Credit: NASA/Ames/JPL-Caltech
For life to develop on a planet, two chemical elements are needed in sufficient quantities: phosphorus and nitrogen. Phosphorus is vital for the formation of DNA and RNA. Nitrogen is an essential component of proteins. Without these two elements, life can’t develop out of matter.
A study published in Nature Astronomy, and led by Craig Walton, postdoc at the Centre for Origin and Prevalence of Life at the Federal Institute of Technology (ETH) Zurich, and ETH professor Maria Schönbächler has now shown that there must be sufficient phosphorus and nitrogen present when a planet’s core is formed. “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. That amount was present when Earth was forming around 4.6 billion years ago.
The core is the key
Earth-like planets form out of molten rock and metal. The heavier metals, such as iron and nickel, sink down and form the core. Lighter metals form the mantle, the layer above the core. The lightest metals make up the crust, a planet’s outermost layer.
If there is too little oxygen present at this time, phosphorus will fuse with heavy metals such as iron and become locked up in the core. Phosphorus is then no longer available to form DNA or RNA. But if too much oxygen is present when the core forms, that leads to phosphorus remaining in the mantle and nitrogen escaping into the atmosphere, ultimately being lost.
The chemical Goldilocks zone
Walton and his co-authors demonstrated through computer modeling that only in a narrow range of medium-level oxygen conditions — known as a chemical Goldilocks zone — will both phosphorus and nitrogen remain in the mantle in sufficient quantities.
“Our models clearly show that the Earth is precisely within this range. If we had had just a little more or a little less oxygen during core formation, there would not have been enough phosphorus or nitrogen for the development of life,” says Walton.
The researchers also showed that, during the formation of other planets such as Mars, oxygen levels were outside this Goldilocks zone. On Mars, there was more phosphorus in the mantle than on Earth, but less nitrogen, creating challenging conditions for life as we know it.
New limits for life elsewhere
The new findings could change how scientists look for life elsewhere in the universe. Until now, the focus was on whether a planet had water. According to Walton and Schönbächler, this falls short.
The critical component is the amount of oxygen available during the formation of a planet. So, according to this new study, many planets couldn’t support life even if water is present and they otherwise have the right conditions.
Astronomers can look for the right chemistry for life (especially the amount of oxygen present in a solar system) by studying the host star. The star’s chemical structure shapes the planetary system around it because planets are mostly composed of the same material.
Solar systems that have different chemical compositions than ours are therefore not good places to look for 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.
