Artist conception of early Earth. (NASA/JPL-Caltech)
We all know that life has not been found so far on any planet beyond Earth — at least not yet. This lack of discovery of extraterrestrial life has long been used as a knock on the field of astrobiology and has sometimes been put forward as a measure of Earth’s uniqueness.
But the more recent explosion in exoplanet discoveries and the next-stage efforts to characterize their atmospheres and determine their habitability has led to rethinking about how to understand the lessons of life of Earth.
Because when seen from the perspective of scientists working to understand what might constitute an exoplanet that can sustain life, Earth is a frequent model but hardly a stationary or singular one. Rather, our 4.5 billion year history — and especially the almost four billion years when life is believed to have been present — tells many different stories.
For example, our atmosphere is now oxygen-rich, but for billions of years had very little of that compound most associated with complex life. And yet life existed.
The same with temperature. Earth went through snowball or slushball periods when most of the planet’s surface was frozen over. Hardly a ...