Can a planet have a larger diameter than Jupiter?

Question

I have heard it said that if mass were added to Jupiter, then only its mass and density would increase but the diameter would stay about the same. Is this the case for Jupiter or some property of Jupiter specifically, or is 150,000 km pretty much the largest diameter that a non-fusing object can achieve?I am specifically interested in the case of a physically larger object (planet) orbiting a smaller but more massive object (star).

Answer 1

I enjoy these types of questions, as they allow me to ramble. But I will contain myself.

Now to the answer. If I add matter to Jupiter, its diameter will stay the same because Jupiter's immense gravity... UP TO A POINT. There is currently not enough free matter in the solar system to increase Jupiter's diameter. But, if we were to take say, the other 3 gas giants, Saturn, Uranus, and Neptune and add them to Jupiter, well Jupiter would become a much larger planet. Now, there are planets out there (in other solar systems) that are larger than Jupiter, so Jupiter is not as large as a planet can get.

Now if we keep getting larger we run into a new class of celestial body known as a Brown Dwarf. These objects are too small to become a star, meaning they do not fuse or create nuclear fusion, but they are not planets because their composition is basically 100% the same as a star, and they typically have used up the overwhelming majority of mass in there local area. Planets do not do this. Planets clear the area around their orbit, or they cause the things around them to orbit them. Planets also orbit stars. Brown dwarfs may have a small dwarf planet orbiting them. They typically arent orbiting another star unless it is a binary system.

Also, larger objects orbit more massive but smaller objects all the time. Stars orbit black holes.