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 over the surface to deep underground, from the bottom of the sea to miles above during some bird migrations.

So why is it that outside our blue world, everything else looks so… empty?

The intrigue in finding life beyond Earth has been overwhelming for scientists since they first understood that our close neighbors had solid surfaces to land on. The idea of other intelligent creatures living on the Moon or Mars filled the media, only for us to come up empty even after sending a series of life-hunting missions to their surfaces.

The truth is that many of the planets and moons we have explored don’t have the ideal conditions required for life to exist. Yet, with over 5,000 confirmed exoplanets, many scientists agree that at least one of them likely does. Even some moons in our solar system contain promising potential, if only we are able to look closer.