* Earth: Earth is the only planet in our solar system definitively known to have both a solid inner core and a liquid outer core.
* Other Terrestrial Planets: Mars, Venus, and Mercury likely have solid inner cores, but their outer cores are probably more of a partially molten, viscous layer rather than a completely liquid one.
* Gas Giants: Jupiter, Saturn, Uranus, and Neptune are made primarily of gas and ice. While they have very dense cores, the nature of those cores is still debated, and it's unlikely they have the same "inner/outer core" structure as Earth.
In short, only Earth has a solid inner core surrounded by a fully liquid outer core. Other planets might have some kind of core differentiation, but the details are still under investigation.
