The term 'Fourth Rock from the Sun' is a reference to Earth, which is the fourth planet from the Sun in our Solar System. The crossword solution for this term is 'MARS', which is the fourth planet from the Sun. MARS is a terrestrial planet, meaning it is composed of rocks and metals, and is the second smallest planet in the Solar System. It is known for its red-orange hue, which is caused by iron oxide on its surface.