Plate tectonics, oceans, and continents might just be the secret ingredients for complex life on Earth. And if these geological features are rare elsewhere in the universe, then perhaps that explains why we haven’t yet discovered intelligent alien life. New research from American and Swiss Earth scientists suggests that these ingredients represent missing variables in the famous Drake equation, devised more than half a century ago to estimate the chances of finding advanced civilizations in our galaxy. Including these new variables could completely rewrite the probability of detecting intelligent life in the Milky Way.
The impetus for this research, with its galaxy-spanning implications, began with a mystery right here at home – why did life take so long to move beyond simple organisms?
“Life has been around on Earth for about 4 billion years, but complex organisms like animals didn’t appear until about 600 million years ago, which is not long after the modern episode of plate tectonics began,” said Robert Stern of the University of Texas at Dallas. “Plate tectonics really jump-starts the evolution machine, and we think we understand why.”
Stern and his collaborator, Taras Gerya of the Swiss Federal Institute of Technology, propose that plate tectonics – the grinding movement of the upper layers of the planet at long geologic time scales – helped speed up the transition to complex life.
Early in Earth’s history, simple organisms formed in the ocean, but humanity – an advanced civilization capable of communicating across outer space – couldn’t exist if ancient life hadn’t transitioned to land. Vast, resource-rich continents were therefore a vital prerequisite for what Stern and Gerya call Active Communicative Civilizations (ACCs) like humanity to develop. But that alone wasn’t enough: the continents needed to move.
The geologic record on Earth suggests that plate tectonics accelerated evolution on land through five distinct processes: it increased the supply of nutrients; sped up the oxygenation of both the atmosphere and the ocean; tempered the climate; caused a high turnover rate of habitat formation and destruction; and offered non-catastrophic environmental pressure that forced organisms to adapt.
The end result of all these environmental pressures: us.
If Stern and Gerya are right, plate tectonics were a requirement for eventual innovations like the wheel, the smartphone, and the Apollo program.
And for other civilizations in the galaxy to develop similar technological marvels, perhaps their planets need plate tectonics too. But as far as we know, they’re rare.
Earth is the only planet in our solar system to feature plate tectonics. Volcanism exists on some other worlds, like Venus, Mars, and Io, but these worlds have a singular solid shell, rather than multiple moving plates. Similarly, ocean worlds like Enceladus and Europa are bound within an icy coating, forbidding any hypothetical life there from transitioning to land.
We don’t know for sure whether distant solar systems feature planets with plate tectonics – current space telescopes don’t have the resolution to make such determinations. But knowing that they might not enables a more accurate version of the Drake equation.
There are two essential factors proposed in the revised equation: the fraction of habitable exoplanets with large continents and oceans, and the fraction of those that have plate tectonics lasting more than 500 million years.
This version is much more nuanced than the original Drake equation, which simply took into account the fraction of habitable planets on which intelligent life had developed.
The Drake Equation, a mathematical formula for the probability of finding life or advanced civilizations in the universe. Credit: University of Rochester“In the original formulation, this factor was thought to be nearly 1, or 100% — that is, evolution on all planets with life would march forward and, with enough time, turn into an intelligent civilization,” Stern said. “Our perspective is: That’s not true.”
Indeed. Their math reduces the percentage of these planets that develop ACCs to just 0.003% at minimum and 0.2% at maximum – a far cry from the original 100%.
When put together with all the other factors of the Drake Equation: number of stars formed annually, number of those stars with planets, number those planets that are habitable, number of those habitable planets with life, number of civilizations on those planets sending out detectable signals, and how long they send out the signals – well, the chances of finding intelligent alien life shrink considerably.
The implications of the original Drake equation were that ACCs should be common, and we should see them everywhere. But including plate tectonics in the equation changes the result, and makes it clear that it’s perfectly understandable why we don’t see ET all across the galaxy.
So intelligent alien life might be rarer than anyone thought. And Earth may be more special than we knew. All thanks to our planet’s fragmented, unruly, and shifting upper crust.
Learn More:
Amanda Siegfried, “Geoscientists Dig into Why We May Be Alone in the Milky Way.” University of Texas at Dallas.
Robert Stern and Taras Gerya, “The importance of continents, oceans and plate tectonics for the evolution of complex life: implications for finding extraterrestrial civilizations.” Nature Scientific Reports.
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The three-body problem is one of Nature’s thorniest problems. The gravitational interactions and resulting movements of three bodies are notoriously difficult to predict because of instability. A planet orbiting two stars is an example of the three-body problem, but it’s sometimes called a “restricted three-body problem.” In that case, there are some potential stable orbits for a planet.
A new study shows that the nearby Alpha Centauri AB pair could host a Super Jupiter in a stable orbit.
