Most of us have a pretty good idea of what life is...

A parrot, or a penguin, a sloth or a seal, is clearly alive.

A flower in the forest... a cactus in the desert, they're also living.

But researchers looking for life across the universe have to come up with a more sophisticated definition.

Chris Chyba
Carl Sagan Chair for the Study of Life in the Universe
SETI Institute & Stanford University

I would say that life as we know it means three things.

It means life based on liquid water, almost all life on Earth is 80 or 90 percent liquid water by mass.

It means life based on a set of certain elements, sometimes called biogenic elements, most famously carbon, but also nitrogen, oxygen, sulfur, phosphorous, and there's a long list of others.

And it means an energy source. Life requires an energy source because life has to create and maintain order and you need energy to do that.

So when I say "life as we know it," I mean life that's based on water, life that's based on carbon, and some other elements, and life that requires a source of free energy.