The secret of the Earth
One of the questions currently keeping astrobiologists (the people who would like to study life on other planets if only they could find some) awake at night is, what is the crucial difference that allowed the emergence and evolution of life on Earth, while its neighbours remained sterile? In their violent youth, all the inner planets started out with so much surplus heat energy—from planetary accretion and radioactive decay—that their surfaces melted to form magma oceans hundreds or thousands of kilometres deep.