The Rare Chemical Conditions That Build Earth Habitable

Researchers have revealed why only a small number of planets possess the necessary chemical ingredients for life—and highlighted Earth’s remarkable fortune. These findings have significant implications for the ongoing search for extraterrestrial life.

Essential Elements for Life

For life as we grasp it to emerge on a planet, sufficient quantities of certain chemical elements are crucial. Phosphorus and nitrogen are among the most vital. NASA research emphasizes that liquid water, energy, and nutrients are similarly essential pillars for life.

Phosphorus is fundamental for the formation of DNA and RNA, which store and transmit genetic information, and plays a critical role in cellular energy balance. Nitrogen is an indispensable component of proteins, essential for the structure and function of cells. Without these elements, the development of life from non-living matter is impossible.

The “Goldilocks Zone” for Core Formation

A new study, led by Craig Walton, a postdoctoral researcher at the Centre for Origin and Prevalence of Life at ETH Zurich, and ETH professor Maria Schönbächler, demonstrates that sufficient phosphorus and nitrogen must be present during a planet’s core formation.

“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. This precise balance is what allowed Earth to develop life.

Around 4.6 billion years ago, Earth experienced a unique stroke of chemical luck. If the amount of oxygen had been even slightly different during core formation, there wouldn’t have been enough phosphorus or nitrogen for life to develop.

How Oxygen Levels Impact Element Distribution

When planets form, they initially develop from molten rock. A sorting process occurs where heavy metals, like iron, sink to form the core, while lighter metals form the mantle and, eventually, the crust.

Too little oxygen during core formation causes phosphorus to fuse with heavy metals and descend into the core, making it unavailable for life. Conversely, too much oxygen leads to phosphorus remaining in the mantle and nitrogen escaping into the atmosphere, ultimately being lost to space.

Walton and his colleagues, through extensive modeling, found that only within a narrow range of moderate oxygen levels—a chemical “Goldilocks zone”—do both phosphorus and nitrogen remain in the mantle in sufficient quantities.

Implications for Mars and the Search for Extraterrestrial Life

The researchers also found that oxygen levels on other planets, such as Mars, fell outside this Goldilocks zone. On Mars, more phosphorus ended up in the mantle, but less nitrogen was available, creating conditions less conducive to life as we know it.

These findings suggest that the presence of water alone is not enough to determine a planet’s habitability. The amount of oxygen available during planet formation is a critical factor, potentially rendering many planets chemically unsuitable for life from the outset.

Future Directions in the Search for Life

Astronomers can indirectly measure these chemical prerequisites for life by observing other solar systems using large telescopes. The amount of oxygen present during planet formation is linked to the chemical composition of the host star. A star’s chemical structure influences the entire planetary system around it, as planets are primarily composed of the same material as their star.

solar systems with stars significantly different from our Sun are less likely to harbor life. “This makes searching for life on other planets a lot more specific. We should look for solar systems with stars that resemble our own Sun,” says Walton.

This research, published in Nature Astronomy, refines the criteria for identifying potentially habitable planets and focuses the search for life beyond Earth.