Mercury- Closest planet to the Sun; smallest planet; weak magnetic field suggests an iron core; has many craters and high cliffs; no true atmosphere, so the surface temperatures range from very hot to very cold; no moons.

Venus- 2nd planet from the Sun; similar to Earth in size and mass; extremely dense atmosphere of clouds; no moons.

Earth- 3rd planet from the Sun; water exists on the surface as solid, liquid, gas; more than 70% of surface covered with water; atmosphere projects surface from most meteors and Sun's radiation.

Mars- 4th planet from the Sun; called Red Planet because iron oxide in rocks makes them reddish-brown; polar ice caps made mostly of frozen carbon dioxide and frozen water; soil shows no evidence of life; has gullies and deposits of soil and rocks, which may look like the presence of liquid groundwater; thin atmosphere of mostly carbon dioxide; strong winds caused by differences in temperatures between day and night; is tilted on its axis, which causes seasons; two small moons: Phobos and Deimos.