In spite all of that, we've yet to find a single measly microbe off-world. So where is everybody?
In a new study published in Astrobiology, researchers from the Australian National University offer up one possible explanation: Maybe all of the other life in our neighborhood has already come and gone.
It's not a new idea. One of my favorite songs is about this very concept: Space is big and time is long, so it's likely that any life (intelligent or otherwise) would manage to miss us. It's a standard answer to the Fermi Paradox, which is the contradiction between the immense number of Earth-like planets and the apparent rarity of Earth-like life.
The new study goes a step further, suggesting that life, if it evolved, would often be too fragile to survive past its infancy.
In other words, it's possible that other planets in our own solar system had blips of microbial life – tiny emergences that stood no chance of evolving into intelligent organisms.
In fact, you could kind of blame climate change. But in this case, early organisms didn't do enough to change their planet's shifting climate.
"Most early planetary environments are unstable. To produce a habitable planet, lifeforms need to regulate greenhouse gases such as water and carbon dioxide to keep surface temperatures stable," lead author Aditya Chopra said in a statement. The study authors believe that Earth's early microbial life helped to stabilize the planet's climate, while any life that might have existed on the nearby, similarly rocky planets Venus and Mars failed to do so.
"Between the early heat pulses, freezing, volatile content variation and runaway positive feedbacks, maintaining life on an initially wet, rocky planet in the habitable zone may be like trying to ride a wild bull. Most life falls off," the authors wrote in the study.
The researchers have named this phenomenon the "Gaian Bottleneck." If it's true, they point out, our life-hunting robots and astronauts will have to look for fossilized microbes of the most primitive sort. We're not likely to find alien dinosaur bones, which is a bummer.
"The mystery of why we haven't yet found signs of aliens may have less to do with the likelihood of the origin of life or intelligence and have more to do with the rarity of the rapid emergence of biological regulation of feedback cycles on planetary surfaces," Chopra said.
Of course, this work is purely theoretical. It's possible that Earth won some kind of massive cosmic lottery, and that we're the only life ever to have evolved in the whole wide universe. It's also possible that intelligent life is out there somewhere right now. Space is big and time is long, so we may never know for certain.