I was inspired by Victoria’s post to think more about the Fermi Paradox, and specifically, explanations of the uniqueness of intelligent life on Earth. The Rare Earth Hypothesis is one such explanation; it postulates that conditions favorable to life (and particularly intelligent life) are incredibly rare in the universe. It is in opposition to hypotheses that suggest intelligent life is (or has been) abundant but that we haven’t yet detected it for various reasons.
The Rare Earth Hypothesis mentions a number of characteristics that make Earth special. One particularly notable characteristic is our large moon. Of the four terrestrial planets in our Solar System, Earth is the only one that has a moon of decent size, and this moon has shaped Earth life in a multitude of ways. For one, the Moon creates large tides (twice the size of those the Sun creates), and the resultant tidal pools might have been crucial to the evolution of complex life, as these areas are only sometimes submerged in water. The Moon’s tidal forces also could have led to plate tectonics and the formation of oceanic crust. We do not currently know how rare a moon like ours is in the universe, as we have not yet detected any exomoons with certainty.
In addition, the Moon was likely formed due to a giant impact event (Theia, a Mars-sized object, crashed into a very young Earth). This chance event gave the Earth its axial tilt, leading to seasons, which were an impetus for organisms to adapt to different climates. The giant impact also led to the Earth’s rapid rotation, stabilizing daily temperatures and facilitating photosynthesis.
Image credit: NASA/JPL-Caltech
If complex life can only form on a planet that is within its habitable zone and has undergone such a giant impact, then it makes sense that we have not yet detected other intelligent life – it is simply that rare.
However, a common criticism of the hypothesis is that life on other worlds need not take the form of life on Earth, meaning that the exact same conditions are not necessary. This counterargument holds that Earthly conditions were just one of the many ways life could arise. We do not currently have sufficient technology to test the Rare Earth Hypothesis, as we are unable to detect moons, plate tectonics, surface water, or even signs of simple life on planets beyond our Solar System. Perhaps, then, time will tell why we are alone…or reveal that we are not.
Do you agree with the Rare Earth Hypothesis? Do you think there is an alternate explanation for the lack of intelligent life, or do you think there is currently intelligent life we haven’t found yet?
Sources:
“Rare Earth Hypothesis.” The Center for Planetary Science. Web.
“Rare Earth Hypothesis.” Wikipedia, Wikimedia Foundation, 9 Apr. 2019. Web.
Epiphany's Planet 
It’s difficult to imagine what forms of extraterrestrial life are even possible and thus these factors (and their uncertainties) are difficult to quantify. A recent paper from Oxford suggests if you carefully assess all of the uncertainties in the Drake equation calculation, the likelihood of other life in our galaxy is only ~30% (compared to what some would argue should be 100% given the number of planets). I’ll leave a link to the paper here! https://arxiv.org/pdf/1806.02404.pdf
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