AtmosphereNo atmosphere It is possible to have a planet essentially without atmosphere, as is the case with Mercury. Due to its small size and resulting little gravity, it's believed that Mercury's atmosphere was blown away by solar winds. There are some remnants remaining, traces of water vapor and potassium among a few others, but it is unlikely such a planet could support life. An atmosphere is important in stabilizing water and temperature and in protecting organisms on a planet from radiation. Runaway Greenhouse Planets A planet whose atmosphere is too heavily composed of carbon dioxide - such as Venus's is - will often become victim to the runaway greenhouse effect. The atmosphere of Venus is approximately 96.5% carbon dioxide, which trapped heat from the sun on the planet and continually raised the planet's temperature until almost all the water that was on Venus had long since boiled away. This means that Venus's surface temperature, without any ability to regulate itself with the water having boiled away and the heat unable to escape, is 894 °F (480 °C). Not exactly welcoming! Balanced Atmospheres In order to support life in such a form as we know it, an atmosphere must be present, it must have the correct thickness (Venus's atmosphere is much too thick to support life, resulting in problems with greenhouse gases and atmospheric pressure). Earth's atmosphere is estimated to be about 1000km in thickness. The atmosphere also needs to have certain elements present (carbon, nitrogen, hydrogen, and oxygen) to support life, at least the life as we know it. For life forms utilizing alternate forms of respiration or whose primary structure is different from life we know, there may be other requirements instead. Jovian Planets These are the "gas giants", one whose primary element is not a solid material such as rock. It is difficult with such planets to determine where the atmosphere begins. In the ordinary gas giants (Jupiter and Saturn), their atmosphere and much of the planet itself is composed of hydrogen and helium. The "special" gas giants (Neptune and Uranus) are often referred to as ice giants for their atmospheric differences from the other gas giants. Water, ammonia, and methane are the contributors which make them "ice" giants and methane absorbs red lightwaves, giving them their tranquil blue color. As these are not mainly made of a stable substance, life on these planets is unlikely. Spoiler (Move your mouse to the spoiler area to reveal the content) Show Spoiler Hide Spoiler Lunine, Jonathan I. (September 1993). "The Atmospheres of Uranus and Neptune". Annual Review of Astronomy and Astrophysics31: 217–263. Ingersoll, A. P.; Dowling, T. E.; Gierasch, P. J.; Orton, G. S.; Read, P. L.; Sanchez-Lavega, A.; Showman, A. P.; Simon-Miller, A. A.; Vasavada, A. R. "Dynamics of Jupiter's Atmosphere" Ward, Peter D. and Brownlee, Donald, Rare Earth, Copernicus Books, 2000.