As important as the question of
biological conditions favorable for life is
the question of astronomical conditions in which we might expect life to
evolve. Certainly life dependent on chemical reactions similar to those important for Earth life can exist only in
a narrow range of conditions that are not easy to produce. For example, if
the distance of the Earth from the Sun
or the properties of the Sun were very different from what they are, could life have evolved as it has on the Earth?