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If I get the numbers right, at about 17 Jupiter masses a mainly gaseous planet would ignite into some type of star. What about rocky planets? Is there a theoretical limit to their size, so that they can grow up to size x? (I'm imagining a "liquid" planet would congeal into rock after certain gravity value, and would somehow fit into the "rocky class" as well).

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  • $\begingroup$ I think that other question covers it. The issue isn't how massive the heavy/rocky elements can get, the issue is accumulation of too much gas to be classified as a rocky planet. The maximum mass of an Earth like planet is surprisingly small before it begins to resemble something like Neptune. I like your idea of gravity so strong that liquid resembles solid, but I don't think that's realistic. Large planets accrue large atmospheres. The pressure, and the internal temperature, not the gravity becomes the key factors on any liquids. $\endgroup$ – userLTK Nov 4 '17 at 17:42
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Giant planets have an abundance of H and He. Hydrogen burning occurs at relatively low temperatures. Rocky planets, on the other hand, don't have much hydrogen and even less helium. For them, the first relevant process would be C burning, which occurs at about 8 solar masses.

But this is valid only as a thought experiment. Rocky planets loose or don't accrete H/He mainly because of insufficient mass. If they were massive, they would consist of H/He and therefore "ignite" at 17 Jupiter masses.

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  • $\begingroup$ That's "oh wow" :D makes sense, and took me a while to realize how science gives you answers that are true and funny. I'm attempting the most 'realistic' space game ever (in some aspects :P). What you say not only solved a big question (I'm polishing a planet generator), it opened possibilities for new ideas the moment I read it. If I ever pull it off, and star construction is an option, after your permission, the wiki of the game will say that such bodies were nicknamed "olaf b's". Many thanks :) $\endgroup$ – Jorge Al Najjar Nov 5 '17 at 23:37

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