Why Few Planets Are Suitable for Life
Researchers at ETH Zurich have determined that the chemical conditions necessary for life are surprisingly rare and that Earth benefited from a fortunate confluence of events. The study, published today, February 9, 2026, in Nature Astronomy, suggests that the presence of the right amount of oxygen during a planet’s core formation is crucial for ensuring sufficient phosphorus and nitrogen are available in the mantle and crust – elements essential for life as we know it.
For life to develop from non-living matter, phosphorus and nitrogen are indispensable. Phosphorus is a key component of DNA and RNA, the molecules that store and transmit genetic information, and plays a vital role in cellular energy processes. Nitrogen, meanwhile, is a fundamental building block of proteins, which are essential for cell formation, structure, and function. Without adequate supplies of both, the emergence of life is considered impossible.
The research, led by Craig Walton, a postdoctoral researcher at the Centre for Origin and Prevalence of Life at ETH Zurich, and ETH Professor Maria Schönbächler, reveals that the availability of these crucial elements hinges on conditions present during a planet’s core formation. “The decisive factor during core formation is that there is precisely the right amount of oxygen present so that phosphorus and nitrogen remain on the planet’s surface,” Walton explained.
The Earth, approximately 4.6 billion years ago, experienced precisely these conditions, making it a chemical anomaly in the universe. This discovery has significant implications for the search for extraterrestrial life.
Core Formation as a Cosmic Roll of the Dice
When planets form, they initially consist of molten rock. During this phase, a sorting process occurs: heavy metals like iron sink towards the center, forming the core, while lighter materials rise to create the mantle and, eventually, the crust.
If there is too little oxygen present during core formation, phosphorus binds with heavy metals and descends into the core, effectively removing it from the areas where life could potentially develop. Conversely, too much oxygen can cause nitrogen to escape into the atmosphere, also diminishing its availability.
A Chemical “Goldilocks Zone”
Walton and his co-authors demonstrated through numerous simulations that only a narrow range of intermediate oxygen levels – a “chemical Goldilocks zone” – allows sufficient phosphorus and nitrogen to remain in the mantle. “Our models show that Earth lies right in this zone,” Walton stated. “If there had been just a little more or less oxygen during Earth’s core formation, there wouldn’t have been enough phosphorus and nitrogen for life to arise.”
The researchers also found that other planets, such as Mars, likely experienced oxygen levels outside this optimal range during their formation, resulting in insufficient phosphorus and nitrogen in their mantles.
New Criteria for the Search for Life
These findings suggest that the search for life beyond Earth may need to shift its focus. Currently, much attention is given to the presence of water on a planet. However, Walton and Schönbächler argue that this is insufficient. The amount of oxygen available during a planet’s formation can render many planets chemically unsuitable for life, even if they possess water and appear otherwise habitable.
The research emphasizes the importance of seeking solar systems that resemble our own in terms of chemical composition. Astronomers can indirectly assess the oxygen levels present during planet formation by analyzing the chemical fingerprint of a star, as planets are primarily composed of material originating from their host star.
“This makes the search for life on other planets much more specific,” Walton concluded. “We should therefore look for solar systems that are similar to our own.”
Scientific Contact:
Dr. Craig Robert Walton, ETH Zürich, craig.walton(at)eaps.ethz.ch, +44 7940 063 423
Original Publication:
Walton CR, Rogers LK, Bonsor A, Spaargaren R, Shorttle O, Schönbächler M: The chemical habitability of Earth and rocky planets prescribed by core formation, Nature Astronomy, 9 February 2026, DOI: 10.1038/s41550-026-02775-z
Further Information:
https://ethz.ch/de/news-und-veranstaltungen/eth-news/news/2026/02/warum-nur-weni…
