While plate tectonics may not be absolutely necessary for life, they may be necessary for a technological civilization to arise. Habitability may be possible on a static world, but habitability probably won’t persist long enough for a technological civilization like ours to appear. Plate tectonics regulates our planet’s climate, and without it, atmospheric CO2 would rise to catastrophic levels.
New research presented at the Europlanet Science Congress and the American Astronomical Society’s Division for Planetary Science (EPSC-DPS2025) says that plate tectonics are critical for long-term habitability. The authors say that the reason we don’t find any technological civilizations in our searches is because of the strong odds against finding planets like Earth with plate tectonics and carbon-silicate cycles. So while temporary habitability may be possible on many planets, those planets can’t moderate their climate long enough for beings like us to appear and build a technological civilization.
The results are based on a pair of papers published in the journal Astrobiology. One is “Eta-Earth Revisited I: A Formula for Estimating the Maximum Number of Earth-Like Habitats” and the lead author is Helmut Lammer. Lammer is from the Austrian Academy of Sciences, Space Research Institute.
“In this hypothesis article, we discuss the basic requirements of planetary environments where aerobe organisms can grow and survive, including atmospheric limitations of millimeter-to-meter-sized biological animal life based on physical limits and O2, N2, and CO2 toxicity levels,” the authors of the first paper write. In their work, they define Earth-like habitats (EH) as “rocky exoplanets in the habitable zone for complex life that host N2-O2-dominated atmospheres with minor amounts of CO2, at which advanced animal-like life or potentially even extraterrestrial intelligent life can in principle evolve and exist.”
The researchers developed a formula to estimate the maximum number of EH in the Milky Way. The formula contains arguments that can be constrained by characterizing their atmospheres with ground-based and space-based telescopes. Since we’re only in the early days of exoplanet atmospheric characterization, the study looks ahead to the coming decades when our capabilities will be stronger.
The second is “Eta-Earth Revisited II: Deriving a Maximum Number of Earth-Like Habitats in the Galactic Disk” and the lead author is Manuel Scherf. Scherf is also from the Austrian Academy of Sciences, Space Research Institute. Both Lammer and Scherf are co-authors on each others’ papers, and Laurenz Spross from the same institution is the third author for both papers.
This study builds on the first. It takes the EH definition and the formula and applies it to the Milky Way’s galactic disk. In their work, they only consider requirements that we can already quantify scientifically. This includes the initial mass function and the star formation rate. They worked with the known frequency of rocky planet occurrence, and with existing ideas about the galactic habitable zone.
Atmospheric CO2 and plate tectonics are at the heart of this work. In general terms, the more CO2 a planet has in its atmosphere, the longer it can sustain habitability. But it has to be in balance. If CO2 levels rise too high, it can trigger the runaway greenhouse effect. Once levels get too high, a planet can’t shed atmospheric heat into space and it gets hotter and hotter. A planet with plate tectonics, like Earth, has a way to slowly but steadily remove carbon from its atmosphere and sequester it into rock.
Earth’s carbon-silicate cycle keeps too much carbon dioxide from building up in the atmosphere. Image Credit: By John Garrett – https://www.skepticalscience.com/print.php?n=1959, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=74327875
But there’s a downside to that, too.
“At some point enough carbon dioxide will be drawn from the atmosphere so that photosynthesis will stop working,” lead author Scherf said in a press release. “For the Earth, that’s expected to happen in about 200 million to roughly one billion years.”
It’s all about atmospheric balance and how long-lived it can be. Earth has a nitrogen (78%) and oxygen (21 %) dominated atmosphere. It contains trace amounts of other chemicals, including carbon dioxide at 0.042 %.
Oxygen plays critical roles. Large, complex animals like us need a lot of oxygen. So does civilization. If there’s less than about 18% oxygen, then there’s no combustion. Without fire, a species will never become technological. Metal smelting would be impossible. Even worse, without open fire, species may not evolve into technological intelligence because there’s no way to cook food.
Our ability to constrain exoplanet atmospheres is growing, but we’re still only in the early stages. TRAPPIST-1 e is in the news because the JWST has been observing its atmosphere. Unfortunately, the telescope so far hasn’t detected any atmosphere that suggests there’s a biosphere. Image Credit: By NASA/JPL-Caltech – Cropped from: PIA22093: TRAPPIST-1 Planet Lineup – Updated Feb. 2018, Public Domain, https://commons.wikimedia.org/w/index.php?curid=76364487
The combined conclusions of both studies suggest that EHs and the technological civilizations that arise on them are extremely rare. In the second paper, the authors write that “on average, a minimum of ~10^3 to 10^6 rocky exoplanets in the HZCL are needed for 1 EH to evolve.” So between 1000 and 1,000,000 rocky planets in the complex life habitable zone need to exist before one will become a planet like Earth, with long-lived climate stability.
The researchers took their thinking even further. They calculated the necessary scenario for their to be even one more technological civilization in the entire Milky Way at the same time as ours. On a planet with a 10% carbon dioxide atmosphere, a technological species would need to persist for at least 280,000 years to be concurrent with ours.
“For ten civilizations to exist at the same time as ours, the average lifetime must be above 10 million years,” said Scherf. “The numbers of ETIs are pretty low and depend strongly upon the lifetime of a civilization.” That’s an awfully long time. Can even the most optimistic person imagine that our civilization can last that long?
There’s one clear conclusion to this work, and it’s one explored in science fiction. If we ever encounter another ETI (extraterrestrial intelligence), it’s highly likely that it’ll be much older than ours. In any encounter, humanity would be the junior partner.
Their work also allowed the researchers to estimate how far away the closest ETI is to Earth: about 33,000 light-years. Since the Sun is about 27,000 light-years from the Milky Way’s center, the next closest ETI would be on the other side of the galaxy, an interminable distance for a civilization like ours.
A single study on this topic can’t include every possible factor. The authors write that “the origin of life, the origin of photosynthesis, the origin of multi-cellular life and the frequency with which intelligent life develops technology,” are all a part of it. They just have no real way to quantify any of those factors. If these factors have high probabilities, then the number of ETIs goes up. Maybe our closest neighbour would “only” be 20,000 light-years away.
Some of these numbers are staggering. If the authors are correct, and for 10 civilizations to exist concurrently with ours their average age needs to be about 10 million years, a stubborn question comes to mind.
The idea of the Great Filter asks why we haven’t detected another ETI. It says that the reason we don’t detect any is that something stops civilizations from progressing to an interstellar stage. Maybe there’s simply no way to overcome the vast distances between stars with technology. Or maybe intelligent species are dragged down by their dark sides. A quick look at headlines fleshes that idea out.
Much of our understanding of habitability is hampered by the fact that we only have Earth to go by. As our observational technologies improve, we’ll begin to understand more exoplanet atmospheres in greater detail. Maybe we will one day be able to reliably conclude that a rocky exoplanet in the habitable zone around a distant star has plate tectonics and the right atmosphere to sustain habitability for a long time.
Ultimately, it’ll come down to detecting another ETI.
“Although ETIs might be rare there is only one way to really find out and that is by searching for it,” said Scherf. “If these searches find nothing, it makes our theory more likely, and if SETI does find something, then it will be one of the biggest scientific breakthroughs ever achieved as we would know that we are not alone in the Universe.”