# How large can a rocky planet be? [duplicate]

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.

• Hi Bohemian. Yes, you're building a world, but you might consider asking this on Astronomy or Physics. 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
Commented Apr 22, 2019 at 2:26
• You know it would become a star waaay before it would collapse into neutronium, right? Not sure what you're asking. Commented Apr 22, 2019 at 2:27
• 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. Commented Apr 22, 2019 at 2:52
• 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? Commented Apr 22, 2019 at 2:57
• @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. Commented Apr 22, 2019 at 2:59