The universe is full of countless galaxies, stars and planets. Astronomes can find life one day, but they will need extraordinary tests. Credit: Consortium/NASA ESA/EUCLID/EUCLID, Elaboration of the images of J.-C. Cuillandre (Cea Paris-Saclay), G. Anselmi
The detection of life beyond the earth would be one of the deepest discoveries in the history of science. The only galaxy of the Milky Way hosts hundreds of millions of potentially habitable planets. Astronomes are using powerful spatial telescopes to look for molecular biology indicators in the atmospheres of the most similar to these planets.
But so far, no solid proof of life has ever been found beyond the earth. A document published in April 2025 declared that she took over a signature of life in the atmosphere of the planet K2-18 b. And while this discovery is intriguing, most astronomers – including the authors of the document – is not ready to affirm that it means that there is extraterrestrial life. A life detection would be a remarkable development.
The astronomer Carl Sagan used the phrase “extraordinary statements require extraordinary evidence”, as regards the search for alien life. It transmits the idea that there should be a high bar for tests in support of a remarkable claim.
I am an astronomer who wrote a book on astrobiology. During my career, I saw some convincing scientific discoveries. But to reach this threshold of finding life beyond the earth, a result must satisfy several important criteria.
When is it an important and reliable result?
There are three criteria for a scientific result to represent a real discovery and not to be subject to uncertainty and doubt. How is the pretense of life on K2-18 b is measure?
First of all, the experiment must measure a significant and important amount. The researchers observed the atmosphere of K2-18 B with the James Webb spatial telescope and saw a spectral feature that identified as Dimetyl sulfuro.
On earth, the Dimetyl sulfuro is associated with biology, in particular bacteria and plankton in the oceans. However, it can also arise by other means, so this single molecule is not a final test of life.
Secondly, the detection must be strong. Each detector has a little noise from the random movement of electrons. The signal should be strong enough to have a low probability of deriving by chance from this noise.
K2-18 b detection has a meaning of 3-sigma, which means that it has a probability of 0.3 percent to derive by chance.
It looks low, but most scientists would consider weak detection. There are many molecules that could create a function in the same spectral range.
The “Gold Standard” for scientific detection is 5-Sigma, which means that the probability that the discovery takes place by chance is less than 0.00006 percent. For example, CERN physicists collected the data patiently for two years until they had a detection of 5 sigma of the Higgs Bosone particle, leading to a Nobel Prize a year later in 2013.
The announcement of the discovery of Higgs’s Bosone requested decades from the moment Peter Higgs provided for the first time the existence of the particle. Scientists, like Joe Incandela shown here, waited for them to reach that 5-Sigma level to say: “I think we have it”.
Third, a result must be repeatable. The results are considered reliable when they were repeated, ideally corroborated by other investigators or confirmed using a different tool. For K2-18 b, this could mean detecting other molecules that indicate biology, such as oxygen in the atmosphere of the planet. Without more and better data, most researchers are observing the affirmation of life on K2-18 b with skepticism.
Life claims to Mars
In the past, some scientists have said they had found life much closer to home, on the planet Mars.
Over a century ago, the retired Boston merchant transformed the Percival Lowell astronomer claimed that the linear characteristics that saw on the surface of Mars were channels, built by a dying civilization to transport water from the poles to the equator. The artificial navigable streets on Mars would certainly have been a great discovery, but this example failed the other two criteria: strong evidence and repeatability.
Lowell was missed by his visual observations and was committing to a pious desire. No other astronomer could confirm his discoveries.
In 1996, NASA held a press conference in which a team of scientists presented tests for biology in the Martian meteorite Alh 84001. Their tests included an evocative image that seemed to show microphones in the meteorite.
However, scientists have devised explanations for the unusual characteristics of the meteorite that do not involve biology. That extraordinary statement has dissipated.
More recently, astronomers have detected low methane levels in the atmosphere of Mars. As Dimetyl sulphide and oxygen, methane on earth is mainly made – but not exclusively – by life. Several spaces and rover on the Martian surface have returned contrasting results, in which a detection with a space vehicle was not confirmed by another.
The low level and variability of methane on Mars are still a mystery. And in the absence of definitive evidence that this very low level of methane has a biological origin, nobody claims definitive evidence of life on Mars.
Advanced civilization claims
The detection of microbial life on Mars or an exoplanet would be dramatic, but the discovery of extraterrestrial civilizations would be truly spectacular.
The search for extraterrestrial intelligence, or thirst, has been in progress for 75 years. Messages have never been received, but in 1977 a radio telescope in Ohio detected a strong signal that lasted only for a minute.
This signal was so unusual that an astronomer who works on the telescope wrote “Wow!” In the press, giving the signal the name. Unfortunately, nothing similar to that region of the sky has been detected since then, so the WOW! The signal fails the proof of repeatability.
In 2017, a rocky cigar -shaped object called ‘Oumuamua was the first interstellar object known to visit the Solar System. ‘The strange form and trajectory of Oumuamua brought Harvard Avi Loeb astronomer to argue that it was an alien artifact. However, the object has already left the Solar System, so there is no possibility for astronomers to observe it again. And some researchers have collected tests that suggest that it is only a comet.
While many scientists think that we are not alone, given the enormous amount of habitable properties beyond the earth, no detection has eliminated the threshold enunciated by Carl Sagan.
Complaints on the universe
These same criteria apply to research on the entire universe. A particular concern in cosmology is the fact that, unlike the case of the planets, there is only one universe to study.
A precautionary story derives from the attempts to demonstrate that the universe has crossed an extremely rapid period of expansion a fraction of a second after the Big Bang. Cosmologists call the inflation of this event and is invoked to explain why the universe is now smooth and flat.
In 2014, astronomers said they had found tests for inflation in a subtle signal from the microwaves left after the big bang. Within a year, however, the team withdrawn the result because the signal had a trivial explanation: they had a confused powder in our galaxy with a signature of inflation.
On the other hand, the discovery of the acceleration of the universe shows the success of the scientific method. In 1929, astronomer Edwin Hubble discovered that the universe was expanding. So, in 1998, the tests emerged that this cosmic expansion is accelerating. The physicists were surprised by this result.
Two research groups used supernovae to trace the expansion separately. In a friendly rivalry, they used several supernova sets but they achieved the same result. Independent confirmation increased their trust that the universe was accelerating. They called the strength behind this accelerated dark energy expansion and received a Nobel Prize in 2011 for its discovery.
On large and small stairs, astronomers try to set up a high rehearsal bar before claiming a discovery.
Chris Imy is a distinct professor of astronomy at the University of Arizona. He receives funding from the National Science Foundation and the Howard Hughes Medical Institute.
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