Researchers at ETH Zurich have discovered that Earth formed with just the right amount of oxygen during its core development, keeping essential phosphorus and nitrogen accessible for life. Too much or too little oxygen would have trapped or lost these elements. The finding highlights a chemical 'Goldilocks zone' critical for habitability.
Planets start as molten rock, where materials separate by density. Heavy metals like iron sink to form the core, while lighter substances create the mantle and crust. Oxygen levels at this stage prove decisive for phosphorus and nitrogen availability, according to Craig Walton, a postdoc at ETH Zurich's Centre for Origin and Prevalence of Life, and professor Maria Schönbächler. About 4.6 billion years ago, Earth hit the perfect balance, Walton explained: '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.' Phosphorus aids DNA, RNA, and cellular energy, while nitrogen forms proteins essential for life. Models by Walton and colleagues, published in Nature Astronomy, pinpoint a narrow moderate-oxygen range—termed a chemical Goldilocks zone—where both elements stay in the mantle in sufficient quantities. Walton added: 'Our models clearly show that the Earth is precisely within this range. If we had had just a little more or a little less oxygen during core formation, there would not have been enough phosphorus or nitrogen for the development of life.' Mars, by contrast, fell outside this zone, retaining more phosphorus but less nitrogen, complicating life prospects there. The study urges shifting exoplanet habitability assessments beyond water presence. Planets must retain these elements from core formation onward. Walton noted that stellar composition influences oxygen levels, as planets form from similar material. 'This makes searching for life on other planets a lot more specific. We should look for solar systems with stars that resemble our own Sun,' he said.