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In "Star Wars Episode I: The Phantom Menace," Qui-Gon Jinn, Obi-Wan Kenobi and Jar Jar Binks are in the underwater Gungan city on the planet Naboo. They must reach Queen Amidala on the other side of the planet. So, the Gungan leader says that the fastest way is to go through the planet's core and gives them a submarine. As they travel, several sea monsters pursue them and they escape before surfacing on the other side. While this scene seems harmless, it defies what we know about planets. Planets do not have rocky surfaces and watery cores.
In our solar system, we have small rocky planets (Mercury, Venus, Earth, Mars) and gas giants (Jupiter, Saturn, Uranus, Neptune). The planets formed when material from the early solar disk (planetismals) collided and clumped together to form the planets. The material in the inner solar system was mostly rock, dust and metal that could exist in the warm environment. The planetismals in the outer solar system were mostly gas, water ice and dust that could exist in the cold environment. As the planets formed, gravity held the aggregated planetismals together and caused them to spin. Naboo is an Earth-like planet, so let's look at the formation of the Earth.
On the early Earth, collisions of planetismals produced heat that melted the material, which was not uniform. In this molten medium, materials of different densities settled. Iron and nickel within the molten Earth were the densest and sank to the center to form the core. Less dense materials settled above to form layers (outer core, mantle, crust). Geologists refer to this process as differentiation. (Note: In the outer gas giant planets, the cores may be composed of dust and water ice with liquid gas layers around them).
Image courtesy U.S. Geological Survey
Water is less dense than nickel, iron and rocks. It would float on these substances. So, you would not find water in the center of the Earth. Similarly, Naboo would not have a liquid water core.
In her book "The Science of Star Wars," Jeanne Cavelos states that Naboo supposedly is made of irregular rock aggregates with watery caves between them. However, gravity would attract these pieces and they would heat up. Any caves would collapse, water would disappear and Naboo would assume a spherical shape with differentiation just like the Earth.
In "The Core," scientists discover that the Earth's inner core has stopped rotating. This disrupts the Earth's magnetic field leaving the Earth vulnerable to deadly microwave radiation (Bad science alert: the Sun does not put out enough energy in the microwave band to be a danger and the Earth's magnetic field does not deflect microwaves). To correct this situation, the scientists travel through the mantle and the molten layers of the outer core and attempt to jumpstart the core's rotation with nuclear bombs. While the depiction of the Earth's structure is better than other movies, "The Core" has many other science problems (see detailed reviews on Bad Astronomy and Insultingly Stupid Movie Physics).