It is an old question, and a persistent one: Is there life elsewhere in the cosmos? Is the universe more than just an enormous collection of dead rock and glowing gas, with only one inhabited world?

The Viking 1 lander set down on the surface of Mars on July 20, 1976. It had a robotic arm that would scoop up samples of the martian soil and place them in three biology experiments onboard the lander. Although results from these experiments were ambiguous, most scientists believe that no life currently exists on the martian surface.

While speculation about life in space is an old pastime, a serious, scientific search for it is very new. Despite the impression one may get from movies and television, scientists still have not found any conclusive evidence of biology beyond Earth—not even evidence of the simplest...