Life as we know it

A scientist studying a meteor rock might spot a specific shape or feature, or a chemical signature that could indicate a biological origin.

Geocentric life - the much quoted "life as we know it" - Earth life. This would be what Star Trek calls an M Class Planet - worlds where water remains liquid.

When looking at planets from a distance we are looking for atmospheric signals such as the resence of oxygen or methane. Our atmosphere is 78% nitrogen. Oxygen is a by product of life. If all life on earth die, oxygen would quicly disappear. Same goes for methane.