A world can look promising from afar and still be missing the chemical ingredients that biology depends on. Two of the most critical are phosphorus and nitrogen, and they act like gatekeepers for life. Phosphorus is built into DNA and RNA, the molecules that store and pass along genetic information, and it also helps cells manage energy. Nitrogen is a core ingredient in proteins, which living things rely on to build cells and keep them working.

What makes these elements especially interesting is that a planet can lose access to them long before its surface becomes stable. A study led by Craig Walton, a postdoctoral researcher at the Centre for Origin and Prevalence of Life at ETH Zurich, together with ETH professor Maria Schönbächler, found that phosphorus and nitrogen must be available at the moment a planet forms its core.

“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,” explains Walton, lead author of the study.

Earth appears to have hit that chemical balance around 4.6 billion years ago, which may help explain why it ended up with the raw materials life needs. The result could reshape how scientists judge the chances for life elsewhere in the universe.

Core formation as a form of cosmic roulette
Young rocky planets begin as roiling oceans of molten rock. As gravity pulls materials into layers, dense metals such as iron sink inward to form the core, while lighter material remains above to become the mantle and, later, the crust. That physical separation is only half the story. At the same time, chemistry is deciding which elements prefer metal and which prefer rock, and oxygen is one of the biggest drivers of that choice.