Suppose you start with a rocky sphere about 12,742 kilometers in diameter and continuously add more rocky material such that you don't smash this sphere to bits. What is the upper limit for this? What happens as you approach that limit? What happens once this limit is exceeded?

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  • $\begingroup$ It could depend on how much angular momentum you add. $\endgroup$
    – uhoh
    Jan 14 at 10:07
  • $\begingroup$ What are your conditions for the limit? $\endgroup$ Jan 14 at 11:48
  • 6
    $\begingroup$ You may get better answers for this particular question by asking it on the physics or astronomy stack exchanges $\endgroup$ Jan 14 at 12:38
  • 1
    $\begingroup$ If you are careful to keep it from getting too hot you could do it for a while (by which I mean many earth masses). There'd be a phase change when pressure in the core turned it into degenerate matter; don't know if that would heat it up suddenly enough to get an explosion or silicon fusion started. You'd probably end up with something resembling a black dwarf. $\endgroup$
    – antlersoft
    Jan 14 at 16:03
  • $\begingroup$ This answer looks relevant! $\endgroup$
    – Rafa
    Jan 15 at 4:36

An article from 2016, yes old, stated that it was once thought any planet with a radius about 1.6 that of the Earth would become a gas giant, based the 2015 paper, Leslie A. Rogers, MOST 1.6 EARTH-RADIUS PLANETS ARE NOT ROCKY, The Astrophysical Journal, Volume 801, Number 1.

Since then, a rocky exoplanet, TOI 849 b, was discovered in 2020 which supposedly is the largest rocky exoplanet yet discovered. It has a mass 39 times that of Earth and its radius is 0.307 that of Jupiter (3.369 times the radius of Earth). It's calculated average density is 5.638 g/cm3 (1.023 times that of Earth). Its gravity would be 33.826 m/s2 (3.44 times that of Earth) and its escape velocity would be 38.1 km/s (3.41 times that of Earth). The surface temperature is either 1093 °C or 836 °C.

TOI-849b's odd properties suggest that it could probably be the core of a gas giant planet that should have grown more massive than Jupiter.

One possibility why TOI-849b has cleared a gap around its star is because it ran out of material to collect, and stalled. Or this massive planet could have been a huge planet that somehow lost its atmosphere because it was too close to its star.

The third possible explanation is that cataclysms during the planet's early years both bulked up TOI-849b's core and stripped away its atmosphere.

  • $\begingroup$ Nice information, but doesn't quite address the question. OP doesn't ask what is the largest discovered rocky planet, but what is the largest possible rocky planet. OP doesn't put limits on the atmosphere, so presumably the "rocky planet" could be the core of something which looked like a gas giant. $\endgroup$
    – Woody
    Jan 15 at 0:25

In theory (and from a God like perspective putting practical considerations to one side) it should be possible to build a rocky planet of any size you want. The only limiting factor would be when the mass of the planet was so great that the escape velocity exceeded the speed of light and a black hole formed.

  • $\begingroup$ Are you sure it would not become something different than a planet when it got really massive? A star, supernova, black hole, neutron star, etc... Where's the supporting source for "...it should be possible to build a rocky planet of any size you want." $\endgroup$
    – uhoh
  • $\begingroup$ Well it does depend on the composition. Provided the core was made of iron it wouldn't be susceptible to nuclear fission or fusion. But radioactive impurities could melt the surface. If it got hot enough and big enough it might take on some of the characteristics of a brown dwarf, but with less energy. If there was a thick mantle layer with a lot of oxygen, silicon and aluminium they might eventually be forced into some form of nuclear reaction but I doubt there would be enough pressure or a high enough temperature. More likely a lot would boil off into a superheated atmosphere of sorts. $\endgroup$
    – Slarty
    3 hours ago

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