The research is “Stability of the Potential Super Jupiter in Alpha Centauri System.” It’s available on the preprint site arxiv.org. The sole author is Tinglong Feng, an undergraduate at Xi’an Jiaotong University in China.
“The three-body problem, which seeks stable orbit configurations among gravitating bodies, is a longstanding challenge in celestial mechanics,” Feng writes. Feng examines ? Centauri AB, our nearest binary neighbour, to understand if the system could host a super Jupiter and what orbit the giant planet could follow.
Feng isn’t the first astronomer to tackle the problem. “As the closest triple stellar system to Earth, Alpha Centauri system has attracted diverse studies in astronomy, including exoplanet stability,” Feng writes. Though the entire Alpha Centauri system is a triple star system, ? Centauri AB are far enough from the third star that they comprise a binary system.
Size comparisons for the Alpha Centauri A and B, Proxima Centauri, and the Sun. Image Credit: Planetary Habitability Lab/UPR AreciboThere are some solutions to the three-body problem if one of the bodies has a negligible mass compared to the other two. ? Centauri AB is a pair of Sun-like stars. ? Centauri A is a class G star a little more massive than the Sun, and ? Centauri B is a class K star a little less massive than the Sun.
The study compares the ? Centauri AB system with a similar star system named GJ65AB (Gliese 65). It’s a binary pair known to host a Neptune-mass exoplanet. Though Gliese 65 is a pair of M-dwarfs, the comparison is still valuable because it “shares similar mass ratios and orbital eccentricities,” Feng writes. Gliese 65 is also close at only about 8.8 light-years from Earth. Feng also performed simulations of the ? Centauri AB system to test the idea of it hosting an exoplanet.
“The similarities between GJ65AB and Alpha Centauri AB, together with the newly detected stable super Neptune in the GJ65 system, suggest the stability of the corresponding potential super Jupiter in Alpha Centauri AB,” Feng writes. The Gliese 65 and the Alpha Centauri AB systems have nearly identical mass ratios and eccentricities. If GJ65 can host a planet in a stable orbit, can ? Centauri AB also host one?
Feng used the Mean Exponential Growth factor of Nearby Orbits (MEGNO) method to test the potential stability of a super Jupiter at ? Centauri AB. First, he used it to simulate the GJ65AB system and the newly discovered planet to verify the planet’s orbital stability. Then, he did the same with ? Centauri AB. “For this simulation, we restricted the semimajor axis of the planet to range from 0.1 to 5.0 au, and eccentricities less than 0.5,” Feng writes.
The MEGNO simulations for Gliese 65 showed that the newly discovered Neptune mass planet should be stable.
This figure from the research shows MEGNO results for Gliese 65. Dynamically stable regions of e (orbital eccentricity) and a (astronomical units) are shown in green, and the results show that the planet discovered around GJ65 should be stable. We identified the stable zone spanning from 0.1 to ~ 0.35The next step was to find stable orbits for a planet orbiting ? Centauri AB. To do that, Feng used ? Centauri A as the primary star and injected a 350 Earth-mass planet at a distance of 23.336 AU. All of the other parameters were similar to GJ65 but scaled to ? Centauri AB. “We figured out the stable zone with ?
spanning from 0.1 to ~ 2.2 au, and ? ranges from 0 to 0.5,” Feng writes.
Feng says that the “potentially stable planet” should have ? about equal to 1.189 and ? about equal to 0.33. Those numbers place the planet in the stable zone in MEGNO results.
This figure from the study is a stability map based on MEGNO values for a Jupiter-mass planet in Alpha Centauri AB. Dynamically stable regions are coloured in green. For a stable planet around ? Centauri AB to “mimic” the stability of the newly discovered Neptune planet around GJ65, the planet would have ? about equal to 1.189 and ? about equal to 0.33, which places it right in the green stability zone. Image Credit: Feng 2024.Of course, none of this means there is a planet there. It just shows that a potential stable orbit is available.
Feng’s work proposes that exoplanets in binary systems with nearly identical mass ratios and eccentricities can exhibit similar stability properties. “From this hypothesis, together with the newly detected Neptune-mass planet in the GJ65 system, which is similar to Alpha Centauri AB, we assume the existence of a potential Jupiter-mass planet with corresponding orbital parameters in Alpha Centauri AB should also be possible,” Feng writes.
No planets have been detected around ? Centauri AB, but that doesn’t mean there isn’t one there. Our planet-hunting methods are far from absolute, and there are bound to be many planets in nearby systems that we haven’t been able to detect yet.
There are many proposals for missions to the region or for telescopes designed to probe the system more deeply. Their neighbour, Proxima Centauri, has two confirmed exoplanets. And there’ve been tantalizing hints that Alpha Centauri A hosts a planet, but it remains only a candidate.
A true detection or emphatic non-detection may be years or decades away. Who knows? But at least Feng’s work shows that there could be a stable orbital home for a super Jupiter in the system.
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