🧬 This new habitability criterion explains life on Earth and its absence on Mars

For decades, scientists have sought to understand how one planet can host life and another cannot. A study published in Nature Astronomy provides a clue: habitability would be conditioned by a delicate chemical equilibrium established during planetary formation, long before the appearance of oceans or a stable atmosphere. This discovery contradicts the idea that the presence of liquid water constitutes the main condition for the emergence of life.

The key elements for life, such as phosphorus and nitrogen, are essential for biological function. Phosphorus, for example, is found in DNA and RNA, the molecules that carry genetic information, and helps cells manage their energy. Nitrogen, on the other hand, is a central component of proteins, necessary for building and maintaining cells. Their availability on a planet's surface is therefore critical, and it can be compromised from the very first moments of planetary formation.


During the formation of a rocky planet, the core is built by the separation of materials under the effect of gravity. Dense metals like iron sink toward the center, while lighter elements form the mantle and crust. Craig Walton, a researcher at ETH Zurich, and his team discovered that the amount of oxygen present during this phase is decisive. With too little oxygen, phosphorus binds to iron and sinks into the core; with too much oxygen, nitrogen becomes volatile and is lost. Thus, a precise balance is needed to retain both of these elements at the surface, where life can emerge.

Earth seems to have benefited from ideal conditions about 4.6 billion years ago, falling into a narrow range where both phosphorus and nitrogen are available. In contrast, Mars did not have this luck. According to computer models, oxygen levels during its formation were outside this favorable zone. Mars has more phosphorus in its mantle than Earth, but much less nitrogen, making its environment less conducive to the development of life as we know it.

This discovery changes the way scientists search for life in the Universe. Until now, the presence of water was the main criterion. However, even with water, a planet can be chemically unsuitable if essential elements like phosphorus and nitrogen are not accessible. Therefore, the chemical composition during formation must be considered, which adds a new dimension to habitability studies.

To verify these conditions remotely, astronomers can estimate these parameters by observing stars. The chemical composition of a star is an indicator of that of the planets forming around it, as they generally share the same materials. Thus, to find habitable planets, it becomes relevant to target solar systems with stars similar in composition to our Sun, as Walton indicates.

This approach makes it possible to narrow the search to more promising environments, although this is not the whole story, as can be seen with the differences between Earth and Mars, yet located in the same system.

By better understanding the chemical processes at work during planetary formation, scientists can thus refine their strategies for detecting habitable worlds. The study of our own Solar System, with its very different planets, serves as a valuable reference point in this quest.