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Scientists discover the “Goldilocks” secret behind life on Earth

Earth may be habitable because it got unbelievably lucky with its chemistry from the very start.

Date:

April 6, 2026

Source:

ETH Zurich

Summary:

Earth may have won a cosmic chemistry lottery. Researchers found that during the planet’s earliest formation, oxygen had to be in an extremely narrow “Goldilocks zone” for two life-essential elements, phosphorus and nitrogen, to stay where life could use them. Too much or too little oxygen, and those ingredients could be lost or trapped deep inside the planet. This could reshape the search for life by showing that water alone is not enough.

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Life on Earth may have been possible only because the planet formed under an astonishingly precise chemical balance. Credit: Shutterstock

Life cannot begin on a planet unless certain chemical elements are available in large enough amounts. Two of the most important are phosphorus and nitrogen. Phosphorus helps build DNA and RNA, which store and pass along genetic information, and it also plays a key role in how cells manage energy. Nitrogen is a major part of proteins, which are essential for building cells and helping them function. Without enough phosphorus and nitrogen, life cannot emerge from nonliving matter.

New research led by Craig Walton, a postdoc at the Centre for Origin and Prevalence of Life at ETH Zurich, and ETH Zurich professor Maria Schönbächler shows that these elements must already be available in the right amounts when a planet's core forms. "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. On Earth, that appears to have happened about 4.6 billion years ago, giving our planet an unusually fortunate chemical starting point. The result could influence how scientists search for life beyond Earth.

How Planet Core Formation Affects Habitability

Planets begin as bodies of molten rock. As they form, their materials separate by weight. Heavy metals such as iron sink inward and create the core, while lighter material remains above and eventually becomes the mantle and later the crust.

Oxygen levels during this stage are critical. If there is too little oxygen when the core forms, phosphorus bonds with heavy metals such as iron and gets pulled down into the core. Once that happens, it is no longer available in the parts of the planet where life might develop. If there is too much oxygen, phosphorus stays in the mantle, but nitrogen becomes more likely to escape into the atmosphere and be lost.

The Chemical Goldilocks Zone

Using extensive modeling, Walton and his co-authors found that both phosphorus and nitrogen remain in the mantle in large enough amounts only within a very narrow range of moderate oxygen conditions. They describe this as a chemical Goldilocks zone.

"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 team also found that other planets, including Mars, formed under oxygen conditions outside this Goldilocks zone. On Mars, that meant more phosphorus in the mantle than on Earth, but less nitrogen, producing difficult conditions for life as we know it.

A New Way to Search for Life Beyond Earth

These findings may change how scientists think about habitability. So far, much of the focus has been on whether a planet has water. Walton and Schönbächler argue that this is not enough.

A planet may have water and still be chemically unfit for life from the very beginning. If oxygen levels were wrong while the core was forming, the planet may never have kept enough phosphorus and nitrogen in the places where life could use them.

Why Sun-Like Stars May Matter Most

Astronomers may be able to estimate these chemical conditions by studying other solar systems with large telescopes. The oxygen available during planet formation depends on the chemical makeup of the host star. Because planets form mostly from the same material as their star, the star's composition helps shape the chemistry of the entire planetary system.

That means solar systems whose chemistry is very different from ours may be poor candidates in the search 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.