Earth may owe its biological success to a cosmic chemistry lottery that occurred during its infancy. A new study from ETH Zurich suggests that the planet’s habitability was determined by oxygen levels during its formation, which dictated whether essential life-building elements remained on the surface or were lost to the core.
Phosphorus and nitrogen are fundamental to life as we know it. Phosphorus is a building block for DNA and RNA, while nitrogen is critical for protein synthesis. Without these elements, life cannot emerge from nonliving matter.
The Goldilocks zone for life
Lead author Craig Walton and professor Maria Schönbächler found that these ingredients were only preserved if oxygen levels remained within a very narrow, moderate range 4.6 billion years ago. If oxygen levels were too low, phosphorus bonded with iron and sank into the planet’s core, effectively locking it away from the surface. If oxygen levels were too high, nitrogen escaped into the atmosphere and was lost.
"During the formation of a planet's core, there needs to be exactly the right amount of oxygen present so that phosphorus and nitrogen can remain on the surface of the planet," Walton said. "Our models clearly show that the Earth is precisely within this range."
The researchers used extensive modeling to compare Earth’s history with other planetary bodies. They determined that planets like Mars formed outside this specific chemical window, resulting in conditions that make the emergence of life significantly more difficult.
These findings shift the focus of astrobiology. For years, the search for life beyond Earth has prioritized the presence of water as the primary indicator of habitability. The ETH Zurich team argues that water is not enough; a planet must also have undergone the correct chemical sorting during its birth.
Because a planet’s chemical composition is largely inherited from its host star, the team suggests that astronomers should refine their search for life. By identifying solar systems with stars similar to our Sun, scientists can better predict which planets might have inherited the necessary chemical balance to support life.
"This makes searching for life on other planets a lot more specific," Walton said. "We should look for solar systems with stars that resemble our own Sun."
