The reason why the Earth has become a planet that nurtures life may be due to the fact that it reached a delicate chemical balance just right at the beginning of its birth, so that the key life elements were neither sealed deep in the core of the planet nor escaped into space. A new study led by a research team from the Swiss Federal Institute of Technology in Zurich (ETH Zurich) points out that liquid water alone is far from enough. In the early stages of forming a metallic core, a planet must meet extremely narrow oxygen-containing conditions in order to retain phosphorus and nitrogen that are critical to life.
In the distant starry sky, a planet may seem to have a suitable temperature and may have an ocean on its surface, but without a suitable chemical "recipe", life will still be difficult to emerge. The researchers emphasized that phosphorus and nitrogen play the role of "gatekeepers" in living systems: phosphorus is an important component of genetic information carrier molecules and cellular energy molecules, while nitrogen is the core element of protein and is related to the construction and maintenance of cell structure and function. More importantly, whether these two elements can be available on the planet's surface for a long time has been "predetermined" to a large extent when the planet is still in the hot molten stage and the core is still being formed.
The research was led by postdoctoral researcher Craig Walton and professor Maria Schönbächler at the Center for the Origin of Life and Universality at ETH Zurich. They found that the key to preserving phosphorus and nitrogen near a planet's surface is that the oxygen content during the formation of the planet's metallic core must be controlled within extremely precise limits. Walton pointed out that if there is too little oxygen at this stage, phosphorus will tend to combine with heavy metals such as iron and sink into the core, thereby almost "disappearing" from the surface environment; if there is too much oxygen, although phosphorus can remain in the mantle, nitrogen will more easily escape into the atmosphere and eventually be lost to space. In other words, conditions that protect one element are likely to make another element scarce, making it difficult to have both.
To quantify this chemical equilibrium window, the research team used a number of computer models to simulate the partitioning behavior of elements between metals and rocks under different oxygen contents. The results show that only within an extremely narrow range of intermediate oxidation states can both phosphorus and nitrogen remain in the mantle in abundance suitable for the evolution of life. Researchers call this condition a "chemical version of the Goldilocks zone" - it must be neither too "oxygen-deficient" nor too "oxygen-rich," but it must be just right. Walton said the model results clearly show that the Earth falls within this narrow window: If the oxygen content was slightly higher or lower, there may not be enough phosphorus or nitrogen for life on Earth, making the emergence of life extremely unlikely.
The research also shows that other Earth-like planets under different formation conditions may not be so "lucky". In the case of Mars, simulations show that oxygen levels at the time of its formation fell outside this chemical "habitable zone." So while the Martian mantle may retain more phosphorus than Earth's, it's also significantly deficient in nitrogen, a combination that means it's not hospitable to life as we understand it. From this perspective, the reason why Mars has difficulty maintaining a stable and rich biosphere is not just because of the lack of climate and water. Its deep chemical conditions are unfavorable to life from the beginning.
This discovery is quietly changing the scientific community's strategy for searching for extraterrestrial life. In the past, when people evaluated whether an exoplanet might have life, they often regarded "whether there is liquid water" as the primary criterion. As long as the planet is located in the "habitable zone" of the star and the temperature allows water to remain liquid, it is regarded as a potential cradle of life. However, Walton and Schönbeckler pointed out that this standard is far from enough, because if the oxygen-containing conditions are not suitable during the core formation stage, many planets are chemically incapable of harboring life from the beginning, even if they have oceans on their surfaces and suitable temperatures.
It is worth noting that these chemical prerequisites are not completely unobservable. Astronomers can use large telescopes to indirectly infer the oxygen content and overall chemical composition of planets when they formed by observing the spectra of other stars and planetary systems. A planet's "raw material recipe" is largely determined by its parent star, since planets are mostly made of the same stuff as the star. Therefore, if a star in a planetary system has a very different distribution of chemical elements from the Sun, then the chance that the planet in that system has a suitable balance of phosphorus and nitrogen is greatly reduced, making it an ideal target for the search for life.
Walton said this research makes the search for extraterrestrial life more specific and focused. Rather than casting a net across the universe to find all planets in the traditional habitable zone, it would be better to prioritize those star systems whose parent stars are chemically similar to the sun. In these systems, planets have a higher probability of obtaining Earth-like oxygen conditions at birth and retaining sufficient amounts of phosphorus and nitrogen, making life more likely.
The related paper was titled "The Core Formation Process Determines the Chemical Habitability of the Earth and Earth-like Planets" (tentative translation) and was published in "Nature Astronomy" on February 9, 2026. The research team believes that with the continuous advancement of observation technology, humans are expected to not only detect water and atmosphere on exoplanets in the next few decades, but also further determine whether these worlds have the "underground" of life similar to Earth at the chemical level.