For decades, the search for extraterrestrial life has revolved around a simple rule: follow the water. If a distant planet had liquid water and perhaps oxygen, it would be flagged as potentially habitable. But new research led by scientists at ETH Zurich shows that this long-term strategy may not be complete. Researchers believe that a planet can have oceans and continents and still be chemically unable to support life. The real limit may lie deeper in the chemistry of planet formation.
a chemical goldilocks zone below the surface
The study was published in natural astronomy under title “Chemical habitability of Earth and rocky planets as determined by core formation”led by postdoctoral researchers Dr. Craig R. Walton and Professor Maria Schönbächler and colleagues at the Center for the Origin and Epidemic of Life at ETH Zurich. Their core claim is precise: Life depends not just on water and oxygen, but also on whether two key elements – phosphorus and nitrogen – were still present in the Earth’s mantle during the planet’s earliest formation. Phosphorus is needed to build DNA and RNA (molecules that store and transmit genetic information). It also plays a key role in cellular energy systems. Nitrogen, meanwhile, is an important component of proteins, the structural and functional building blocks of cells. Without both, “life as we know it simply could not have formed.”
Phosphorus and nitrogen are essential for life: phosphorus forms DNA, RNA and ATP to provide energy, while nitrogen forms proteins. / AI illustration
“During the formation of the planet’s core, the right amount of oxygen needs to be present so that phosphorus and nitrogen can remain on the planet’s surface,” Walton explained. Young rocky planets start out as melts. As they cooled, heavier elements such as iron sank to form the core, while lighter materials formed the mantle and crust. At the same time, oxygen content determines how elements are chemically distributed between metals and rocks. If oxygen is scarce, phosphorus binds to iron and sinks into the core, effectively removing it from the surface environment. If oxygen is too abundant, phosphorus will remain in the mantle, but nitrogen is more likely to escape into the atmosphere and eventually be lost in space. “Too much or too little oxygen throughout the Earth—not the atmosphere itself—makes the planet inhospitable to life because it traps life’s key nutrients in its core,” Walton told Science . daily mail. “The different oxygen balance means that when the Earth cooled and rock formed, you had nothing on the surface to take advantage of.” Using numerical models, the team identified what they describe as a very narrow “chemical blond zone,” an intermediate oxygen range in which phosphorus and nitrogen remain in the mantle in sufficient quantities to support life.
A planet’s “habitable zone” for life requires the right amount of oxygen to keep phosphorus and nitrogen available / Image: X
“Our model clearly shows that the Earth is right within this range,” Walton said. “If we had a little more or a little less oxygen during core formation, there wouldn’t be enough phosphorus or nitrogen to develop life.” Earth appears to have reached this equilibrium about 4.6 billion years ago.
Rethinking what makes Earth habitable
The findings suggest that many planets previously thought to be promising may not be chemically suitable for life from the start, even if they contain water. While no known life can survive without liquid water, researchers believe using oxygen or water alone as a sign of habitability can be misleading. The total oxygen balance during planet formation, not just atmospheric oxygen, determines whether key elements for life are still available. Walton warned that this could significantly reduce the number of habitable worlds in the universe. He said the number of habitable planets may be only one to 10 percent of previous estimates. “It would be very disappointing to come all the way to colonize a planet like this only to find out there is no phosphorus to grow food,” he said. “We would be better off trying to check the conditions under which the planet formed first, like making sure your dinner is cooked correctly before you eat it.” Closer to home, research shows that Mars lies just outside this chemical zone. Mars appears to be relatively rich in phosphorus but significantly lower in nitrogen near the surface. Additionally, harsh salts and other surface chemicals make the soil uninhabitable.
Mars has enough phosphorus but not enough nitrogen to make its surface chemically unsuitable to support life like on Earth / The True Color of Mars / Image: Earth.com
“Mars is very similar to Earth, and its formation conditions meant there was more phosphorus, not less. That means growing food there might be relatively easy,” Walton said. But he added that nitrogen deficiency and surface chemistry posed significant challenges: “It’s not that different, but it’s currently uninhabitable, Elon Musk One has to come up with a clever way to change the ingredients so that food can be grown there. “
Find the right star
Directly measuring the internal chemistry of distant, rocky planets remains extremely difficult. However, astronomers can infer the possible planetary composition by studying the host star. Planets are formed from the same material as their parent stars. Therefore, a star’s oxygen abundance and overall chemical structure determine the composition of its planetary system. Solar systems with stars very similar to our sun may offer better chances. “This makes the search for life on other planets much more concrete,” Walton said. “We should look for solar systems with stars similar to our sun.” The work reframes the long-term search for life beyond Earth. Water is still required. But that may not be enough. A planet’s fate, whether barren or alive, may depend on the delicate chemical balance it reaches in its first moments of melting, long before oceans, atmospheres or continents form.
