Is Seeing Believing?
But how would we convince ourselves anything is living on a Goldilocks planet?
For astrobiologists, life is defined as a chemical system that undergoes Darwinian evolution. That definition works in the lab where you can put matter under a microscope, says geoscientist James Kasting of Pennsylvania State University. But what about on the surfaces of chemically exotic worlds?
As far back as 1964 scientists, who were then called exobiologists, suggested that a planet's atmosphere must be in extreme chemical disequilibrium if life is there. For example, aliens studying Earth from afar would note that oxygen and methane levels are 20 times greater than what would be expected if Earth were lifeless. But dead planets can be in disequilibrium too, it's only a matter of degree, says Kasting. What's more, living planets can look like they are in chemical equilibrium, he adds.
For the chemistry of life to be remotely detected, we must find a planet where photosynthesis is is being conducted by surface organisms. So there must be surface water too - even if it is loaded with arsenic.