Why are the inner planets so different in composition from the outer ones?

Answer 1

The main reason that the inner planets are much more rocky and the outer planets are gaseous has to do with the condensation of materials. When the Solar System was being formed according to the "Solar Nebula Hypothesis", the Solar System began as a rotating disk of gas and dust. The majority of this material formed the Sun, and the rest of it was left to form the planets. The material closest to the Sun was obviously the hottest, and therefore any lighter materials such as gases would have evaporated and been vaporized by the Sun's heat. The materials with a higher melting/condensing point, such as iron, nickel, and other rock-forming substances could remain solid at these higher temperatures and therefore eventually accreted (came together) to form the inner, or Terrestrial planets. The materials with lower condensing points, gases such as methane, ammonia, water etc, were able to condense out further from the Sun where it was cooler and accreted into the gas giants we have in the outer part of the Solar System called the Jovian planets. It is believed that the Jovian planets started with large "seeds" of solid material made of up of rock and ices, and then their gravity attracted all of the gas in the region. This will unfortunately have to remain a theory as we would never be able to visit the centers of the Jovian planets due to the tremendous pressures that exist there.

Reference:

Freedman, R. A., Geller, R. M., and Kaufmann, W. J. 2008, Universe (9th Ed.; New York, NY: W. H. Freeman and Company)

Answer 2

The inner planets are rocky and dense because they formed closer to the Sun where it was hotter, causing lighter materials to be blown away. In contrast, the outer planets are made mostly of gaseous and icy materials because they formed farther from the Sun where it was cooler, allowing these lighter elements to accumulate.