Life needs more than water and the right temperature to exist.
New research suggests that only a small number of planets ever get the chemical conditions needed for life—and Earth may be one of the rare winners of a cosmic lottery.
For life to emerge from nonliving matter, certain elements must be available in sufficient amounts.
Two of the most important are phosphorus and nitrogen. Phosphorus is essential for DNA and RNA, which store and pass on genetic information, and it also plays a key role in how cells manage energy.
Nitrogen is a core building block of proteins, which are necessary for the structure and function of all living cells. Without these elements, life as we know it cannot exist.
A new study led by Craig Walton, a postdoctoral researcher at the Center for Origin and Prevalence of Life at ETH Zurich, together with ETH Zurich professor Maria Schönbächler, shows that the availability of phosphorus and nitrogen depends on a very specific moment in a planet’s early history.
The research was published in Nature Astronomy.
The critical moment occurs when a planet forms its core. Early in a planet’s life, it is largely molten. Heavy elements such as iron sink inward to form the core, while lighter materials remain in the mantle and crust.
Whether phosphorus and nitrogen end up where life can use them depends on how much oxygen is present during this process.
If there is too little oxygen, phosphorus bonds with iron and sinks into the core, permanently removing it from the planet’s surface environment. If there is too much oxygen, phosphorus stays in the mantle, but nitrogen is more likely to escape into the atmosphere and eventually be lost to space. In both cases, life would struggle to get started.
Using detailed computer models, the researchers found that only a very narrow range of oxygen levels allows both phosphorus and nitrogen to remain in the mantle in usable amounts. Walton describes this as a chemical “Goldilocks zone”—not too much oxygen and not too little, but just the right amount.
Earth, it turns out, formed squarely within this narrow window around 4.6 billion years ago. According to the study, even a small difference in oxygen levels during Earth’s core formation would have left the planet without enough of these vital elements to support life. Other planets, such as Mars, appear to have formed outside this chemical sweet spot, leaving them poorer in phosphorus or nitrogen.
These findings could reshape how scientists search for life beyond Earth. Until now, the focus has largely been on finding planets with liquid water. The new study suggests that water alone is not enough. A planet may look promising on the surface, yet still be chemically unsuitable for life from the start.
The researchers also point out that a planet’s chemistry is linked to its parent star. Planets form from the same material as their stars, so stars with chemical compositions very different from the Sun are less likely to host life-friendly planets. As a result, the search for life elsewhere may need to focus more narrowly on solar systems that closely resemble our own.
In short, Earth’s ability to host life may come down to a rare and delicate chemical balance—one that few planets ever achieve.
