Our solar system contains nine planets, which are broken down into 2 classifications known as terrestrial planets and jovian planets. The terrestrial planets are composed primarily of rock and metal. They also generally have high densities, slow rotation, solid surfaces, no rings, and few satellites. These planets include Mercury, Venus, Earth, and Mars. On the other hand, the jovian planets are composed primarily of hydrogen and helium. They generally have low densities, rapid rotation, deep atmospheres, rings, and numerous satellites. These planets include Jupiter, Saturn, Uranus, and Neptune.
The closest terrestrial planet to the sun is Mercury. Mercury is relatively small, and technically the eighth largest of all the planets. It is actually smaller in diameter than the moons Ganymede and Titan. Mercury has been visited by only one spacecraft, and that was the Mariner 10. The temperature variations on Mercury are the most extreme of any in the solar system. Temperatures range from 90K to 700K. Venus is slightly hotter, but much more stable. Mercury is in many ways similar to the moon. The biggest comparison is the surface being heavily cratered and very old. Mercury is also the second densest planet in the solar system, only behind earth. Mercury actually has a very thin atmosphere consisting of atoms blown off the planet by solar winds. Mercury is often visible with binoculars, and sometimes even the naked eye. The best place to find Mercury is always near the Sun.
The next terrestrial planet, and second planet from the sun, is Venus. Venus is the brightest object in the sky except for the Sun and the Moon. The first spacecraft to visit the planet was the Mariner 2 in 1962. It has also been visited by many other spacecrafts, including the Pioneer Venus, Venera 7, Venera 9, and most recently the US spacecraft Magellan. The rotation on Venus is somewhat unusual because it is very slow and also
Cited: Lunine, Jonathon I. "The Occurrence of Jovian Planets and the Habitability of Planetary Systems." Proceedings of the National Academy of Sciences of the United States of America 98n3 (2001): 809-814. Seeds, Michael A. Astronomy: The Solar System and Beyond. 2nd ed. Brooks/Cole 2001.