The upper limit to a planet

See that little dot next to the bright one? Here's an interesting puzzle: is it a planet, or a star? If you're thinking "Here we go again!" then you're not too far off the mark. With all the Pluto nonsense going on, the meaning of the word "planet" is under fire. Mostly, the definition has been causing grief due to how it applies to objects at the small end of the scale. But what about the upper end? Take Jupiter. It's the biggest planet in the solar system. What happens if you dump mass into it? Let's throw Saturn, Uranus, Neptune into it. Of course, Jupiter will get more massive. Oddly, it won't get much bigger: more mass means more gravity, and since Jupiter is mostly gas, it compresses. That extra compression will pretty much balance out the extra volume being added, and the size stays the same. Weird, huh? ...