Solar System: The Major Planets
Nine major planets are currently known. They are commonly divided into two groups: the inner planets ( Mercury , Venus , Earth , and Mars ) and the outer planets ( Jupiter , Saturn , Uranus , and Neptune ). The inner planets are small and are composed primarily of rock and iron. The outer planets are much larger and consist mainly of hydrogen, helium, and ice. Pluto does not belong to either group, and there is an ongoing debate as to whether Pluto should be categorized as a major planet. Mercury is surprisingly dense, apparently because it has an unusually large iron core. With only a transient atmosphere, Mercury has a surface that still bears the record of bombardment by asteroidal bodies early in its history. Venus has a carbon dioxide atmosphere 90 times thicker than that of Earth, causing an efficient greenhouse effect by which the Venusian atmosphere is heated. The resulting surface temperature is the hottest of any planet—about 477°C (about 890°F). Earth is the only planet k