Earth could easily have remained a lifeless, rocky planet if a very narrow combination of conditions hadn’t aligned during its formation. A new study shows that the key factors weren’t just distance from the Sun or the presence of water, but the precise balance of oxygen during core formation. Thanks to this balance, phosphorus and nitrogen—two elements essential for life as we know it—remained in the crust and mantle rather than sinking into the core or escaping into space.

The research was published on February 9, 2026, in Nature Astronomy.

The Oxygen “Goldilocks Zone”

Lead author Craig Walton from ETH Zürich explained the finding:

“During the formation of a planet’s core, the amount of oxygen has to be just right so that phosphorus and nitrogen remain near the surface.”

  • Too little oxygen: Phosphorus binds with iron and sinks into the core. Without phosphorus, DNA, cell membranes, and ATP molecules cannot form—essentially, life’s energy system fails.
  • Too much oxygen: Nitrogen is more likely to escape into space. Without nitrogen, there are no proteins, nucleic acids, or most organic molecules.

Modeling revealed that Earth landed in a surprisingly narrow “chemical Goldilocks zone” for oxygen, where both phosphorus and nitrogen remained in sufficient amounts in the crust and mantle. Walton emphasized:

“Our models clearly show that Earth sits exactly in this range. Slightly more or less oxygen during core formation, and there would not have been enough phosphorus or nitrogen to support life.”

Comparison with Mars

To test the model, researchers applied it to Mars. The results matched real-world observations: Mars had too much oxygen, leaving plenty of phosphorus in the crust but catastrophically little nitrogen. This is one reason why Mars, despite evidence of ancient water, never became habitable in the Earth-like sense.

Stars Decide Planetary Fate

A major takeaway is that a planet’s chemical composition depends directly on its parent star, since planets form from the same material. Systems with Sun-like stars (G-type) with similar oxygen and elemental compositions are far more likely to produce planets with the right chemical balance for life.

“This makes the search for life on exoplanets much more specific,” Walton said. “We should focus on star systems with stars similar to our Sun.”

Changing the Search for Life

Traditionally, astronomers have searched for planets in the “habitable zone,” where liquid water could exist. Now it’s clear that water is only part of the equation. A planet could be at the perfect distance, with oceans and an atmosphere, but if its crust lacks phosphorus or nitrogen, life simply cannot begin.

Earth, in this view, is not a typical habitable planet—it is a lucky chemical exception, a world that hit a rare elemental jackpot early in its history.

In Brief

A study in Nature Astronomy shows that Earth retained the key elements for life—phosphorus and nitrogen—only because the oxygen level during core formation 4.6 billion years ago was in an extremely narrow range. Slight deviations would have sent phosphorus into the core or allowed nitrogen to escape into space. Models applied to Mars confirm the theory. The implication: searching for habitable exoplanets should focus on systems with Sun-like stars. Life on Earth is not the norm, but a rare chemical stroke of luck.