In addition, the most striking difference between the terrestrial planets and outer solar system planets lies in average density . A star's light or photon engine. Every star on the main sequence converts mass into energy through thermonuclear fusion. The process demands high temperature and pressure in order to fuse the hydrogen into helium, due to powerful electromagnetic forces separating neighboring H atoms. The difference between gravitational forces on opposite sides of an object, like a planet or moon.
Gravity is a force that depends on both the involved masses and distance between the masses.
The Moon-Earth gravitational interaction results in sea tides on Earth. Elsewhere in the solar system, the magnitude of gravitational force on Jupiter's moon, Io, is greatest on the Jupiter side of Io, and least on the backside. The difference between the two forces compresses and stretches Io, which in turn heats up Io's interior. As a result, Io is the most geologically active body in the solar system. The Moon's shadow narrowly projects onto the Earth, making this a rare experience.