The planet that matches all the given characteristics is Saturn. Here’s how it fits each description:
- Cold at the surface, hot at the core: Saturn has a cold atmosphere but a hot interior.
- 30 moons orbiting it: Saturn has over 80 known moons, including several notable ones like Titan.
- The largest rings: Saturn is well-known for having the largest and most developed ring system in the solar system.
- A surface that is not solid: Saturn is a gas giant, meaning it does not have a solid surface.
Thus, the correct answer is Saturn.