If a planet is the size of Jupiter does it have to be a gas giant?

Answer 1

If we are going to be particular about the definition of 'planet' then yes, the formation models pretty much restrict something with Jupiter's mass being a gas giant. If we were to have a rocky planet with such a mass then it would acquire a hydrogen atmosphere simply by its attractive gravitational potential.

Now we could think up a few weird situations where a Jovian planet could orbit a star so closely that its atmosphere could be burned off, but if it has Jovian mass all in rock and iron than that is going to have to be one hot star and one tight orbit.

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However, Shoemaker–Levy comet fragments collided with Jupiter's Southern Hemisphere between July 16 and July 22, 1994, at a speed of approximately 134,000 mph. The scars from the impacts persisted for many months.... So, some say its a gas giant, but if a comet collided with it, made plumes of ash and soil for 1000s of miles across, well... Seems like it has some surface some people won't accept.