Chris Impey
,
University of Arizona
The discovery of life outside Earth would rank as one of the most significant findings in scientific history. Consider just our own Milky Way galaxy, which contains an estimated hundreds of millions of planets with potential for hosting life.
habitable planets
. Astronomers are using
powerful space telescopes
to search for molecular signs of life within the atmospheres of the most Earth-like among these planets.
However, up until now, concrete evidence of life outside our planet has never been discovered. In April 2025, a study was released claiming to have identified signs of life within the atmosphere of another celestial body.
planet K2-18b
And although this finding is fascinating, most astronomers—including the paper’s authors—are not prepared to assert that it indicates the presence of extraterrestrial life. Discovering life would be an extraordinary breakthrough.
The
astronomer Carl Sagan
used the phrase, “Extraordinary claims require extraordinary evidence,” in regard to searching for alien life. It conveys the idea that there should be a high bar for evidence to support a remarkable claim.
I’m an
astronomer
who has written a
book about astrobiology
Throughout my career, I have witnessed numerous fascinating scientific breakthroughs. However, uncovering evidence of life outside our planet requires meeting multiple significant standards.
Under what circumstances is a result both significant and trustworthy?
Three conditions must be met for a scientific finding to signify a genuine discovery without lingering ambiguity and skepticism. How does the assertion about life on K2-18b fare against these standards?
Initially, the experiment must quantify a significant and pertinent measurement. The researchers examined K2-18b’s atmosphere using the
James Webb Space Telescope
and observed a ghostly signal that they recognized as dimethyl sulfide.
On our planet, dimethyl sulfide is
associated with biology
, particularly focusing on bacteria and plankton in the seas. Nonetheless, it may originate through various other methods; thus, this individual molecule can
inconclusive evidence of existence
.
Secondly, the detection must be robust. Each detector experiences certain levels of noise due to the random movement of electrons. The signal ought to be sufficiently powerful to minimize the likelihood of occurring randomly amidst this noise.
The detection of K2-18b has a significance level of
3-sigma
, which means it has a 0.3% probability of arising by chance.
That seems low, yet most researchers would view that as
weak detection
There are numerous molecules that could produce features within the same spectral range.
The “gold standard” for scientific detection is
5-sigma
, which means the probability of the finding happening by chance is less than 0.00006%. For example,
physicists at CERN
collected data meticulously for two years until they achieved a 5-sigma detection of the
Higgs boson particle
, leading to a
Nobel Prize
one year later in 2013.
https://youtube.com/watch?v=0CugLD9HF94%3Fwmode%3Dtransparent%26start%3D0
The revelation of the Higgs boson’s discovery spanned many years since Peter Higgs initially theorized its existence. Researchers like Joe Incandela, as seen here, held off until they achieved a 5-sigma confidence level before declaring, “We believe we’ve found it.”
Third, a result
needs to be repeatable
Results are deemed reliable when they have been replicated—ideally substantiated by other researchers or verified with a distinct apparatus. In the case of K2-18b, this could involve identifying additional signals.
substances indicating biological presence, like oxygen
In the planet’s atmosphere. Without additional and higher-quality data, most scientists are approaching the assertion of life on K2-18b with caution.
skepticism
.
Assertions about life on Mars
Previously, certain researchers asserted they had discovered signs of life not too far away, right here within our solar system on Mars.
Over a century ago, retired Boston merchant turned astronomer
Percival Lowell
claimed that linear features he saw on the surface of Mars were canals, constructed by a dying civilization to transport water from the poles to the equator. Artificial waterways on Mars would certainly have been a major discovery, but this example failed the other two criteria: strong evidence and repeatability.
Lowell was misled by his visual observations, and he was engaging in wishful thinking.
No other astronomers could confirm his findings
.
In 1996, NASA held a press conference where a team of scientists presented evidence for biology in the
Martian meteorite ALH 84001
Their proof consisted of a striking picture appearing to depict microfossils within the meteorite.
Nevertheless, researchers have proposed explanations for the meteorite’s peculiar characteristics that
do not involve biology
That remarkable assertion has faded away.
More recently, astronomers detected
low levels of methane
In the environment of Mars, similar to how dimethyl sulfide and oxygen function on Earth, methane is predominantly—but not solely—produced through biological processes. Various orbiters and rovers exploring the Martian surface have provided data on this subject.
conflicting results
where an observation made by one spaceship wasn’t corroborated by another.
The minimal and inconsistent levels of methane on Mars remain an enigma. Without concrete proof indicating that these trace amounts have a biogenic source, scientists refrain from asserting definite signs of life on the planet.
Claims of advanced civilizations
Finding microorganisms on Mars or another planet would be significant, but uncovering alien civilizations would be utterly astounding.
The
search for extraterrestrial intelligence, or SETI
, has been underway for 75 years. No messages have ever 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 working at the telescope wrote “Wow!” on the printout, giving the signal its name. Unfortunately, nothing like it has since been detected from that region of the sky, so the
Wow! Signal
does not pass the repeatability test.
In 2017, a
Rocky, cigar-like entity referred to as ‘Oumuamua’
Was the initial recognized celestial body from outside our solar system to enter it. ‘Oumuamua’s unusual form and path prompted Harvard astronomer Avi Loeb to suggest that it might be artificial.
alien artifact
However, the object has already exited our solar system, thus providing no opportunity for astronomers to revisit it. Some investigators have collected data indicating that
it’s just a comet
.
Although numerous researchers believe we are not solitary beings, considering the vast number of potentially inhabitable places outside our planet, no discovery has met the criteria set forth by Carl Sagan.
Claims about the universe
The exact same standards are applicable when conducting investigations into the whole cosmos. A specific worry within the field of cosmology is that, contrary to what occurs with planets, we have
one universe
to study.
A warning story arises from efforts aimed at demonstrating that the universe underwent an extraordinarily fast expansion for a tiny fraction of a second following the big bang.
Big Bang
. Cosmologists
call this event inflation
, and it is invoked to explain why the universe is now smooth and flat.
In 2014, astronomers purportedly discovered
evidence for inflation
In a subtle indication from microwaves remaining since the Big Bang, within a year though, the group withdrew their finding as the signal was actually due to an ordinary cause: They had mistaken
dust in our galaxy
with a signature of inflation.
On the other hand, the discovery of the universe’s acceleration shows the success of the scientific method. In 1929, astronomer
Edwin Hubble
discovered that the universe was expanding. Later, in 1998, they found evidence suggesting that this universal expansion is speeding up. The physicists were
startled by this result
.
In a congenial competition, two research teams utilized supernovae independently to track cosmic expansion. Despite employing distinct collections of supernovae, both arrived at identical conclusions. This independent verification bolstered their trust in the findings.
universe was accelerating
They referred to the driving force behind this rapid growth asExpansion.
dark energy
and received a
Nobel Prize
In 2011 due to its discovery.
Astronomers aim to maintain rigorous standards of proof when making claims, whether the phenomena they study are vast or minute.
Chris Impey
, University Distinguished Professor of Vmeetsolutions News,
University of Arizona
This article has been republished from
The Conversation
under a Creative Commons license. Please read the
original article
.
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What level of proof do researchers require to assert they’ve discovered extraterrestrial life?
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