Finding life beyond Earth would be extraordinary. The first clue, however, might look more like an ambiguous chemical signal than a moment in a science-fiction film.
In 2021, NASA scientists proposed a way to communicate that uncertainty more clearly. Their idea was to describe progress towards a life detection through several levels of evidence, instead of forcing every result into a simple yes-or-no headline.
A scale for confidence
The Nature commentary led by Jim Green introduced a proposed framework with seven levels. NASA’s explanation of the proposal describes it as a starting point for discussion among researchers and communicators.
Early evidence might be a signal that could have a biological origin. Later work would need to check contamination, establish the environmental context, test non-biological explanations and obtain additional supporting observations.
The scale was a proposal for clearer communication. It should not be described as a binding, universally adopted rulebook or as an announcement that life had been discovered.
Jim Green, NASA’s chief scientist at the time, and his co-authors framed their 2021 proposal as a responsibility accompanying the possibility of discovery:
"Our generation could realistically be the one to discover evidence of life beyond Earth. With this privileged potential comes responsibility."
Their comparison was with NASA’s Technology Readiness Level scale, which follows technology from an initial idea toward a tested mission component. The proposed confidence-of-life-detection scale applied the idea of successive evidential steps to biosignatures. It was a discussion framework, not a declaration that extraterrestrial life had been detected. NASA’s Technology Readiness Level scale tracks spaceflight hardware from the moment it is conceived to its use on an actual mission; on the proposed Confidence of Life Detection (CoLD) scale, the lowest levels would cover a possible biosignature and the higher levels would require ever more specific evidence, so that every candidate could be placed in a standard context that helps scientists interpret what they have found.
"History includes many claims of life detection that later proved incorrect or ambiguous when considered in exclusively binary terms. If, instead, we recast the search for life as a progressive endeavor, we convey the value of observations that are contextual or suggestive but not definitive and emphasize that false starts and dead ends are an expected part of a healthy scientific process."
Why a promising signal can remain ambiguous
Suppose an instrument detects a chemical associated with life on Earth. That is a reason to investigate, but other questions immediately follow. Was the chemical identified correctly? Could an unfamiliar geological process produce it? Could the instrument or sample have introduced contamination?
Each question can lead to a useful experiment. Ruling out one explanation is progress even when the result remains undecided.
Read the evidence in layers
When encountering a life-discovery headline, look for three things: the actual measurement, the interpretation offered by the authors, and the next observation that could distinguish between competing explanations.
Our account of the Venus phosphine debate shows why those layers matter. A disputed spectral signal and a confirmed organism are very different findings.
Likewise, a model estimating possible communicating civilisations is a calculation built from assumptions. It is not an observation of inhabited worlds.
Uncertainty can make a discovery story better
A careful account lets readers follow the investigation as it develops. New measurements can strengthen an idea, change it or send researchers in another direction without making the earlier work pointless.
That is a more interesting journey than repeatedly announcing that the biggest question has already been settled. The next clue deserves excitement—and enough context to understand what it genuinely adds.
The scale meets a real Mars sample
Four years after the commentary, the language it proposed appeared in a real announcement. On 10 September 2025 NASA said that a sample called “Sapphire Canyon”, drilled by the Perseverance rover from a rock nicknamed “Cheyava Falls” in Jezero Crater, contained a potential biosignature: “leopard spot” patterns holding the minerals vivianite and greigite, described in a paper led by Joel Hurowitz in Nature. NASA defined a potential biosignature as something that might have a biological origin but needs more data or further study before anyone can say whether life is present. Mission scientist Katie Stack Morgan said non-biological explanations had become less likely but could not be ruled out, and NASA’s release explicitly pointed to the CoLD scale and to its Standards of Evidence. It did not place the finding on a specific level.
That is the kind of report the 2021 authors hoped for: a striking result, stated with its limits, and a clear sense of what would have to come next.
The original commentary also explained why communication belongs within that process:
"Establishing best practices for communicating about life detection can serve to set reasonable expectations on the early stages of a hugely challenging endeavor, attach value to incremental steps along the path, and build public trust by making clear that false starts and dead ends are an expected and potentially productive part of the scientific process. Whatever the outcome of the dialogue, what matters is that it occurs… In doing so, we can only become more effective at communicating the results of our work, and the wonder associated with it."
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A convincing claim must survive alternatives
Imagine a signal that initially looks biological. The next useful question is not how exciting it would be if true, but what else could produce it and how those alternatives might be tested. Independent observations can matter more than a dramatic announcement. This makes uncertainty productive: it identifies the measurements still needed rather than turning every promising clue into a discovery of life.
Explore the related article: Multiverse Ideas: What the Theories Claim and Leave Open.
Credits and further reading: technology readiness level · science alert.
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