What if we found life on another world?

Are there living things on other worlds? Nobody knows yet.

Scientists are looking very carefully. What if they found some? Let's explore how they search.

People have wondered for thousands of years whether we are alone in the universe. NASA says the honest answer today is that we have no evidence of life beyond Earth yet. But the search is really just getting started.

So what if scientists did find life on another world? What might it be like, how would they be sure, and what would change for us? This is a thought experiment. Choose a path to explore how scientists search, what counts as a clue, and why they check every clue so carefully.

Astrobiology asks how life began, how it evolved on Earth and where it might exist elsewhere; NASA has funded it since its first exobiology grant in 1959. Despite thousands of confirmed exoplanets, NASA's position is clear: so far there is no evidence of life beyond Earth.

This thought experiment imagines a discovery and follows how it would be tested. The evidence is the real search: Perseverance's rock samples, missions to ocean moons, and telescopes that study planetary atmospheres. Anything about what we would find, or how it would change us, is an idea, not a prediction.

Earth is full of life, from tiny germs to giant whales.

It is the only place where we know life lives.

Earth is the only world where we know life exists. Scientists use what they know about life here to decide what to look for elsewhere. NASA describes life as a chemical system that keeps itself going and can change over generations through evolution.

Life on Earth is built mostly from a handful of elements, including carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur, and it works in water. Scientists pay special attention to places with water, now or long ago, such as ancient Mars and Jupiter's icy moon Europa.

NASA's working definition is that life is a self-sustaining chemical system capable of Darwinian evolution. NASA's astrobiology program notes the features of the one example we have: terran life runs metabolism directed by inherited molecules, builds its biomolecules from carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur, and depends on interactions with water.

Those traits shape the search. Missions target environments with past or present liquid water and chemical energy, and NASA has collected resources on how to detect existing life beyond Earth within the limits of robotic missions, including the Ladder of Life Detection (2018).

If scientists found life, it might be very, very tiny. Too small to see without a microscope!

They would check and check again to be sure.

Much of the search looks for signs of tiny, simple life. NASA's Perseverance rover, for example, is looking for signs of ancient microbes on Mars. So a first discovery might be a trace left by microbes long ago, not a living creature you could see.

Scientists would test it many ways and share their data so others could check it. NASA has even made a scale to show how confident scientists are about a clue. A discovery might take years to confirm. These are ideas about the future, not predictions.

Current missions mostly target microbial life: Perseverance seeks signs of ancient microbes, and NASA says scientists don't expect to find life thriving on Mars today. A first claim might therefore be a chemical or mineral pattern rather than an organism.

NASA's 2021 Confidence of Life Detection (CoLD) scale, with seven benchmarks, was proposed to guide how scientists communicate how far evidence has progressed, on a subject NASA says has a high potential to be sensationalized, alongside community work on standards of evidence and false positives. A real claim might move up that scale slowly, through peer review and independent tests. What any discovery would mean for society is an idea, not a forecast.

Some rocks and gases look like signs of life. But they can be made without life too.

So scientists test every clue many times.

A sign that might have been made by living things is called a biosignature. The tricky part is that many of these signs can also be made without life, by ordinary chemistry.

That's why NASA calls a promising clue a potential biosignature: something that might be biological but needs more data before anyone can decide. As a scientist at NASA's Jet Propulsion Laboratory put it, claims about life beyond Earth require extraordinary evidence.

NASA defines a potential biosignature as a substance or structure that might have a biological origin but needs more data or study before a conclusion about life can be reached. The difficulty is false positives: abiotic processes can mimic biology.

The Sapphire Canyon core from Mars shows the problem. Its vivianite and greigite can form through microbial reactions, but also without life under high temperatures, acidic conditions or binding by organic compounds. Perseverance project scientist Katie Stack Morgan noted that such claims require extraordinary evidence, and that while abiotic explanations are less likely given the findings, they cannot be ruled out.

A rover on Mars collects rocks. A spacecraft is flying to an icy moon. Telescopes study faraway planets.

Scientists keep looking, and nothing has been found yet.

On Mars, Perseverance drills rocks in Jezero Crater, which once held a lake. In 2025, scientists reported that one sample, Sapphire Canyon, holds a potential biosignature. It is being studied, and it might have formed without life.

NASA's Europa Clipper is heading to Jupiter's moon Europa, where scientists think a salty ocean under the ice might hold the ingredients for life. Far beyond, telescopes study planets around other stars. NASA's planned Habitable Worlds Observatory would be the first telescope designed specifically to search for signs of life on them.

Three strategies run in parallel. In situ: Perseverance had cored 27 samples in Jezero Crater by September 2025, including Sapphire Canyon with potential biosignatures reported in Nature in September 2025, for possible return to Earth. Ocean worlds: Europa Clipper will make a detailed study of Europa, whose subsurface salty ocean scientists predict may hold the building blocks for life.

Remote sensing: NASA notes thousands of exoplanets, many Earth-sized and in habitable zones, and a galaxy likely holding at least 100 billion planets. The proposed Habitable Worlds Observatory, recommended by the Astro2020 decadal survey, would be the first telescope designed specifically to look for signs of life on exoplanets.