The inner planets Venus, Earth and Mars are excellent examples of why some planets become habitable while other do not. Venus had a similar origin to the planet earth but it lies on the inner border of the ecosphere. It probably had oceans when the sun was less luminous before entering the main sequence. The temperature rose to six thousand degrees kelvin. Volcanism in the planet`s past pumped much carbon dioxide into the atmosphere. Since the planets’ oceans had boiled off there was no water to transform the carbon dioxide back into rock causing a runaway greenhouse effect to occur.
Mars probably had oceans in it’s early days when it had a thicker atmosphere of carbon dioxide due to greater geological activity. Low gravity caused the carbon dioxide gas to escape and the greenhouse effect weakened. The oceans evaporated and the planet was left cold and dry (Gore,R p 24 National Geographic January 1985). Conversely, Earth, by being far enough away from the sun was able to retain it’s oceans and therefore support life.