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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 . Astronomers are using 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 Earth. A paper published in April 2025 claimed to detect a signature of life in the atmosphere of the planet K2-18b. 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 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 who has written a . 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 and saw a spectral feature that they identified as dimethyl sulfide.

On Earth, dimethyl sulfide is , in particular bacteria and plankton in the oceans. However, it can also arise by other means, so this single molecule is .

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 , which means it has a 0.3% probability of arising by chance.

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

The "gold standard" for scientific detection is , which means the probability of the finding happening by chance is less than 0.00006%. For example, gathered data patiently for two years until they had a 5-sigma detection of the , leading to a one year later in 2013.

Third, a result . 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 in the planet's atmosphere. Without more and better data, most researchers are viewing the claim of life on K2-18b with .

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 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. .

In 1996, NASA held a press conference where a team of scientists presented evidence for biology in the . 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 . That extraordinary claim has dissipated.

More recently, astronomers detected in the atmosphere of 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 , 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 , 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 fails the test of repeatability.

In 2017, a was the first 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 . 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 .

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 to study.

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 . Cosmologists , and it is invoked to explain why the universe is now smooth and flat.

In 2014, astronomers claimed to have found 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 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 found that the universe was expanding. Then, in 1998, evidence emerged that this cosmic expansion is accelerating. ÃÈÃÃÉçÇøicists were .

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 . They called the force behind this accelerating expansion and received a in 2011 for its discovery.

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

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