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Jupiter is a "gas giant". If it was (significantly) bigger the pressure from gravitation would ignite a fusion process and it would become a star, which is basically what happened to the sun.

However, what if a body the size of Jupiter or the Sun was made of rocks like Earth and Mars are - what would happen then? Somewhere around iron (lead?) fusion can no longer take place and there is plenty of heavier-than-iron material on Earth.

Or is there something that would prevent such a large body of rocks to form?

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  • $\begingroup$ Are you asking what would happen if accretion of solely high-proton-count atoms were to occur? That might be an interesting process to describe. My guess is it'd be somewhere in the black hole /neutron star family of weird objects. $\endgroup$ – Carl Witthoft Oct 2 '20 at 12:55
  • $\begingroup$ Something like that yes, similar to the process that formed Earth or Mars. $\endgroup$ – d-b Oct 3 '20 at 6:17
  • $\begingroup$ The Sun is almost 98% hydrogen & helium, but that other 2% is around 6,600 Earth masses. So (for example) it contains over 300 Earth masses of iron. $\endgroup$ – PM 2Ring Oct 23 '20 at 13:55
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As far as I understand, rocky planets can only grow up to a certain size. This has to do with planetary formation period. A planet cannot grow indefinitely. It can grow only as long as there are particles around the star that can contribute to its increase of mass. During the formation period, dust particles collide and coalesce to form chunks, which further grow in size by gathering more dust particles or by combining with other chunks. This can go on only as long as there are supply from the dust disk surrounding the star. Eventually this gets depleted and planet can no longer grow much. Also note that the rocky planets are usually found to be closer to the star. Due to this, the amount of material available to form the planet is relatively smaller than the gas giants. The gas giants being formed at the outer regions, have larger circumference and thus more material gets fed into it. This cannot be the case with inner rocky planets.

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  • $\begingroup$ It's unreasonable to assume there never was or will be a large enough cloud of dust to form a larger "rocky" planet. The correct answer so far as I can tell is that the increased density will lead to fusion ignition long before the volume approaches that of Jupiter. $\endgroup$ – Carl Witthoft Oct 2 '20 at 12:53
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    $\begingroup$ I am not aware of any observation where a planet of the size of Jupiter is rocky. Obviously that does not mean there won't be any observation in future. There is every chance of such a planet to exist. Now whether a planet of that size is sustainable or not is a matter of calculation. Anyone working on astrophysics might be able provide the correct answer. The critical volume required to initiate fusion is not yet determined completely. We still discover rocky planets that break the records. So I agree, only more observation can settle this issue. $\endgroup$ – Samapan Bhadury Oct 2 '20 at 13:14
  • $\begingroup$ This doesn't really answer the Title question; it simply indicates that the condition is unlikely to occur naturally. I could strap rockets onto a bunch of large rocky planets, then wait a few hundred million years for them to get into position, then blow them up and allow the debris to slowly join to form one very large rocky planet. The original Title question would then be valid for this situation, yet remains unanswered. $\endgroup$ – Ray Butterworth Oct 23 '20 at 13:50
  • $\begingroup$ The problem with building a planet with the mass of Jupiter out of rock is that such a body has enough gravity to hold onto hydrogen & helium. And since the interstellar medium is around 98% H + He, it's pretty hard to avoid having a high percentage of those elements once the planet's mass is large enough. OTOH, if the planet forms close to the (proto)star then a lot of the light elements can get "baked" out of it, and dispersed by the stellar wind, but it will still retain a lot of H + He. $\endgroup$ – PM 2Ring Oct 23 '20 at 14:07
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I hope it is ok to link to other stackexchange communities, as there is an excellent answer to be found in the worldbuilding community: Is there a theoretical maximum size for rocky planets. The consensus seems to be that the maximum size for Earth-like planets is at around twice the radius of Earth.

