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HAVE WE FOUND LIFE ON PLANET K1-18B?  
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Chris Impey
April 25, 2025
The Conversation
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_ Astronomer Carl Sagan asserted that ‘Extraordinary claims require
extraordinary evidence.’ What precisely does that mean? And does the
evidence of life on a distant planet recently detected by the James
Webb Space Telescope meet that challenge? _

The universe is filled with countless galaxies, stars and planets.
Astronomers may find life one day, but they will need extraordinary
proof, ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C.
Cuillandre (CEA Paris-Saclay), G. Anselmi

 

The detection of life beyond Earth would be one of the most profound
discoveries in the history of science. The Milky Way galaxy alone
hosts hundreds of millions of potentially habitable planets
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Astronomers are using powerful space telescopes
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to look for molecular indicators of biology in the atmospheres of the
most Earth-like of these planets.

But so far, no solid evidence of life has ever been found beyond the
Earth. A paper published in April 2025 claimed to detect a signature
of life in the atmosphere of the planet K2-18b
[[link removed]].
And while this discovery is intriguing, most astronomers – including
the paper’s authors – aren’t ready to claim that it means
extraterrestrial life exists. A detection of life would be a
remarkable development.

The astronomer Carl Sagan
[[link removed]] 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
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written a book about astrobiology
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Over my career, I’ve seen some compelling scientific discoveries.
But to reach this threshold of finding life beyond Earth, a result
needs to fit several important criteria.

When is a result important and reliable?

There are three criteria for a scientific result to represent a true
discovery and not be subject to uncertainty and doubt. How does the
claim of life on K2-18b measure up?

First, the experiment needs to measure a meaningful and important
quantity. Researchers observed K2-18b’s atmosphere with the James
Webb Space Telescope [[link removed]] and saw
a spectral feature that they identified as dimethyl sulfide.

On Earth, dimethyl sulfide is associated with biology
[[link removed]], in particular bacteria
and plankton in the oceans. However, it can also arise by other means,
so this single molecule is not conclusive proof of life
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Second, the detection needs to be strong. Every detector has some
noise from the random motion of electrons. The signal should be strong
enough to have a low probability of arising by chance from this noise.

The K2-18b detection has a significance of 3-sigma
[[link removed]], which means it has a
0.3% probability of arising by chance.

That sounds low, but most scientists would consider that a weak
detection [[link removed]].
There are many molecules that could create a feature in the same
spectral range.

The “gold standard” for scientific detection is 5-sigma
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probability of the finding happening by chance is less than 0.00006%.
For example, physicists at CERN [[link removed]] gathered data
patiently for two years until they had a 5-sigma detection of the
Higgs boson particle
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leading to a Nobel Prize
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later in 2013.

The announcement of the discovery of the Higgs boson took decades from
the time Peter Higgs first predicted the existence of the particle.
Scientists, such as Joe Incandela shown here, waited until they’d
reached that 5-sigma level to say, ‘I think we have it.’

Third, a result needs to be repeatable
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Results are considered reliable when they’ve been repeated –
ideally corroborated by other investigators or confirmed using a
different instrument. For K2-18b, this might mean detecting other
molecules that indicate biology, such as oxygen
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Without more and better data, most researchers are viewing the claim
of life on K2-18b with skepticism
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Claims of life on Mars

In the past, some scientists have claimed to have found life much
closer to home, on the planet Mars.

Over a century ago, retired Boston merchant turned astronomer Percival
Lowell [[link removed]]
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
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[An image of Mars in space]
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Mars, as taken by the OSIRIS instrument on the ESA Rosetta spacecraft
during its February 2007 flyby of the planet and adjusted to show
color. ESA & MPS for OSIRIS Team
MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA
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CC BY-SA [[link removed]]

In 1996, NASA held a press conference where a team of scientists
presented evidence for biology in the Martian meteorite ALH 84001
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Their evidence included an evocative image that seemed to show
microfossils in the meteorite.

However, scientists have come up with explanations for the
meteorite’s unusual features that do not involve biology
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That extraordinary claim has dissipated.

More recently, astronomers detected low levels of methane
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Mars. Like dimethyl sulfide and oxygen, methane on Earth is made
primarily – but not exclusively – by life. Different spacecraft
and rovers on the Martian surface have returned conflicting results
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where a detection with one spacecraft was not confirmed by another.

The low level and variability of methane on Mars is still a mystery.
And in the absence of definitive evidence that this very low level of
methane has a biological origin, nobody is claiming definitive
evidence of life on Mars.

Claims of advanced civilizations

Detecting 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 SETI
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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
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fails the test of repeatability.

[An illustration of a long, thin rock flying through space.]
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‘Oumuamua is the first object passing through the solar system that
astronomers have identified as having interstellar origins. European
Southern Observatory/M. Kornmesser
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In 2017, a rocky, cigar-shaped object called ‘Oumuamua
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known interstellar object to visit the solar system. ‘Oumuamua’s
strange shape and trajectory led Harvard astronomer Avi Loeb to argue
that it was an alien artifact
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However, the object has already left the solar system, so there’s no
chance for astronomers to observe it again. And some researchers have
gathered evidence suggesting that it’s just a comet
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While many scientists think we aren’t alone, given the enormous
amount of habitable real estate beyond Earth, no detection has cleared
the threshold enunciated by Carl Sagan.

Claims about the universe

These same criteria apply to research about the entire universe. One
particular concern in cosmology is the fact that, unlike the case of
planets, there is only one universe
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A cautionary tale comes from attempts to show that the universe went
through a period of extremely rapid expansion a fraction of a second
after the Big Bang [[link removed]].
Cosmologists call this event inflation
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it is invoked to explain why the universe is now smooth and flat.

In 2014, astronomers claimed to have found evidence for inflation
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in a subtle signal from microwaves left over after the Big Bang.
Within a year, however, the team retracted the result because the
signal had a mundane explanation: They had confused dust in our galaxy
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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 [[link removed]] found that the
universe was expanding. Then, in 1998, evidence emerged that this
cosmic expansion is accelerating. Physicists were startled by this
result
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Two research groups used supernovae to separately trace the expansion.
In a friendly rivalry, they used different sets of supernovae but got
the same result. Independent corroboration increased their confidence
that the universe was accelerating
[[link removed]]. They called
the force behind this accelerating expansion dark energy
[[link removed]] and received a Nobel
Prize
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in 2011 for its discovery.

On scales large and small, astronomers try to set a high bar of
evidence before claiming a discovery.[The Conversation]

Chris Impey [[link removed]],
University Distinguished Professor of Astronomy, _University of
Arizona
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This article is republished from The Conversation
[[link removed]] under a Creative Commons license. Read
the original article
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