Are we alone?

Is there life anywhere else? It is one of the oldest questions people have asked, and the honest answer today is that nobody knows.

So far, life is known on exactly one world: this one. No sign of living things has been confirmed anywhere else, and no message from anyone has ever been picked up.

That is not because we have looked everywhere. We have barely started. The space we could search has been compared to all the Earth’s oceans, and what we have searched so far to a bathtub of water.

All life on Earth needs liquid water, so that is the first thing to look for. Mars had rivers and lakes long ago. Several moons of Jupiter and Saturn have whole oceans hidden under their ice. And among the thousands of planets found round other stars, some are at the right distance from their star for water to stay liquid.

Spacecraft are on their way to some of these places now.

This lesson has more open questions than any other on the site. It sets out what is known, what is being done, and how to judge the claims that appear in the news.

One example. Everything we know about life comes from a single case. All living things on Earth are related, built from the same few chemical elements, and dependent on liquid water. The oldest widely accepted fossils, layered mats made by microbes, are about 3.5 billion years old, and there are chemical hints older still. So life appeared within the first billion years of a 4.5 billion year old planet. For most of the time since, it has been microbes only. Animals are a late arrival, in the last 600 million years or so.

Two lessons are usually drawn. Life may start readily where conditions allow, though one example cannot prove it. And if life elsewhere is like life here, most of it will be microscopic.

What to look for. The search is guided by what life here needs: liquid water, a supply of energy, and the right chemical ingredients, carbon above all. Where all three are found together, a place is called habitable. That means life could survive there. It does not mean anything lives there.

Where, in the Solar System.

Place	Why	What is happening
Mars	Had lakes and rivers more than 3 billion years ago. Now cold and dry, with ice below ground	The Perseverance rover has explored an ancient lake bed since 2021 and has set aside rock samples. In 2024 it found a rock with chemical patterns that on Earth are often, but not always, made by microbes. Settling it needs the samples in a laboratory, and there is at present no agreed plan to bring them back
Europa (moon of Jupiter)	A salt water ocean under the ice, holding about twice the water of all the Earth’s oceans	Europa Clipper, launched in 2024, arrives in 2030. Europe’s Juice mission arrives at Jupiter in 2031
Enceladus (moon of Saturn)	Sprays its ocean into space through cracks. The Cassini spacecraft flew through the spray and found salts, carbon compounds, hydrogen and phosphates	No mission on its way yet
Titan (moon of Saturn)	Thick air, lakes of liquid methane, and rich carbon chemistry	Dragonfly, a flying lander, is due to launch in 2028 and arrive in 2034

None of these missions is designed to find life outright. They are designed to find out whether each place could support it.

Round other stars. No spacecraft can reach another star’s planets in any useful time, so the search is done with light. A biosignature is something measurable from far away that would be hard to explain without life. The best example is in our own air. Oxygen and methane destroy one another, so finding both together means something keeps supplying them. On Earth that something is life.

Reading the air of a small rocky planet is at the edge of what the Webb telescope can do, and no biosignature has been confirmed. There have been claims: a gas called phosphine on Venus in 2020, and a sulphur compound on the planet K2-18 b in 2025. Other teams looking at the same or newer data did not agree, and neither claim is accepted.

Listening. Since 1960, radio telescopes have listened for signals from other civilisations, a search known as SETI. Nothing confirmed has been heard. The search so far has covered a tiny part of the sky, of the radio dial and of time, so silence does not yet tell us much.

How to read a headline. A claim of life elsewhere has to pass several tests. Is the measurement real, and has another instrument repeated it? Could anything that is not alive produce the same result? Volcanoes, sunlight and ordinary chemistry can make many of the same gases and patterns. So far every claim has stopped at one of these steps. Scientists in this field expect the first true discovery to arrive not as one announcement but as years of evidence adding up.

If the answer is no. That would be worth knowing too. A galaxy of several hundred billion stars with life on one planet, and a galaxy full of it, are both hard to imagine. At present nothing rules out either.

Three piles. No life has been found anywhere but the Earth. Everything written on the subject falls into one of three piles: things that have been measured, claims that were made and then disputed, and guesses. This level keeps the three apart, and each heading says which pile it is.

Measured: planets are common. One survey used gravitational lensing, in which a star and its planets briefly magnify the light of a more distant star behind them. It concluded that stars have planets as a rule. It found that 17 per cent of stars have a planet of about Jupiter’s mass between 0.5 and 10 times the Earth’s distance from the Sun, and that smaller planets are commoner still.

Measured, with wide error bars: rocky planets at the right distance. The Kepler telescope’s survey gives the number of planets between 0.5 and 1.5 times the Earth’s radius, in the habitable zone, for each star of roughly the Sun’s temperature (4,800 to 6,300 K):

Definition of the habitable zone	Planets per star
Conservative	Between 0.37 (+0.48, −0.21) and 0.60 (+0.90, −0.36)
Optimistic	Between 0.58 (+0.73, −0.33) and 0.88 (+1.28, −0.51)

The two ends of each range come from two extreme ways of allowing for planets Kepler would have missed, and the errors are large because few such planets were found. The same study estimates that the nearest one, round a star of the Sun’s kind or the kind a little cooler, is about 6 parsecs away, and that about four lie within 10 parsecs. By our own sum those distances are 20 and 33 light years.

What this figure does not say matters as much. A planet in the habitable zone gets the right amount of starlight for liquid water. Whether it has water, air or life has not been measured for any of them.

