Why nuclear reactions on an exoplanet won’t imply alien life | by Ethan Siegel | Starts With A Bang! | Apr, 2025

Planets can create nuclear power on their own, naturally, without any intelligence or technology. Earth already did: 1.7 billion years ago.

If you were hunting for alien intelligence, looking for a surefire signature from across the Universe of their activity, you’d have a few options.

• You could look for an intelligent radio broadcast, like the type humans began emitting in the 20th century.
• You could look for examples of planet-wide modifications, like human civilization displays when you view Earth at a high-enough resolution.
• You could look for artificial illumination at night, like our cities, towns, and fisheries display, visible from space.

And there are many other options: you can look for a unique chemical “fingerprint” of biological processes in an exoplanet’s atmosphere by performing spectroscopy on the planet’s atmosphere, or rapid long-term evolution of certain chemical species. After all, there’s a tremendous amount you can learn even from a single pixel if you can directly image an Earth-sized exoplanet.

But there are technological achievements that we’ve realized here on Earth as human civilization has…