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Jupiter is the largest planet in the solar system. In terms of mass, Jupiter towers over the other planets. If you were to gather all the other planets together into a single mass, Jupiter would still be 2.5 times more massive. It is hard to understate just how huge Jupiter is. But as we’ve discovered thousands of exoplanets in recent decades, it raises an interesting question about how Jupiter compares. Put another way, just how large can a planet be? The answer is more subtle than you might think.
The simple answer is that a large planet is anything too small to be a star. The usual definition for a star is that it must be large enough to fuse hydrogen into helium in its core. A main-sequence star is one where the heat and pressure generated by fusion are balanced by the gravitational weight of the star.
Stars - Hydrogen - Helium - Planets - Composition
Stars are mostly made of hydrogen and helium, and it’s safe to assume that the largest planets would have a similar composition. The Sun is made of about 75% hydrogen and 24% helium, the other 1% being heavier elements. Jupiter is roughly 71% hydrogen, 24% helium, and 5% other. So let’s figure any large planet is 3 parts hydrogen to 1 part
Fusion - Planet - State - Equilibrium - Weight
As long as there’s no fusion going on, a large planet will be in a state of hydrostatic equilibrium. That means the weight of all that gas trying to collapse on itself is balanced by the pressure of the gas not wanting to be squeezed. The more mass you have, the more the interior is squeezed, and the hotter it gets. With enough mass, the interior gets hot enough for hydrogen to start fusing into helium. That critical mass is about 80 Jupiters. Anything with more mass than that must be a...
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