Is anyone else out there?

More than six thousand exoplanets have been confirmed, a count the archive cited here revises upward every few weeks. A statistical analysis of microlensing surveys found that stars in our galaxy have planets as a rule rather than as an exception, and NASA states that planets outnumber stars. Rocky planets in the temperate zone around Sun-like stars are not rare: one analysis of Kepler data puts their number at between about 0.37 and 0.60 per star, with wide uncertainties.

The NASA Exoplanet Archive maintains the confirmed list and it grows continuously.

Saturn’s moon Enceladus has liquid water, organic molecules and a source of chemical energy, the three things life on Earth draws on. Cassini flew through the plume it vents from an ocean under its ice and found large organic molecules in the ice grains, and hydrogen in the gas at a level that signals chemistry out of equilibrium in the ocean, which is a source of energy. Silica grains 2 to 8 nanometres in radius, traced to the same ocean, point to water above 90 °C reacting with rock on its floor, which is evidence of hydrothermal activity under the ice.

Mars is the other case, and one group argues that its surface may not be the best place to look. By the time oxygenic photosynthesis arose on Earth, Mars had been a hyperarid frozen desert under solar and cosmic radiation for more than a billion years, so photosynthetic surface life may never have happened there at all; ancient hydrothermal deposits in its subsurface may be the best targets for evidence of early life, and for clues about the origin of life here. That is a perspective piece rather than a measurement, and it is offered as one. In 2025 the Perseverance rover team reported mudstones in Jezero Crater, laid down in water, whose minerals and organic carbon “warrant consideration as potential biosignatures”. The same paper sets out how those minerals could form without life, and says deciding between the two needs the sample brought to Earth.

Radio searches have produced no confirmed signal of artificial origin. Breakthrough Listen’s survey of nearby stars, which its team called the most comprehensive of its kind when it was published, looked at 1,327 of them with the Green Bank and Parkes telescopes between 2016 and 2019, searched for narrowband engineered signals across 1.10 to 3.45 gigahertz, and after removing terrestrial interference was left with zero candidates. In a 2023 search that counted every star in the telescope’s beam, about 11,680 of them, every detection was either rejected automatically or confirmed as human-made.

No biosignature has been established in an exoplanet atmosphere either. A candidate reported on the planet K2-18 b is contested between the team that reported it and others who reanalysed the data.

Detecting a gas in an exoplanet atmosphere is difficult. In 2025 one team reported dimethyl sulfide or dimethyl disulfide, both possible biosignature gases, in James Webb observations of K2-18 b at three standard deviations; another team, analysing all of the planet’s Webb spectra together, found insufficient evidence for either. Establishing that only life could produce a gas is harder again. The 1996 claim of past life on Mars in the meteorite ALH84001 is now generally judged not to be evidence of life, after other researchers built the case for an abiotic origin of its features.

In 2021 a group of NASA scientists called in Nature for an agreed scale for reporting evidence of life, so that a result could be placed on it rather than announced as a yes or a no. They pointed to a history of life-detection claims that later proved incorrect or ambiguous, and wanted the scale agreed before a result arrived, not in response to one.

Frank Drake’s 1961 formulation organises the question into factors: how fast stars form, how many have planets, how many of those planets could support life, on how many life arises, and so on. The first three are now estimated from observation: the rate of star formation was known in 1961, and exoplanet surveys have since supplied the fraction of stars with planets and, less firmly, how many of those planets could support life. The ones involving life and intelligence are not, and cannot be estimated from a single example.

It is best understood as a way of structuring ignorance rather than as a calculation.

This is the load-bearing unknown. Life began on Earth quickly, which is suggestive, but a sample of one, observed by beings who could only exist if it happened, is weak evidence. A Bayesian analysis of exactly this found that what it implies depends mostly on the assumptions brought to it, and that it is consistent with life arising only very rarely.

If life and intelligence were common, the absence of any detection would need explaining. Proposed explanations range from the sobering to the speculative, and no observation has yet decided between them. The honest position is that the searches have covered a small fraction of the relevant parameter space, of frequencies, of directions, of time: the Breakthrough Listen survey watched each of its 1,327 stars for three five-minute spells.

Most of our detection methods assume life like Earth’s: carbon chemistry, liquid water and, in the case of intelligence, mostly radio. These assumptions are reasonable, and with one example of life to go on they cannot yet be tested. A biosphere unlike Earth’s might be invisible to every instrument we currently operate.

Life on Earth started early. One molecular-clock study puts the last common ancestor of all cells before 3.9 billion years ago and a 2024 study at about 4.2 billion, while the ancestor of every modern complex cell lived less than 1.84 billion years ago, more than two billion years later. If that second step is the bottleneck, the galaxy could be full of microorganisms and empty of anyone to talk to. A 2021 model of the timing of these transitions reaches a version of that conclusion: intelligent life rare even if life itself starts easily.

Radio searches have covered a small fraction of the stars, frequencies and times they could. The survey cited here reached 1,327 of the 1,702 nearby stars on its list. One 2018 estimate put everything searched by then at about a large hot tub’s worth of water out of all of Earth’s oceans; an earlier one, using a different measure, at a drinking glass.

One proposal for the first evidence is not a signal but an atmosphere held away from chemical equilibrium. The appeal of the test is that it assumes nothing about which gases a biosphere would make: it asks only how far the whole atmosphere sits from the state it would relax into. Measured that way, Earth’s air on its own is unremarkable, with less of this disequilibrium than Mars’s. Counted together with the ocean it touches, Earth is between about twenty and two million times further from equilibrium than the other atmospheres of the solar system, and most of that is the coexistence of nitrogen, oxygen and liquid water. Its authors add that whether it works as a test on exoplanets, with false positives and noisy observations, is still to be established.