These findings highlight the overwhelming odds against discovering Earth-like planets that possess both plate tectonics and a nitrogen-oxygen atmosphere containing the right balance of oxygen and carbon dioxide.

Taking these planetary requirements into account, the chances of success for SETI (Search for Extraterrestrial Intelligence) appear slim, say Dr. Manuel Scherf and Professor Helmut Lammer of the Space Research Institute at the Austrian Academy of Sciences in Graz.

"Extraterrestrial intelligences, ETIs, in our galaxy are probably pretty rare," says Scherf.

A planet's carbon dioxide level plays a key role in sustaining life. Higher levels of CO₂ help maintain photosynthesis and keep the atmosphere from leaking into space, but too much can trigger a runaway greenhouse effect or make the air toxic. Plate tectonics are essential because they regulate carbon dioxide through the carbon-silicate cycle, recycling the gas between the atmosphere and the planet's crust. Over time, however, carbon dioxide becomes trapped in rocks and is no longer returned to the atmosphere.

"At some point enough carbon dioxide will be drawn from the atmosphere so that photosynthesis will stop working," says Scherf. "For the Earth, that's expected to happen in about 200 million to roughly one billion years."

Today, Earth's atmosphere consists mostly of nitrogen (78 percent) and oxygen (21 percent), with only a trace of carbon dioxide (0.042 percent). Scherf and Lammer modeled what would happen on other worlds. A planet with ten percent carbon dioxide -- if located farther from its sun or orbiting a dimmer, younger star -- could support a biosphere for up to 4.2 billion years. By comparison, a planet with one percent carbon dioxide would remain habitable for about 3.1 billion years.

For advanced life to emerge, such planets would also need at least 18 percent oxygen. Complex animals require higher oxygen levels, and earlier studies have shown that if oxygen drops below this threshold, there would not be enough free oxygen for open-air combustion. Without fire, metalworking would be impossible, preventing the rise of any technological civilization.

Scherf and Lammer compared these potential biosphere lifetimes with how long it took life on Earth to evolve technology -- about 4.5 billion years -- and with the expected longevity of intelligent species. The longer a civilization endures, the higher the likelihood that it overlaps in time with another.

From these calculations, the researchers concluded that a technological species on a planet with ten percent carbon dioxide would need to persist for at least 280,000 years for even one other civilization to exist in the Milky Way at the same time as ours.

"For ten civilizations 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 civilization."

This means that if we do detect an ETI, it is almost certainly going to be much older than humanity.

It's these numbers that also lead to the estimate that the next closest technological civilization is about 33,000 light years away. Our Sun is about 27,000 light years from the galactic center, which means that the next closest technological civilization to our own could be on the other side of the Milky Way.

These numbers are not absolutes - Scherf points out that there are other factors that should be included, such as the origin of life, the origin of photosynthesis, the origin of multi-cellular life and the frequency with which intelligent life develops technology, but they cannot be quantified at present. If each of these factors has a high probability, then ETIs might not be as rare. If each of these factors has a low probability, then a more pessimistic outlook is required.

Nevertheless, Scherf strongly believes that SETI should continue the search.

"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."

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