Scientists hunt for life by searching for what living things destroy

The hunt for life beyond Earth has always been a hunt for traces—the fingerprints that only living things leave behind. Scientists now search for two kinds of fingerprints in the atmospheres of distant planets: the presence of gases that life produces, and the absence of gases that life consumes.

Oxygen is the most obvious marker. On Earth, oxygen was not always abundant. When our planet formed, its atmosphere held carbon dioxide, nitrogen, and methane, but almost no free oxygen. Life existed then—hardy organisms that thrived in that airless world. But around 2.4 to 2.7 billion years ago, something shifted. A new form of life appeared: cyanobacteria, the ancestors of plants. Unlike their predecessors, these organisms harnessed sunlight to power themselves, converting carbon dioxide and other compounds through a process we now call photosynthesis. The waste product was oxygen.

As these photosynthetic organisms multiplied, they flooded the oceans and atmosphere with oxygen—a poison to most of the life that had come before. The older organisms largely vanished, though their descendants still survive in airless pockets on Earth today. The rising oxygen transformed the chemistry of the oceans themselves. Iron that had been dissolved in the water became insoluble, sinking to the seafloor as thick layers of red sediment that eventually hardened into rock. Once the oxidizable materials were exhausted, oxygen accumulated in the atmosphere and oceans while carbon dioxide nearly disappeared. This event, known as the Great Oxidation Event, reshaped the world into the one we inhabit now.

The logic is elegant: if life emerged on other worlds, it might have triggered similar transformations. A planet with abundant oxygen in its atmosphere, or one where carbon dioxide has been depleted, could be a world where life has taken hold. This gives astronomers a second way to search—not just looking for what life produces, but for what life consumes.

Detecting these signatures requires technology that has only recently become possible. Most of the thousands of planets discovered orbiting distant stars have been found through a simple method: watching for the tiny dimming of a star as a planet passes in front of it. When a planet has an atmosphere, some of the starlight passes through it, and that light carries the chemical fingerprints of whatever gases surround the world. By analyzing this light, scientists can identify which elements and compounds are present.

Mars offers a tantalizing case study. The planet's rusty appearance—visible through a telescope as red or orange—comes from iron-rich soils and rocks, the same kind of iron oxide that accumulated on Earth's ocean floors during the Great Oxidation Event. Mars also bears the scars of water: dry riverbeds, deltas, ancient lakebeds, and enormous canyons carved by flowing water. Billions of years ago, Mars was warmer, wetter, and had a thicker atmosphere. The geological evidence suggests it might have undergone its own oxidation event, driven by the emergence of life. But something changed. Today, Mars is a cold, arid desert with an atmosphere so thin it barely exists. That thin atmosphere is 95 percent carbon dioxide, with only traces of oxygen. And despite decades of searching, no signs of abundant life have been found.

Venus, by contrast, offers little hope. Its atmosphere is thick with carbon dioxide, but its surface is hot enough to melt lead and tin—conditions under which life as we understand it could not emerge or persist. The search for extraterrestrial life now has two tools instead of one: the presence of oxygen, and the absence of carbon dioxide. Having two markers to look for makes the search more reliable, and as observation technology continues to improve, the hunt across the stars has begun in earnest.