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.
A very high excess of oxygen, methane, and nitrous oxides would certainly be suggestive of life. But the only way to declare the world inhabited would be to rule out all possible abiological explanations.
No doubt our modeling of exoplanet atmospheres is simplistic. And when the real data trickle in from something as big as an 18-meter aperture space telescope, astrobiologists will be befuddled interpreting it. They may never reach a consensus.
So my caution is that you'll be reading again and again and about so-called "Earth-like" planets. But convincing ourselves of a place where something is actually living is very problematic. I predict that when the first science paper does come out claiming discovery of an extrasolar biosphere, there will once again be intense skepticism and allegations of hype.
The risk and reward in answering the question "are we alone?" is so high, it is the single biggest motivation for an extraterrestrial civilization to invest in interstellar travel (unless they just want to eat us). Their space program's overarching goal would be to "seek out new life," as TV's Captain Kirk so succinctly put it 45 years ago.
But we have no evidence of such alien visitations. We haven't found extraterrestrial life, and apparently it hasn't found us.
Photo Credits: NASA, ESA