Jan Hendrik Bredehöft from UK's Open University proposes there may be four types of habitable planets in the universe: Earth-like, Mars-like, Europa-like and water-worlds.
Taking each of these in turn, he considered their potential for hosting complex life. Earth-like words are the first class, and a kind of "control" since we already know such worlds are capable of sustaining complex life. Earth-like worlds feature an appropriate atmosphere, liquid water, moderate temperature ranges, and stable climates.
The second class of planets are those that were once much like Earth, such as Mars and Venus. "For some reason these planets left the classical habitable zone," said Bredehöft. "Mars became too dry, there's very little water left, at least not liquid water. Venus became just so enormously hot due to the greenhouse effect."
Still, Bredehöft believes there is some chance for life to exist on this type of world. He reasons that organisms could have developed when the planet was more hospitable, and this life could maintain a grip even through the hard times. "Once life has established itself it is really hard to kill off," said Bredehöft. "There have been absolutely devastating events in Earth's history that might have wiped out all kinds of life, but usually these served to further enhance biodiversity, rather than destroy it."