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  • $\begingroup$ FWIW, HDE 226868, the author of the (currently) accepted answer to that question, is not only a mod on Worldbuilding, they're also a mod on Astronomy. $\endgroup$ – PM 2Ring Oct 23 '20 at 13:32
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The universe is roughly 98% hydrogen and helium (by mass), so it's very unlikely for a large body to form without it retaining a lot of those gases. (See Wikipedia for a table of the abundances of the ten most common elements in the Milky Way). In the modern universe, the molecular clouds that give birth to stars are enriched with heavier elements, but they are still primarily hydrogen and helium.

So while we know a lot about the various nuclear fusion processes that occur in stars, there's not a lot of info about fusion commencing in large rocky bodies composed primarily of heavier elements, simply because it's impossible for such a large body to avoid accumulating a lot of hydrogen & helium as it forms.

However, fusion does happen in some situations where there isn't much hydrogen. When large stars have fused most of the hydrogen in their cores, they start fusing heavier elements. These reactions require much higher temperatures than hydrogen fusion.


The main proton-proton chain which powers the Sun starts at around 4 MK (megakelvin). The triple alpha process, which fuses helium into carbon, needs 100 MK. The various carbon fusion processes require 500 MK and core density above 3 billion kg/m³, such conditions normally only occur in older stars heavier than 8 solar masses.

The final stages of stellar core fusion, the silicon-burning ladder, occur in massive stars with a minimum of about 8-11 solar masses. They require 2.7-3.5 billion kelvin (GK). The exact temperature depends on mass. At these temperatures, the ambient thermal radiation is so energetic (in the hard x-ray / gamma region of the spectrum) that it can disrupt nuclei, producing free protons, neutrons, and alpha particles, i.e., helium nuclei, and those helium nuclei can then participate in the silicon fusion ladder. At that stage, there isn't much primordial helium, or helium produced by fusion, left in the core.

However, the silicon burning processes only run for a few days. When a star has completed the silicon-burning phase, no further fusion is possible. The star catastrophically collapses and it may explode in a Type II supernova, leaving a neutron star or black hole remnant.

Carbon fusion can also occur when a white dwarf accretes mass from a companion, but that process also tends to be rather violent, leading to a Type Ia supernova. Such supernovae can occur when two white dwarfs collide, creating a much more energetic Type Ia supernova than usual.


While it's theoretically possible for a large carbonaceous or rocky body to undergo fusion, it's not possible for such a body to form naturally, except as the core of a star. But let's say you did somehow manage to get a huge mass of rocky material together without also gathering a lot of hydrogen. You'd need roughly a solar mass for it to reach the temperature and density necessary for fusion (so more than 50 times the heavy element content of our solar system). And then it'd blow itself up in a week or so. I think it might be difficult to raise funding for such a project. ;)

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  • $\begingroup$ Can't hydrogen be used to form rocks and minerals? $\endgroup$ – d-b Oct 27 '20 at 9:14
  • $\begingroup$ @d-b Not really. Hydrogen is a gas at room temperature. It solidifies at 259°C, only 14° above absolute zero. (Under high pressure it does solidify at higher temperatures). Hydrogen compounds can be solid at higher temperatures, but those compounds require some heavier elements. One very common example is ice, as long as the temperature isn't too warm. Note that the process of forming a planet (or star) releases heat, so if you build a planet from lots of small ice pebbles it will melt, and even if it's far from a star it will take a long time to refreeze. $\endgroup$ – PM 2Ring Oct 27 '20 at 9:28
  • $\begingroup$ But aren't hydrocarbon and similar reasonably frequent building blocks in rocks and minerals? $\endgroup$ – d-b Oct 28 '20 at 0:09
  • $\begingroup$ @d-b No. On Earth, most hydrocarbons have biological origins, and the hydrocarbons found in the ground are primarily found in petroleum & natural gas. Light hydrocarbons are gases & liquids at typical Earth surface temperatures. The heavy hydrocarbons tend to be waxy or tar-like, not rocky. But in all hydrocarbons, most of the mass is in the carbon, not the hydrogen. Methane, the lightest hydrocarbon, is 75% carbon (by mass), heavier hydrocarbons (even fully saturated ones) are more carbon-rich. Eg, $\rm C_{20}H_{42}$ is over 85% carbon. $\endgroup$ – PM 2Ring Oct 28 '20 at 5:02

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