<P> In a review of the factors which are important for the evolution of habitable Earth - sized planets, Lammer et al. proposed a classification of four water - dependant habitat types: </P> <P> Class I habitats are planetary bodies on which stellar and geophysical conditions allow liquid water to be available at the surface, along with sunlight, so that complex multicellular organisms may originate . </P> <P> Class II habitats include bodies which initially enjoy Earth - like conditions, but do not keep their ability to sustain liquid water on their surface due to stellar or geophysical conditions . Mars, and possibly Venus are examples of this class where complex life forms may not develop . </P> <P> Class III habitats are planetary bodies where liquid water oceans exist below the surface, where they can interact directly with a silicate - rich core . </P>

How does the size of a planet affect its ability to harbor life