Planets that are close to the sun are made mostly of rocks and minerals, because minerals can solidify at higher temperatures than gases.  It is hot close to the sun, but rocks and dust can form there, while the more volatile elements, the gases, do not condense until they are beyond the zone of habitability, the live zone.

     Elements that we know as gases, such as oxygen and nitrogen, do not become solid until they are very cold. The outer planets are made of very cold gases, frozen gases, and water ice.  It is believed that they may have rocky cores. Their moons are also cold, and seem to be made up of ice mixed with rocks and minerals.

     The planets and moons of the inner solar system, Mercury, Venus, Earth, and Mars, have solid surfaces. They are very interesting to geologists because they are made of the same elements as the earth, but have different histories and characteristics.

       As a planet forms, gravity draws the the rocks and dust from which a planet is made together. When enough matter has been gathered, heat is released, and the planet may become a molten ball of melted material. While the planet is molten, the heaviest elements sink toward the center (although some proportion of the heavy material stays on the surface), and the lighter elements float to the top.

     Eventually the outside of the planet cools and begins to solidify and form a crust. As the crust solidifies it contracts, forming an uneven surface, with mountains, cliffs, rifts, and flat areas. Cracks appear in the shell of the planet. Hot material from the inside of the planet may escape, forming volcanoes, and meteors from space may crash on the planetary surface, creating craters. Steam may escape from the volcanic vents, and the icy parts of meteors may melt and add water. While the planet is hot, water exists as steam and rises in the atmosphere until it can form clouds, and, later, rain. Some of the water escapes into space, but on our planet, and possibly others, enough remains to support living beings.

     Meanwhile, the interior of the planet cools more slowly. Pressure causes the iron core of the planet to solidify despite its high temperature. The mantle, under the crust, remains more plastic, allowing the plates of solid rock on the surface to float and slowly move about. The places where these tectonic plates meet become sites of volcanism and mountain building.

     Other processes also affect the appearance of landscapes. Erosion by wind and water wears the rocks away: sand and pebbles are carried in rivers and floods. Low lying plains may become bogs and wetlands.  Ice freezes in rocky cracks, splitting the rocks apart. Planets are constantly undergoing change.

     Mapping your planet will set the stage for the development of your world. How much of the world will be covered by water? Earth is about 70 per cent covered by water, which ensures plenty of evaporation for clouds and rainfall. Will your oceans be shallow or deep? Deep oceans permit better circulation of the water, but shallow seas can be warm and life nurturing. If your world is a drier world, will life be restricted to just a few areas? How warm will your world be? Will there be ice at the poles?

     Something else to consider is the influence of gravity. If the mass of your world is greater than that of the earth, it will have greater gravitational forces. Gravity greater than our earth's will require more force to push mountains up. Mountains will probably not be so high as they are on earth. Rain will fall faster, and hit the ground harder. Will this speed up erosion? On the other hand, a planet with less mass than earth will have less gravity, and mountains may be higher and steeper. Have fun thinking about this!