Aliens haven't contacted us. Scientists found a compelling reason why.

The universe may teem with tiny alien microbes.

After all, scientists suspect such primitive organisms could even dwell nearby on other worlds in our very solar system — in briny oceans beneath shells of ice. We can't be sure, of course. Microbes can't beam us any messages. (Though we have plans to look for them.)

But scientists have been getting a clearer picture of why no far-off intelligent civilizations — among the trillions and trillions of planets in the universe — have called us, or why we haven't picked up even a hint of their existence. A compelling new idea, published in the journal Scientific Reports, shows how challenging it would be for a planet to gradually evolve intelligent, communicating life. Such a world, they argue, would need both oceans and continents, and the surface must be in geologic motion (which we call "plate tectonics") for at least some 500 million years.

When other factors are considered — such as the fraction of hospitable planets that host any life at all and how long a signal-emitting civilization might last — the possibility of many active, communicating civilizations in space looks implausible.

"It's like winning the lottery," Taras Gerya, a geophysicist at the research university ETH Zurich in Switzerland and an author of the study, told Mashable. "It can be so rare that we don't have much of a chance to be contacted," added Gerya, who coauthored the study with Robert Stern, a geoscientist at the University of Texas at Dallas.

Estimates of the number of advanced communicating civilizations in our galaxy range widely, but are generally high. They all use an elegant, simple formula, called the Drake Equation (shown below), to guide the prediction. The Drake equation itself, created by astronomer Frank Drake in 1961, doesn't predict anything — but calculates an answer based on several inputs. A recent proposal suggests there should be at least 36 civilizations in the Milky Way galaxy. Drake surmised there could be some 10,000 civilizations scattered around our galaxy — which would make for a bustling, Star Wars-like realm.

"It's like winning the lottery."

The new research estimates, however, are considerably lower.

"It can be as low as only four out of 10,000 galaxies having one civilization," Gerya said. "But having one or two per galaxy is still not impossible," he added.

Indeed, this galaxy has at least one. But other galaxies might not be so lucky.

An artist's conception of the exoplanet Kepler-186f, an Earth-size world 500 light-years away orbiting in its solar system's "habitable zone," meaning the planet could host liquid water.

The scarcity of alien civilizations

It's not unusual for experts to suspect that communicating civilizations are scarce. Pascal Lee, a senior planetary scientist at the SETI Institute, an organization researching the origins and prevalence of life in the universe, thinks the number of intelligent, communicating civilizations in our galaxy, which NASA estimates has some 100 to 400 billion stars (and many more planets), is around one.

"It's not too surprising that we could be alone in our galaxy," Lee, who had no role in the new research, told Mashable.

Both Lee and the new study have separately focused on a specific component of the Drake Equation, which they find dramatically lowers the likelihood of a communicating civilization: It's the variable "fi" (pronounced "f sub i"), which SETI describes as the fraction of life-bearing planets on which intelligent life emerges.

Here's the Drake equation with the important variable fi:

N = R? × fp × ne × fl × fi × fc × L

• N = the number of civilizations in the Milky Way galaxy that can communicate
• R? = the average rate of star formation in our galaxy
• fp = the fraction of those stars with planets
• ne = the number of planets around each star harboring suitable environments for life
• fl = the fraction of planets where any life emerges
• fi = the fraction of planets with life that develops intelligent life / civilizations
• fc = the fraction of civilizations with technology to emit detectable signs of their existence
• L = the average length of time a civilization produces signs of their existence

What made Earth special, the new research argues, isn't just that it harbored the relatively pleasant conditions for temperate oceans to exist among sprawling continents. The outer part of Earth also operated geologically for hundreds of millions of years — meaning the tectonic plates that make up our planet's crust gradually moved, altering the continents and seas. These conditions stoked the biological evolution that eventually spawned our civilization, the researchers say.

But these planetary factors occurring together, which they propose make up the fraction of planets that develop civilizations (fi), is probably an extreme rarity (for example, it's probably uncommon for a planet to have the right recipe of materials, size, and gravity for plate tectonics to occur). The researchers estimate it's a range between under?0.00003 to 0.002 percent of all planets. That means, quite optimistically, two out of 1,000 planets where life actually emerges might develop a civilization.

A deep view of the cosmos captured by the James Webb Space Telescope. Every object, except for the six-pointed foreground stars, is an entire galaxy.

Why might shifting plates on Earth's surface, such as the Pacific and North American plates, be so critical to the evolution of advanced communicating civilizations? Imagine two planets with large continents and oceans: one with a billion years of plate tectonics, and one without. On the planet with no movement, like Mars when it harbored seas, the land masses do not move. Everything is largely static. "As a result, life stays as is or evolves very slowly," Gerya explains. "You're happy as is, so why should you evolve?"

Yet dramatic changes in continents — and their coastlines, climates, and beyond — drive species to evolve. "That's what plate tectonics does," Gerya emphasized. "It's always pushing complex species to change." On Earth, during the Neoproterozoic Era (around 1 billion to 540 million years ago), modern plate tectonics "dramatically accelerated" the emergence of complex species, the authors argue. "Species are pushed toward adaptations or dispersal," Gerya said. "It creates a very dynamic situation."

After 3 billion years of microbes dominating Earth, the first fossil record of animals appears during this period. The seas teemed with life. The first dinosaurs emerged. Sharks, reptiles, insects, birds, and mammals came into existence.

"That's what plate tectonics does. It's always pushing complex species to change."

This was the start of the long, uncertain highway towards a civilization. We arrived, but not long ago: human civilizations have only been around for some 5,500 years.

Yet the long-lived tectonics argument is far from the only thing, or things, that might have allowed rare intelligent life to eventually blossom on Earth.

"The problem is we don't know what factually allowed life to really emerge and become intelligent," said SETI's Lee, emphasizing there are a number of intriguing possibilities.

Plumes of water ice shooting out of Saturn's moon Enceladus. The briny ocean below could potentially sustain life.

A giant asteroid wiping out most dinosaurs (but not the avian ones!) may have helped the human cause. In the aftermath of the cataclysmic event, mammals and burrowing animals could now dominate a surface largely devoid of dinosaurs. "They became the apex predators on the surface of Earth, and eventually led to us," Lee noted. Some have made the argument that our unusually large moon (relative to Earth) is responsible for creating the balanced environs needed for life to thrive. The gravitationally-influential moon stabilized our world's chaotic spin, so it would no longer wobble dramatically over time, like on Mars, igniting climate chaos. Or, perhaps, the reason it took so long for intelligent life to evolve on Earth was simply because for eons there was no evolutionary need for life to become smart enough to build interstellar-voyaging craft and beam signals through space.

"Dinosaurs never needed to come up with a radio telescope or an FM radio," Lee said. (Dinosaurs persisted for 165 million years. Our species has been around for some 300,000 years, and only invented the lightbulb 145 years ago.)

"Dinosaurs never needed to come up with a radio telescope or an FM radio."

Indeed, it might be that our world is exceptional. Perhaps, on average, only one communicating civilization exists in a galaxy at any one time. Or four for every 10,000 galaxies. Or somewhere in between. The only certainty we have is a communicating civilization, for all of its woes and flaws, exists on Earth. And it's achieved great things.

"It would be a big loss for the universe if our civilization failed," Gerya said.