Life cannot form on Earth unless certain chemical elements are available in sufficient quantities. Two of the most important are phosphorus and nitrogen. Phosphorus helps build DNA and RNA, which store and transmit genetic information, and also plays an important role in how cells manage energy. Nitrogen is a major part of proteins and is essential for building cells and helping them function. Without sufficient phosphorus and nitrogen, life cannot emerge from inanimate matter.
The new study, led by Craig Walton, a postdoctoral researcher at the Center for the Origin and Spread of Life at ETH Zurich, and Maria Schönbechler, a professor at ETH Zurich, shows that these elements must already be available in adequate amounts when a planet’s core forms. “During the formation of the planet’s core, exactly the right amount of oxygen needs to be present for the phosphorus and nitrogen to remain on the planet’s surface,” explains Walton, lead author of the study. On Earth, that appears to have happened about 4.6 billion years ago, giving our planet a very lucky chemical starting point. The results could have implications for how scientists search for extraterrestrial life.
Impact of planetary core formation on habitability
Planets begin as masses of molten rock. Once formed, the material separates by weight. Heavy metals such as iron sink into the interior and form the core, while lighter materials remain at the top, eventually becoming the mantle and later the crust.
Oxygen levels are important at this stage. If there is too little oxygen during core formation, phosphorus combines with heavy metals such as iron and is drawn into the core. When that happens, it becomes unavailable to areas on Earth where life could occur. When there is too much oxygen, phosphorus remains in the mantle, but nitrogen escapes into the atmosphere and is more likely to be lost.
chemical goldilocks zone
Using extensive modeling, Walton and his co-authors found that both phosphorus and nitrogen remain in sufficient quantities in the mantle only within a very narrow range of moderate oxygen conditions. They call this the chemical Goldilocks zone.
“Our model clearly shows that Earth is exactly within this range. If there had been a little more oxygen or a little less oxygen during nucleation, there would not have been enough phosphorus or nitrogen for life to develop,” Walton says.
The researchers also discovered that other planets, including Mars, formed under oxygen conditions outside this Goldilocks zone. On Mars, that means there is more phosphorus in the mantle than on Earth, but less nitrogen, creating difficult conditions for life as we know it.
A new way to search for life beyond Earth
These discoveries could change the way scientists think about habitability. So far, the focus has been on whether the planet has water. Walton and Schönbächler argue that this is not enough.
Even if a planet has water, it may not be chemically suitable for life to exist in the first place. If oxygen levels were wrong during core formation, the planet might not have retained enough phosphorus and nitrogen for life to use.
Why are Sun-like stars the most important?
Astronomers may be able to estimate these chemical states by studying other solar systems with large telescopes. The availability of oxygen during planet formation depends on the chemical composition of the host star. Because planets are formed primarily from the same material as stars, the composition of stars helps shape the chemistry of the entire planetary system.
This means that solar systems with significantly different chemical properties than our own may be poor candidates in the search for life. “This makes the search for life on other planets more concrete. We need to look for solar systems with stars similar to our sun,” Walton says.
