A new paper that addresses the Fermi Paradox using the Drake equations has proposed an uncomfortable solution: we may be alone in the galaxy.
If you haven’t heard of the Fermi Paradox, it goes something like this: given the vastness of the universe and the likelihood that life will evolve elsewhere, how is it that no alien civilization has ever made contact? In the short time we’ve been looking, we’ve found many exoplanets. Surely there must be someone else out there who, like us, is desperate to find others?
Since it was posed by Enrico Fermi in 1950, there have been a variety of responses, from the benign to the downright terrifying. One is that there just hasn’t been enough time yet. Alien civilizations may prefer, as we do, to search for techno signatures that we simply haven’t broadcast long enough.
At the other end of the spectrum, it could be that in space civilizations tend to destroy themselves before they 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 place known or best guesses as to the number of stars that contain (for example) planets in habitable zones, or best guesses as to how many of them will be able to support intelligent life.
Using these equations, scientists attempt to estimate the number of intelligent civilizations in the universe and, depending on their input, have come up with answers ranging from 30 to 100,000. Drake himself estimated the number to be between 1,000 and 100,000,000 in our galaxy.
As we learn more about exoplanets and how life began here on Earth, we can at least refine our estimates, as a new paper attempts to do. These high estimates don’t match what we see — ie, no active, communicative civilizations (ACCs) — the researchers note, so we may be missing some important variables.
The team tried to solve this by looking at how life evolved on Earth. Like many others, they suggest that plate tectonics is essential to the evolution of complex animals. Plate tectonics, the team explained, likely accelerated biological evolution in several key ways. This involves delivering key elements for life, such as phosphorus, to the surface.
“Tectonic processes exposing fresh rocks at the surface are key to improving supply [phosphorus] and other inorganic nutrients, as the shielding of fresh rock surfaces by soil reduces nutrient fluxes due to chemical weathering,” the team explains in their paper, adding that evidence for this can be found in Earth’s ancient history, where the emergence of plate tectonics created a more life-friendly environment.
“Addition [phosphorus], [iron] and other nutrients from erosion and weathering of the Ediacaran collision mountains broke the Mesoproterozoic nutrient drought and stimulated life and evolution.”
The transition to plate tectonics may have been essential in other areas as well, including increasing oxygen levels in the atmosphere and ocean, moderating climate (e.g. by carbon subduction), and creating complex landscapes and climates that can stimulate the diversity of life.
“We further suggest that both continents and oceans are required for the ACC because the early evolution of simple life must have occurred in water, but the late evolution of advanced life capable of creating technology must have occurred on land.”
It’s possible that plate tectonics—like enough oxygen for a planet to have fire—is necessary for the emergence of intelligent, communicative life. So we should look for planets with continents and plate tectonics that can be sustained for long enough time periods for life to evolve.
The team then tried to put constraints on the amount of water that would have to be present on exoplanets 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 more). ), making them potentially suitable for ACC development.
They came up with numbers ranging from less than 0.006 to less than 100,000. But that’s not the only limiting factor for ACC, with other potential “big filters” coming later in life, such as potential extinction events or societal collapse.
Factoring this in, 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 that range, given that potential catastrophes could limit how long alien civilizations will be around communicate.
“It is possible 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 planets with life, ACCs may be very rare.”
There are, of course, a number of uncertainties within the Drake equation that can be updated as new information becomes available. Large planets are much easier to detect than Earth-sized terrestrial planets due to the increased amount of dimming and wobble they produce on their host stars.
There may be a number of Earth-like planets to be found as detection improves, or other planets capable of hosting life. Or we might find that early life is more likely than we thought, making life more likely to pass through these big filters somewhere in the universe.
Although plate tectonics may play a huge role in our own evolution, let’s not lose hope that there are others out there with intelligence.
The study is published in Scientific Reports.