The Inner Planets are the four closest planets to the Sun: Mercury, Venus, Earth, and Mars. These planets are characterized by their rocky surfaces and relatively small sizes compared to the outer gas giants. They are also known for having higher densities and are primarily composed of silicate rocks and metals. Each of the inner planets has unique features. For instance, Mercury has extreme temperature variations, while Venus is shrouded in thick clouds of sulfuric acid. Earth is the only planet known to support life, and Mars is often explored for signs of past life and potential human colonization.