seriously? There are no gas-planets. Sure gas-giants exist, but they all have rocky cores, like I said. Seriously worth reading: https://en.wikipedia.org/wiki/Planet – Escaped dental patient. Apr 22 '19 at 04:54
  • @Agrajag: Mass is not the only thing that makes a star. You'd need sufficient light elements to support fusion. (You'd initially get heating by gravitational contraction, but this doesn't last long in astronomical terms.) If you had a sufficiently large mass of iron, it could contract without fusion, then the iron core could accrete a rocky mantle & crust. – jamesqf Apr 22 '19 at 05:10
  • You maybe are thinking about the far far future of the universe then, when stars are no longer able to form. @jamesqf Things would look quite different I'd agree in those days. – Escaped dental patient. Apr 22 '19 at 12:13
  • @Agrajag: No, physics will not (as far as we know) change in the far future. If you collect a sufficiently large mass of iron, it will form a spherical planet. You can keep adding more iron, and it will grow larger up to the point where the interior pressure is high enough to collapse into degenerate matter, but AFAIK it won't undergo fusion. – jamesqf Apr 22 '19 at 16:28
  • @jamesqf The relative abundance of the elements will however change in the future (my point) which would allow a super large agregation of matter without hydrogen/helium starting to fuse. The planet would be dead and cold and not orbiting a star because there wouldn't be any. – Escaped dental patient. Apr 22 '19 at 16:31
  • @jamesqf Unless the OP is refering to an engineered planet - in which case it can be made from candy canes and diamond broatches if wished. – Escaped dental patient. Apr 22 '19 at 16:40

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