When planets form, they initially develop out of molten rock. A sorting process occurs during this time: heavy metals such as iron sink down and form the core, while lighter metals form the mantle and, later, the crust.
If there is too little oxygen present during the formation of the core, phosphorus will fuse with heavy metals such as iron and move to the core. This element is then no longer available for the development of life. On the other hand, too much oxygen present during core formation leads to phosphorus remaining in the mantle and nitrogen being more likely to escape into the atmosphere, ultimately being lost.
Walton and his co-authors demonstrated through numerous modeling that only in an exceptionally narrow range of medium-level oxygen conditions—known as a chemical Goldilocks zone—will both phosphorus and nitrogen remain in the mantle in sufficient quantities.
“Our models clearly show that 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,” says Walton.
Why only a small number of planets are suitable for life
Another lottery win for life. The numbers of things like this for there to be life on the planet just keeps increasing.