The more mass the planet has the bigger it should be, right? Well, it turns out the answer is no! Let's take a look at why this happens. During the formation of a planet the size of its radius slowly increases as more more massive it gets. Once a rocky planets becomes heavy enough, it will attract a lot more hydrogen and helium gas due to its gravity, its radius now grows faster. However there comes a point when adding more mass compresses the gas more and the radius actually begins to shrink. So the answer to our question is that a planet doesn't get much bigger than Jupiter as we can see from this chart of simulated planet sizes along with some measured planets and exoplanets:
(Credit: Christoph Mordasini)
If we continue to add mass to a planet, it will keep shrinking turning into a brown dwarf until it reaches the critical point where fusion takes place, turning it into a real star. At this point the outward pressure from the fusion reactions will increase the radius again.
If we continue to add mass to a planet, it will keep shrinking turning into a brown dwarf until it reaches the critical point where fusion takes place, turning it into a real star. At this point the outward pressure from the fusion reactions will increase the radius again.