Measured: some gases in one distant atmosphere. K2-18 b is a temperate planet smaller than Neptune. The Webb telescope found methane in its air at 5 sigma and carbon dioxide at 3 sigma. Neither gas needs life to explain it, and the team that found them did not claim it did. The dispute described below is about a different gas.

Claimed and disputed. Each of the following was reported in a serious journal, and each ran into trouble when others checked it.

Claim	What was measured	What happened next
The Martian meteorite ALH84001	Carbonate globules, magnetite grains and carbon compounds, argued to be the work of microbes	Others built a fuller case that all of it could form without life. The consensus now is that the features are probably not signs of life
Phosphine on Venus, 2020	One absorption line at radio wavelengths, read as about 20 parts in a billion of a gas with no known way of forming there. Its authors offered unknown chemistry, or life, as the source	An independent team found that the line cannot be told apart from one of sulphur dioxide beside it, and set upper limits that disagree with the claim
Dimethyl sulphide on K2-18 b, 2023 and 2025	A hint in 2023 that its authors called of low significance. In 2025, 3 sigma evidence for that gas or a close relative, from a different instrument	A joint analysis of all the data found insufficient evidence for either gas. Other molecules, such as ethane, fit equally well. It estimated that about 25 more passes of the planet in front of its star would be needed to tell the claimed feature from a flat line at 3 sigma
Spots on a Martian rock, 2024 and 2025	Two iron minerals, vivianite and greigite, in a pattern that on Earth microbes can produce, in rock made of clay and silt and rich in organic carbon	NASA calls it a potential biosignature. The minerals can also form without life, and the team says that cannot be ruled out. The sample is one of 27 rock cores the rover has collected, and it is still on Mars
A radio signal from the direction of Proxima Centauri, 2019	A narrow signal near 982 MHz	Traced to interference from electronics near the telescope

A second study of the K2-18 b claim made a wider point. Whether a faint gas appears to be detected depends on which other gases the computer model is allowed to try. When more were allowed, the claim vanished.

A scale for claims. In 2021 a group of NASA scientists proposed that evidence for life should be reported on a ladder of confidence, not as yes or no. Their example has seven steps. The wording below is ours, following their paper.

Step	What has been shown
1	A signal of a kind that life is known to produce
2	Contamination ruled out
3	The setting is one where a biological explanation makes sense
4	Every source that is not life ruled out
5	A second, independent signal, or checking by independent investigators
6	Several independent lines of evidence, with rival explanations dismissed
7	Follow-up observations, made because of the earlier steps, that find what life would predict

By the authors’ own reckoning the meteorite reached step 3 and no higher. They judged in advance that a rover on Mars could reach step 3, and that only samples studied on Earth could reach step 6. They single out step 4 as especially hard for planets that can only be studied from afar. NASA has not placed the Martian rock of 2024 on the scale. But its team says explanations without life cannot be ruled out, and ruling them out is step 4, so by the scale’s own terms it has not passed that step.

Why one gas is never enough. Oxygen was once thought a sure sign of life. A review of the question found it fails in both directions. Life on Earth may have gone several billion years without filling the air with oxygen, so a living planet can show none. And there are ways for a lifeless planet to build up a good deal of it. Oxygen counts as evidence only when the rest of the planet’s circumstances are known well enough to rule those out.

Guesswork: the number of civilisations. In 1961 Frank Drake wrote the question as a product of seven factors:

$$N = R_* \, f_p \, n_e \, f_l \, f_i \, f_c \, L$$

$N$ is the number of civilisations in the Milky Way giving out signals that could be detected.

Factor	Meaning	State of knowledge
$R_*$	Suitable stars formed each year	Measured. It was the only factor known in 1961
$f_p$	Fraction of those stars with planets	Measured. The 1961 guess, close to all of them, agrees with modern surveys
$n_e$	Planets in each system fit for life	Partly measured. The Kepler figure above counts planets of the right size at the right distance, not planets known to be fit for life
$f_l$	Fraction of those where life appears	Unknown. One example, the Earth
$f_i$	Fraction of those where intelligence arises	Unknown
$f_c$	Fraction of those that give out detectable signals	Unknown
$L$	Years for which they go on doing so	Unknown. We have been doing it for less than a century

Four of the seven have never been measured, and the answer is only as good as the worst of them. Published values of $N$ run from 1, meaning only us, to several million. Drake’s own figure of 10,000 comes from supposing one new civilisation a year and a lifetime of 10,000 years. The SETI Institute says plainly that the equation cannot be solved or even accurately calculated. It is a list of what would need to be known. Any number put on $N$ today is speculation.

Measured: how much silence. Between 2016 and 2019 one search listened to 1,327 nearby stars at radio frequencies from 1.10 to 3.45 GHz, the most thorough search of that band up to then. After setting aside interference from Earth it was left with no candidates at all. That is a real measurement with a real limit. For those stars, in that band, while the telescopes were pointed at them, there was no transmitter stronger than about $2 \times 10^{12}$ watts, or $9 \times 10^{12}$ for the stars watched with the smaller of the two telescopes. It says nothing about weaker transmitters, other frequencies, other times or the rest of the galaxy’s stars.

Where it stands.

• Measured: planets are common, and rocky ones at the right distance for liquid water are probably not rare.
• Measured: no confirmed sign of life, past or present, on any other world, and no confirmed signal.
• Disputed: a potential sign in one Martian rock, which needs a laboratory on Earth, and a claimed gas on K2-18 b, which the fuller analysis does not support.
• Speculation: how often life starts, how often it becomes intelligent, and how long civilisations last. Nothing measured so far bears on these.