That question is at the center of a new paper published in Nature Astronomy, where researchers examine the often-overlooked problem of "false negatives" in the search for extraterrestrial life. These are cases in which life exists, or once existed, but scientists fail to detect it.

"We are currently investing a great deal of money in missions that might need to be designed differently."

The Overlooked Challenge of Finding Alien Life

One of the primary goals of astrobiology is determining whether life exists elsewhere in the universe. Researchers search for clues that could point to living organisms, but interpreting those clues is rarely straightforward.

Scientists have long been concerned about so-called false positives, which occur when observations appear to indicate life but later turn out to have another explanation. False negatives present the opposite problem. In those cases, evidence of life is present, yet it goes unnoticed.

"We should be aware of these false-negative results," says lead author Inge Loes ten Kate, professor of astrobiology at Utrecht University and the University of Amsterdam. "It means there are shortcomings in recognizing the existence of life. These shortcomings are not yet high on the research agenda."

Why Signs of Life Can Be Missed

There are several reasons why evidence of life might escape detection. Traces left behind by organisms may not survive over time, observable signals may be too faint to identify, or existing technologies may simply be unable to detect them.

Ten Kate argues that these risks deserve much greater attention.

"We therefore advocate for the development of a targeted research strategy that systematically addresses these risks, in which we must combine laboratory experiments with modelling research and fieldwork. Space missions and instruments are designed to detect potential signs of life, but the risk of overlooking something is not taken into account. The search for signs of life should go hand in hand with better-defined questions and testable hypotheses to justify specific measurement or observation targets."

The researchers also point to artificial intelligence as a potentially valuable tool. AI systems trained to identify patterns could reveal relationships or signals that human observers might never notice.

"Because then you might well uncover things that we would never be able to see on our own. And with new observations, you can then work out how and where they fit into such a pattern."

The Cost of a False Negative

Missing evidence of life could have major consequences.

First, scientists might shift attention away from promising targets or reduce support for instruments that could detect forms of life beyond current capabilities. As a result, potentially habitable environments could be overlooked.

"A simplified example: if there is life under a rock, and you only look at that rock from above, that life will go unnoticed. So, investigate thoroughly whether the conditions for the existence of life forms are present in the environment, and whether you can recognize patterns on the surface of a celestial body."

A second concern involves future exploration and resource extraction. If life exists on another world but remains undetected, policymakers could approve mining or other activities that unintentionally destroy it.

"Secondly, there is a danger that policymakers will approve the premature exploitation of raw materials on planets, with the risk of irreversibly destroying unnoticed life."

Hidden Causes of False Negatives

False negatives can arise in several ways.

For example, life may be widespread and active on a planet, yet the traces it leaves behind might not be recognized. Atmospheric processes can also complicate the search. On some worlds, gases produced by living organisms may be removed or masked by other processes, making them much harder to detect.

Researchers note that these kinds of false negatives are particularly challenging because scientists often recognize them only after the fact.

Looking Beyond What We Already Know

One of the biggest challenges is searching for something that may not resemble any known form of life.

"But how do you investigate things you can't find?" asks Ten Kate. "That question goes to the heart of our problem, because we tend to look for things we already know. We therefore need to understand very clearly what kind of life is possible in a particular place, what the conditions for that life are, and how we can recognize the traces of that life. And even then, we might overlook things."

Ten Kate points to iron-bearing minerals discovered on Mars last year. These minerals show a type of oxidation that differs from nearby materials.

"On Earth, we only see such differing oxidation as a result of the presence of life. But does that necessarily mean that we are dealing with life in an extraterrestrial context?"

Researchers stress that these Martian minerals do not represent a known false negative. Instead, they highlight how much remains unknown.

"To be clear: these minerals do not mean that we are dealing with false-negative results in this case. We simply do not yet understand what is going on here. But if we do not investigate this further, it could indeed result in a false negative. So we need to understand even better how the geochemistry works, and how the underlying chemical reactions operate in such situations. That will also help you try to rule out false-negative results."

The researchers conclude that careful preparation is essential before sending missions to other worlds.

And, last but not least: know what you're getting into before you send an expedition anywhere. "So make sure you've studied the situation in the landing zone meticulously in advance."