Is life likely to be rare or common in the universe?

What have we actually measured?

In 1961, the astronomer Frank Drake wrote an equation as the agenda for the first meeting on the search for extraterrestrial intelligence, held that November at the Green Bank Observatory in West Virginia. The Drake equation multiplies seven factors, listed in the optional detail below. According to the SETI Institute, in 1961 essentially only the first factor, the rate at which stars form, was known.

Since then, astronomers have filled in the planet factors. NASA’s Exoplanet Archive listed 6,372 confirmed exoplanets on September 25, 2026. Most of them, 4,709, were found by the transit method: watching a star dim slightly as a planet passes in front of it. Nine years of data from NASA’s Kepler telescope showed that our galaxy has more planets than stars.

The key number for life is how many planets are rocky and sit in the habitable zone. A 2020 study led by Steve Bryson at NASA’s Ames Research Center combined Kepler’s final data with star measurements from ESA’s Gaia mission. For stars similar to the Sun, it found between about 0.37 and 0.60 rocky habitable-zone planets per star, using a cautious definition of the zone. NASA summed this up as about half of Sun-like stars, and at least 300 million potentially habitable worlds in our galaxy on the most conservative reading. The authors stress that the uncertainties are large, because Kepler found only a few small planets in the habitable zone.

Does life’s early start on Earth mean life is common?

Life arose on Earth sometime in the first few hundred million years after the planet cooled enough for water-based life. (For how that may have happened, see How Did Life on Earth Begin?) In 2002, Charles Lineweaver and Tamara Davis calculated that on rocky planets where the odds are the same as they were on Earth, and that are older than about a billion years, the chance of life starting is more than 13%, at 95% confidence. They added that this does not necessarily mean life is common in the universe.

The strongest objection is a selection effect. Life appeared early here, but a technological species arrived only billions of years later. If intelligence always takes that long, an early start may simply be needed for anyone to be here asking, rather than a sign that life starts easily, as the astronomer David Kipping puts it. David Spiegel and Edwin Turner showed that with only this one data point, the answer depends mostly on the assumptions you start with. The early start is consistent with life being very rare. In 2020, Kipping tried a method meant to reduce those assumptions. He found that a fast origin of life is at least 2.8 times more likely than a slow one, but that intelligence may be rare, at odds of about 3 to 2.

Spiegel and Turner point to the clearest way forward. Finding even one case of life that arose separately from ours, on Earth, elsewhere in the solar system or on another planet, would be much stronger evidence that life is not extremely rare.

Is Earth rare, or ordinary?

In their 2000 book Rare Earth: Why Complex Life Is Uncommon in the Universe, Peter Ward and Donald Brownlee argued that simple life might be widespread, but complex life like animals is uncommon. Their chapters cover conditions such as plate tectonics, the Moon and Jupiter, and mass extinctions.

The opposite instinct is Copernican. In cosmology, the Copernican principle is the working assumption that, as the Stanford Encyclopedia of Philosophy puts it, there are no special locations. Applied to life, it becomes an argument by analogy: if our planet is ordinary, life may be too. In 2016, Adam Frank and W. T. Sullivan III put a number on the hopeful side. Unless the odds of a technological species arising on a habitable-zone planet are worse than about one in a trillion trillion, they found, at least one other technological species has evolved at some point in the history of the observable universe.

The catch is that both sides reason about odds nobody has measured. The SETI Institute notes that estimates for the number of communicating civilizations in our galaxy range from one to several million. In a 2018 paper posted online, Anders Sandberg, Eric Drexler and Toby Ord argued that our uncertainty about several factors, including the origin of life, spans many powers of ten. Taking that seriously, they found a substantial chance that there is no other intelligent life in the observable universe.

Did the Webb telescope find signs of life on K2-18b?

What was observed. K2-18b is a planet about 8.6 times Earth’s mass and 2.6 times its width. It orbits a cool red dwarf star, in the habitable zone, roughly 120 light-years away. When it passes in front of its star, a little starlight filters through its air, and gases leave fingerprints in that light. In 2023, Webb found methane and carbon dioxide there. NASA said a hint of a gas called dimethyl sulfide (DMS) was less robust and needed more checks.

What was inferred. On Earth, DMS is made by life, mostly by marine plankton. In April 2025, a team led by Nikku Madhusudhan at the University of Cambridge used another Webb instrument and reported DMS and/or a related gas at the “three-sigma” level. The team said an ocean teeming with life was the scenario that best fit its data, while stressing that more data were needed.

Does the evidence support it? Not yet. The Cambridge team explained that three sigma means a 0.3% probability that the signal arose by chance, while the accepted bar for a discovery is five sigma, below 0.00006%. Independent teams then re-analyzed the data. One, in Nature Astronomy, showed the claim fades when more possible gases are allowed into the models. A joint analysis of Webb’s full spectrum of the planet, in Astronomy & Astrophysics, found insufficient evidence for DMS and showed that ethane fits equally well. A study of instrument noise concluded that K2-18b does not meet the standards of evidence for life. And a team led by Renyu Hu at NASA’s Jet Propulsion Laboratory, including Madhusudhan, found only marginal DMS signals in four new Webb observations, in a 2025 preprint not yet peer-reviewed. Its models also suggested DMS might form without life in some hydrogen-rich atmospheres.

What is unresolved. In a search of 661 molecules published in 2026, Madhusudhan’s group found DMS to be the only one its data consistently favored, by a modest margin, and called for more observations. Other recent studies reproduce the spectrum with a hydrogen-rich atmosphere, without needing DMS. NASA says that confirming a true biosignature takes hundreds of hours of observing, follow-up studies and multiple converging lines of evidence. Our label for “Webb found signs of life on K2-18b” is Not supported. What is in its air is still an open question.

What do people often get wrong?

“Habitable” means “inhabited.” It means only that the temperature could allow liquid water. NASA calls the habitable zone a first cut. A planet also needs a suitable size and atmosphere, and a stable star not prone to sterilizing flares.

“With billions of planets, aliens must exist.” Big numbers multiplied by unknown odds give unknown answers. If the chance of life starting is tiny enough, even billions of planets may not be enough.

“A potential biosignature is a discovery of life.” In 2025, NASA announced a potential biosignature in a Mars rock sample called Sapphire Canyon. NASA defines that as something that might have a biological origin but needs more data before anyone can conclude life was there. Scientists grade such claims on tools like the seven-step Confidence of Life Detection scale.

Could life be built on something other than carbon?

A 2007 National Research Council report, written to help NASA, explored whether life might use chemistry not based on carbon compounds, or a liquid other than water. Silicon, one alternative, fares poorly. In 2020, Janusz Petkowski, William Bains and Sara Seager reviewed silicon chemistry and found that life based mainly on silicon was not a plausible option in any environment they studied. In water, silicon tends to lock up as silica, which limits its chemistry. So silicon looks like a poor stand-in for carbon, while other kinds of chemistry remain a research question.

How can you check this yourself?

• Watch the planet count. NASA’s Exoplanet Archive shows the number of confirmed planets on its home page, with the date it was updated.
• Look for the sigma. Three sigma is a hint. Five sigma is the usual bar for a discovery claim.
• Ask who re-checked it. A claim of life should survive independent teams using their own methods, as the K2-18b story shows.
• Watch the small words. “Potential,” “possible” and “hints” do not mean “confirmed.”
• Try the Drake equation yourself. Put in your own guesses for each factor. Then make the biology guesses 1,000 times smaller and see how much the answer swings.

After all of this, the verdict is unchanged. Homes for life look common. Whether life itself is common is an Open question, and the most useful thing anyone could find is a second example.