The universe is full of countless galaxies, stars and planets. Astronomers may get life one day, but they will need extraordinary evidence. Credit: Image Processing by ESA/Euclid/Euclid Consortium/NASA, J-C. Cuillandre (CEA Paris-Saclay), G. Anselmi
Life beyond Earth will be one of the most intensive discoveries in the history of science. The Milky Way Galaxy hosts hundreds of crores of potential living planets alone. Astronomers are using powerful space telescopes to see the molecular indicators of biology in the Earth’s atmosphere of these planets.
But till now, no concrete evidence of life has ever been found beyond the earth. A paper published in April 2025 claimed to have detected the signature of life in the atmosphere of K2–18B. And while this discovery is complicated, most astronomers – including paper writers – that is not ready to claim that there is an extraterrestrial life. Life detection would be a remarkable development.
Astronomer Carl Sagan, in relation to the discovery of foreign life, “exceptional claims require extraordinary evidence,” used the phrase. This explains the idea that there should be a high bar for evidence to support a remarkable claim.
I am an astronomer who has written a book about astrobiology. During my career, I have seen some compelling scientific discoveries. But to reach this threshold of finding life beyond the earth, the result needs to fit several important criteria.
When is the result important and reliable?
There are three criteria to represent a true discovery for a scientific result and are not subject to uncertainty and doubt. How does the claim of life measures on K2-18 B?
First, the experiment needs to measure a meaningful and significant amounts. Researchers visited the atmosphere of K2–18B with James Web Space Telescope and saw a spectral feature that they recognized as dimethyl sulfide.
On Earth, dimethyl sulfide is associated with biology, in special bacteria and plankton in oceans. However, it can also arise in other ways, so this single molecule is not a decisive proof of life.
Second, the detection must be strong. Each detector has some noise at the random speed of electrons. The signal must be quite strong to be less likely to be produced by coincidence with this noise.
K2–18 B detection is the importance of a 3-igma, which means that it has 0.3 percent likely to be produced by coincidence.
It seems less, but most scientists will consider that a weak identity. There are many molecules that can create a feature in the same spectral range.
The “Gold Standard” for scientific identity is 5-Sigma, which means that the possibility of coincidence is less than 0.00006 percent. For example, CERN physicists collected patiently data for two years, until they were not detected by the Higgs Boson particle, was the lead to the Nobel Prize after a year in 2013.
Higgs Boson’s discovery announced for decades when Peter Higgs first predicted the existence of the particle. Scientists, such as Inkandela shown here, waited until they reached that 5-igma level, to say, ‘I think we have it.’
Third, a result must be repeated. The result is considered reliable when they are repeated – ideally confirmed by other investigators or using a separate device. For K2–18 B, it may mean other molecules that indicate biology, such as oxygen in the planet’s atmosphere. Without more and better data, most researchers are looking at the claim of life on K2–18B with suspicion.
Claim of life on Mars
In the past, some scientists have found very close life on Mars, very close to the house.
A century ago, the retired Boston merchant turned the astronomer Purchail Lowell that the linear features seen on the surface of Mars were canals, to reach the equator from the poles by a civilization built by a dying civilization. Artificial waterways on Mars would definitely be searching a major, but this example failed the other two criteria: strong evidence and repetition.
Lowell was misled by his visual comments, and he was engaged in a wishful thinking. No other astronomer 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. His evidence included a stimulating image that showed microfosils in the meteorite.
However, scientists have come up with clarification to the unusual characteristics of meteorite that does not include biology. That extraordinary claim has been destroyed.
Recently, astronomers discovered low levels of methane in the atmosphere of Mars. Like dimethyl sulfide and oxygen, methane on earth is mainly formed – but not especially – by life. Separate spacecrafts and Rovers have refunded conflicting results on the Martian surface, where the detection with a spacecraft was not confirmed by the other.
The low level and variability of methane on Mars is still a mystery. And in the absence of definite evidence that this low level of methane has a biological origin, no one is claiming a definite evidence of life on Mars.
Claims of advanced civilizations
It would be dramatic to detect microbial life on Mars or exoplanets, but the discovery of extractic civilizations would be really luxurious.
Supernatural intelligence, or Seti has been discovered for 75 years. No message has ever been received, but a radio telescope in Ohio in 1977 detected a strong sign that lasted for only one minute.
This indication was so unusual that an astronomer working in binoculars wrote “Wow!” On printout, the signal is being named its name. Unfortunately, there is nothing since that area of the sky has been revealed, so wow! The signal fails in repetition examination.
In 2017, a rocky, cigar -shaped object called ‘OUMUAMUA’, the first known intersteller object to visit the solar system. The strange shape and trajectory of ‘OUMUAMUA inspired Harvard astronomer AV LOB to argue that it was a foreign artifacts. However, the object has already left the solar system, so there is no chance for astronomers to see it again. And some researchers have gathered evidence that it is just a comet.
While many scientists feel that we are not alone, given the huge amount of living real estate beyond Earth, any detection has not cleaned the threshold by Carl Sagan.
Claims about the universe
These same criteria apply for research about the entire universe. A special concern in the universe is that, unlike the case of planets, there is only one universe to study.
The story of a precaution comes from efforts to show that the universe passed through a period of extremely rapid expansion of a part of a second after Big Bang. Cosmologist calls this phenomenon inflation, and is invited to explain why the universe is now smooth and flat.
In 2014, astronomers claimed that the microwave left after the Big Bang found evidence for inflation in the subtle signal. Within a year, however, the team withdrew the result because the signal had a worldly explanation: they confused the dust in our galaxy with the signature of inflation.
On the other hand, the discovery of the acceleration of the universe reflects the success of the scientific method. In 1929, astronomer Edwin Hubble found that the universe was expanding. Then, in 1998, evidence came out that this cosmic expansion was intensifying. Physicists were shocked by this result.
Two research groups used Supernova to detect detail separately. In a friendly rivalry, he used various sets of supernova, but got the same result. Independently, cohabitation increased his belief that the universe was accelerating. He called this quick expansion a force behind the dark energy and received the Nobel Prize in 2011 for its discovery.
On large and small scales, astronomers try to set a high bar of evidence before claiming a discovery.
Chris Empe is a reputed professor of astronomy at the University of Arizona, He receives funding from the National Science Foundation and Howard Hughs Medical Institute.
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