Scientists at ETH Zurich have discovered a new rule for life on other planets. It's not enough for a planet to have water and oxygen on its surface. The key lies in how much oxygen was present when the planet formed. This balance affects if phosphorus and nitrogen, two elements vital for life, stay near the surface or get locked away deep inside. Phosphorus builds DNA and energy cells need, while nitrogen makes proteins. Without both, life "as we know it simply cannot form," said Dr. Craig R. Walton, lead researcher. When rocky planets are young, they are molten. Heavy elements sink to form cores, and oxygen controls where phosphorus and nitrogen end up. Too little oxygen means phosphorus bonds with iron and disappears into the core. Too much oxygen causes nitrogen to escape into space. Both cases make the planet lifeless. ETH Zurich’s team used models to find a tiny “chemical Goldilocks zone” for oxygen. Earth fits perfectly into this zone, explaining why it supports life. If Earth had even a bit more or less oxygen early on, life might never have started. The study warns that many exoplanets with water might still be chemically unable to support life. Walton said, "We’d better try to check the formation conditions of the planet first, much like ensuring your dinner was cooked properly before you go ahead and eat it." They also looked at Mars, which has enough phosphorus but lacks nitrogen. Combined with harsh surface chemicals, Mars is likely unsuitable for Earth-like life now. Finding planets in the right chemical balance will take looking at their stars. Planets form from star material, so stars like our Sun are better candidates to have life-supporting planets. This research shifts the search for extraterrestrial life. Water is needed but not enough. The secret to life may lie hidden deep inside a planet, in the chemistry set at its fiery birth billions of years ago.