Mercury is the smallest of the inner planets, and is the closest to the sun. It is heavily cratered and has huge cliffs crossing all over the planet's surface. Mercury is visible only just after sunset or before dawn. Mercury has no natural satellites, as it is approximately the size of Earth's moon.
Venus:
Venus is the second closest planet to the sun, and Earth's closest planetary neighbor. It is also the planet which is closest in size to Earth. Venus's atmosphere, is filled with huge layers of clouds of sulfuric acid, which obscure the planet's surface when viewed from Earth. The surface of the planet is covered with volcanic features, suggesting that the planets surface is geologically young. Venus also has no natural satellites.
Earth:
Earth is the third planet from the sun, and the only planet currently known to support life.
Mars:
Mars is the fourth planet from the sun, and the seventh largest in the solar system. Mars can be seen from earth as a red star-like object that moves through the sky. It is often called the evening star because it is easily seen during the evening and early night hours. The surface of Mars is complex, with river valleys and extinct volcanoes.