Your Alien Neighbors Live 33,000 Light Years Away

The search for extraterrestrial intelligence just got a reality check. According to new research presented at the EPSC-DPS2025 Joint Meeting in Helsinki, the closest technological alien civilization to Earth is likely sitting about 33,000 light years away – roughly on the opposite side of our galaxy.

Even more sobering: that alien society would need to be at least 280,000 years old, and possibly millions of years old, to exist at the same time we do.

These calculations emerge from a detailed analysis of what it actually takes for a planet to support technological life. Dr. Manuel Scherf and Professor Helmut Lammer from the Space Research Institute at the Austrian Academy of Sciences didn’t just consider whether planets could host life – they dug into the specific atmospheric chemistry and geological processes needed for civilizations capable of interstellar communication.

The Goldilocks Problem Gets More Complicated

The researchers focused on a atmospheric component often overlooked in discussions about alien worlds: carbon dioxide levels. While too much CO2 creates a runaway greenhouse effect (think Venus), too little means photosynthesis eventually shuts down entirely.

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

Earth’s atmosphere contains just 0.042 percent carbon dioxide, balanced against 78 percent nitrogen and 21 percent oxygen. But Scherf and Lammer modeled planets with higher CO2 concentrations – up to 10 percent – positioned farther from their stars to avoid overheating. These worlds could maintain photosynthesis for 4.2 billion years, compared to Earth’s remaining billion-year window.

The oxygen requirement presents another bottleneck. Complex technological life needs at least 18 percent atmospheric oxygen – not just for breathing, but for fire. Below that threshold, open-air combustion becomes impossible, making metal smelting and advanced technology unfeasible.

But here’s where plate tectonics enters the equation as the ultimate planetary requirement. Without moving continental plates to regulate atmospheric CO2 through the carbon-silicate cycle, planets can’t maintain the delicate atmospheric balance needed for long-term habitability. Most rocky worlds lack this geological engine entirely.

The Longevity Challenge

Even planets that meet these atmospheric and geological requirements face a timing problem. On Earth, technological civilization emerged after 4.5 billion years of evolution. For alien civilizations to overlap with humanity’s brief existence, they need extraordinary longevity.

The mathematics are stark. For just one other technological civilization to exist simultaneously with ours in a galaxy of 400 billion stars, that alien society must survive at least 280,000 years after achieving interstellar communication capability.

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

This longevity requirement explains why any detected extraterrestrial intelligence would almost certainly be much older than human civilization. We’re likely the cosmic equivalent of mayflies compared to our potential galactic neighbors.

The 33,000 light-year distance estimate assumes these civilizations are distributed randomly throughout the Milky Way’s habitable zones. Since our solar system sits about 27,000 light years from the galactic center, the nearest technological neighbors could be on the galaxy’s far side.

Scherf acknowledges these calculations represent maximums rather than certainties. The analysis doesn’t quantify other crucial factors like the probability of life’s origin, the development of photosynthesis, multicellular evolution, or the likelihood that intelligence develops technology. Each additional filter could make alien civilizations even rarer – or potentially more common if these transitions prove highly probable.

The implications for SETI programs are sobering but not discouraging. Radio telescopes scanning for alien signals face astronomical odds, but the potential payoff remains unprecedented.

Despite the daunting numbers, Scherf advocates continuing the search with characteristic scientific optimism: “Although ETIs might be rare there is only one way to really find out and that is by searching for it. 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 research suggests that if alien civilizations exist, they represent some of the universe’s most precious and ancient achievements – technological societies that have somehow survived for hundreds of millennia or longer while maintaining their ability to communicate across the cosmos.

EPSC-DPS Joint Meeting: 10.5194/epsc-dps2025-1512

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