Could There Be Life on Other Planets?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Nobody knows whether there is life on other planets, and anyone claiming certainty in either direction is going beyond the evidence. What has changed is that the question is now a research programme with real instruments and specific targets rather than pure speculation.
The case for optimism: scale
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.
What scientists actually look for
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.
Promising places nearby
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
Looking at exoplanet atmospheres
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.
Why we have found nothing
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.
The Drake equation
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 takeaway
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.