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.