Could There Be Life on Other Planets?

The argument for optimism is scale. There are hundreds of billions of stars in our galaxy alone, and observations since the 1990s have confirmed that planets are common, with over five thousand exoplanets now catalogued.

The argument for caution is that we have exactly one example of life arising, and no idea how likely that event was. It could be near inevitable given the right conditions, or it could be so improbable that it has happened once. A single data point cannot distinguish between those possibilities.

Rather than looking for life directly, researchers look for conditions and chemical signatures. Liquid water is the central requirement, since all known biochemistry depends on it as a solvent.

The habitable zone is the orbital range where a planet could hold liquid water on its surface. It is a useful first filter but a crude one, since it ignores atmosphere, magnetic fields and the possibility of subsurface oceans warmed by other means.

Several targets in our own solar system are more interesting than most people realise:

• •Europa, a moon of Jupiter with a liquid ocean beneath its ice shell
• •Enceladus, a moon of Saturn venting water and organic molecules into space
• •Mars, which had liquid water and may retain life underground
• •Titan, with lakes of liquid methane and a rich organic chemistry

The most promising method for distant planets is transit spectroscopy. When a planet passes in front of its star, some starlight filters through its atmosphere, and the wavelengths absorbed reveal which gases are present.

Researchers look for biosignatures: combinations of gases that should not coexist without something continuously producing them. Oxygen alongside methane is the standard example, since they react with each other and would disappear without replenishment. The James Webb Space Telescope can now do this for some planets, though results so far are tentative and heavily debated.

The absence of detection so far is not strong evidence either way. Our instruments can only examine a tiny fraction of nearby systems, and only for particular kinds of signal.

The related puzzle is why we see no sign of intelligent civilisations despite the galaxy's age, sometimes called the Fermi paradox. Proposed explanations range from life being extremely rare, to intelligence being rare, to civilisations being short lived, to us simply not having looked properly. All remain speculative, which is the honest state of the field.

In 1961 the astronomer Frank Drake wrote down an equation breaking the question of communicating civilisations into factors: how many stars form, how many have planets, how many are habitable, how often life arises, how often it becomes intelligent, and how long such civilisations last.

Its value is not that it produces an answer, since several terms are entirely unconstrained and the result can be adjusted from one to millions by reasonable choices. Its value is organisational. It identifies exactly which unknowns matter, and the first few terms have gone from complete guesses in 1961 to measured quantities today, which is real progress on a question that once seemed unapproachable.

The history of the search contains useful warnings. In 1976 the two Viking landers ran experiments on Martian soil designed to detect life. One of them, which added nutrients and watched for released gas, produced results that looked at first like the work of microbes. Most scientists concluded that reactive chemicals in the soil could explain the signal without biology, especially because another instrument found no organic molecules in the samples. Later missions confirmed that Martian soil contains strongly oxidising compounds such as perchlorates.

In 1996 a team announced possible signs of fossil bacteria in ALH84001, a meteorite from Mars found in Antarctica. The claim made headlines worldwide, but further study showed that each feature, including tiny chain-like structures, could form through non-living processes, and most researchers now regard the case as unproven. Neither episode was a failure of science. Both show how extraordinary claims get tested: by asking whether chemistry or geology alone could produce the same evidence. Any future biosignature, from Mars or an exoplanet, will face exactly that question.

Ideas about where life could exist changed sharply in 1977, when scientists using the submersible Alvin explored hydrothermal vents on the Galapagos Rift in the Pacific. Around hot, mineral-rich springs on the dark seafloor they found thriving communities of giant tube worms, clams and crabs. The food chain was not powered by sunlight. Bacteria were extracting energy from chemicals such as hydrogen sulfide in the vent water, a process called chemosynthesis, and the animals depended on them. Many scientists now think vents like these may resemble the kind of place where life on Earth first began.

The discovery matters for astrobiology because Europa and Enceladus have oceans in permanent darkness. In 2017 scientists reported that the Cassini spacecraft had detected molecular hydrogen in the plumes of Enceladus, a sign of chemical reactions between water and rock of the kind that feed some microbes on Earth. Other organisms have widened the limits further, with microbes found in boiling hot springs, in acid, in brine and in rock more than a kilometre underground. None of this shows that life exists elsewhere, but it means the places worth searching are far more varied than sunlit, Earth-like planets.

Alongside the hunt for chemical signs of life, some scientists look for signs of technology. The first modern search, Project Ozma, took place in 1960, when Frank Drake used a 26 metre radio telescope at Green Bank in West Virginia to listen to two nearby Sun-like stars, Tau Ceti and Epsilon Eridani, close to the radio frequency naturally given off by hydrogen. He heard nothing artificial. In 1977 the Big Ear radio telescope in Ohio recorded a strong, narrow signal that so impressed the astronomer Jerry Ehman that he wrote 'Wow!' beside it on the printout. Despite many later searches, it has never been detected again.

Modern searches are far larger. Breakthrough Listen, launched in 2015, uses major radio and optical telescopes to survey a million nearby stars and many galaxies, sifting huge volumes of data for narrow-band signals that natural sources rarely produce. Researchers have also widened the list of possible technosignatures to include brief laser flashes, unexplained waste heat from enormous structures, and industrial gases in a planet's atmosphere that nature would not make. Most candidate signals so far have turned out to be interference from human technology, such as satellites, phones and once even a microwave oven, so ruling out local sources is the first and hardest step.

Life elsewhere is plausible but unproven, the search focuses on liquid water and atmospheric biosignatures rather than direct detection, and the most promising nearby targets are the ice-covered ocean moons of Jupiter and Saturn.