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.