Plate Tectonics And CO2 On Planets Suggest Alien Civilizations “Are Probably Pretty Rare”

Earth is pretty special, and as far as we know, the only place in the universe with life. But how special is it really? There are billions upon billions of planets in the Milky Way alone; shouldn’t life be common? Maybe, but even if life is common, how common are advanced civilizations? Some new work argues that the answer is: pretty rare.

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The new research comes from Dr Manuel Scherf and Professor Helmut Lammer of the Space Research Institute at the Austrian Academy of Sciences in Graz. They considered the composition of the atmosphere and the geology of a planet and came to a concerning realization. Statistically, there might be just a handful of civilizations in the galaxy. The closest might be 33,000 light-years away, and that civilization is at least 280,000 years, if not millions of years old. 

"Extraterrestrial intelligences – ETIs – in our galaxy are probably pretty rare," Scherf said in a statement.

Their reasoning is this. If Earth is a reasonable model for intelligent life, a planet needs to be in the habitable zone, start with a significant amount of carbon dioxide to build a biosphere, and then get enough oxygen for technology. Crucial to the reduction of carbon dioxide are life and plate tectonics, which can sequester the carbon inside the planet over eons.

On Earth, it took over 3 billion years for intelligent life to evolve. The planet went from having at least 6 percent carbon dioxide in its atmosphere to a fraction of a percent, as more was trapped by tectonic plates. Maybe in one billion years, there won’t be enough to make photosynthesis happen. 

Oxygen went up dramatically thanks to photosynthetic organisms, reaching today’s level of 21 percent in an atmosphere dominated by nitrogen (78 percent). But for the smelting of metal, assuming that to be a crucial part of technology development, it can’t be done if oxygen is below 18 percent.

A planet with an atmosphere that starts at 10 percent carbon dioxide could have a stable biosphere for 4.2 billion years. At 1 percent, for about 3 billion years. Then you consider how long intelligent life takes to evolve and how long a civilization lasts. Statistically, this data can tell us that if we want two civilizations to be alive at the same time, they would need to exist for at least 280,000 years.

"For 10 civilisations to exist at the same time as ours, the average lifetime must be above 10 million years," says Scherf. "The numbers of ETIs are pretty low and depend strongly upon the lifetime of a civilisation."

If this work is correct, does this mean we shouldn’t look for them? The researchers think quite the opposite.

"Although ETIs might be rare there is only one way to really find out and that is by searching for it," says 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."

The study was presented at the EPSC–DPS2025 Joint Meeting in Helsinki last week.