How Do We Know if Other Worlds Harbor Life?

In a universe vast and seemingly empty, where do we look, and what do we look for?

Concept art of exoplanet Kepler-186f, an Earth-sized planet located within its star’s habitable zone. Credit: NASA Ames/SETI Institute/JPL-Caltech.

Earth is teeming with life. It is so commonplace that your body alone has nearly 40 trillion microbial cells living in and on it — little bits of life that are in you, but not exactly you. Our planet is swarming with large and small lifeforms, from all…