Planets and Life

The Earth is the only planet in the Solar System where liquid water is present on the surface, a condition that seems necessary for the development of life. Its sisters Venus and Mars are extremely different. Why did these three planets, born under fairly comparable conditions, evolve to the conditions we observe today ? Understanding the physical or chemical factors that are at the origin of such divergent evolutions is a first step in an approach to the problem of the origin of life on Earth.