A new paper that takes a look at the Fermi Paradox using the Drake Equations has suggested an uncomfortable solution: maybe we’re alone in the galaxy.
If you haven’t heard of the Fermi Paradox, it goes something like this: given the infinity of the universe and the implied probability of life evolving elsewhere, how is it possible that no alien civilization has ever made contact? We’ve found a lot of exoplanets in the short time we’ve been looking. Surely there must be someone else out there who, like us, is desperate to find others?
Since it was posited in 1950 by Enrico Fermi, there have been a variety of responses, from the benign to the absolutely terrifying. One is that there simply hasn’t been enough time yet. Alien civilizations may prioritize, as we do, searching for signatures of technology that we simply haven’t transmitted in a long time.
On the other end of the spectrum, it may be that the trend throughout the universe is for civilizations to destroy themselves before they can make enough progress to make contact.
After the Fermi paradox came the Drake Equation, which attempts to quantify the number of intelligent civilizations in our galaxy, or universe. In it, we can put known or best guesses at the number of stars that contain (for example) planets in habitable zones, or best guesses at how many of them will be able to support intelligent life.
Using these equations, scientists try to estimate the number of intelligent civilizations in the universe, and depending on their contribution, they have come up with answers ranging from 30 to 100,000. Drake himself estimated the figure at between 1000 and 100,000,000 in our galaxy alone.
As we get more information about exoplanets and how life began here on Earth, we can at least refine our estimates, which is what a new paper tries to do. These high ratings do not match what we see – ie. there are no active communicating civilizations (ACCs) – the researchers point out, and thus we are probably missing some important variables.
The team tried to address this by looking at how life evolved on Earth. Like many others, they suggest that plate tectonics is central to the evolution of complex animals. Plate tectonics, the team explained, likely accelerated biological evolution in several key ways. This involves sending elements essential for life such as phosphorus to the surface.
“Tectonic processes that expose fresh rocks to the surface are essential for increased scattering [phosphorus] and other inorganic nutrients, because protecting fresh rock surfaces from soil reduces nutrient fluxes due to chemical weathering,” the team explains in their paper, adding that evidence for this is found in Earth’s ancient history, where the occurrence of plate tectonics created a more hospitable environment for life.
“The addition of [phosphorus], [iron] and other nutrients from erosion and weathering of the Ediacaran crash mountains broke the Mesoproterozoic nutrient drought, stimulating life and evolution.”
The transition to plate tectonics may have been decisive in other areas as well, including increasing oxygen levels in the atmosphere and ocean, moderating climate (e.g. through carbon subduction), and creating complex landscapes and climates that could to stimulate the diversity of life.
“We further suggest that both continents and oceans are necessary for ACCs because the early evolution of simple life must have occurred in water, but the late evolution of advanced life that could create technology must have occurred in earth.”
It’s possible that plate tectonics—as well as enough oxygen for a planet to have fire—is necessary for intelligent, communicative life to emerge. Thus, we should look for planets with continents and plate tectonics that can persist for long enough periods of time for life to evolve.
The team then tried to put limits on the amount of water that would have to be present on exoplanets in order to have surface water and continents, before trying to use the Drake Equation to estimate how many planets in the galaxy contain these conditions (and others), making them potentially suitable to develop ACC.
They came up with a figure that ranged from less than 0.006 to less than 100,000. But this is not the only limiting factor for ACCs, with other possible “big filters” coming later in life, such as potential extinction events or societal collapse.
Factoring this out, they put the number between less than 0.0004 and less than 20,000. The team points out that we should probably be looking at the lower end of this range, given that potential catastrophes could limit the amount of time that alien civilizations can communicate.
“It may be that primitive life is quite common in the galaxy,” the team concluded. “However, due to the extreme rarity of long-term (several hundred million years) coexistence of continents, oceans, and plate tectonics on life-bearing planets, ACCs may be very rare.”
There are, of course, a host of uncertainties within the Drake Equation that can be updated as we learn new information. Giant planets are much easier to detect than Earth-sized terrestrial planets, due to the increased amount of dimming and wobble they produce in their host stars.
Perhaps there are an abundance of Earth-like planets that we will find as discovery improves, or other planets capable of hosting life. Or we may find that primordial life is more likely than we thought, making it more likely that life could pass through these great filters somewhere in the cosmos.
Although plate tectonics may play a large role in our evolution, let’s not lose hope that there are others out there with intelligence yet.
The study is published in Scientific Reports.