No planet beyond Earth has been confirmed to harbor life. Despite decades of searching, scientists have not yet found definitive proof of living organisms on any other world in our solar system or beyond it. But the search has never been more promising. Several places, both nearby and light-years away, show tantalizing chemical hints that life could exist or could have existed in the past.
Why the Answer Is So Hard to Pin Down
Finding life on another planet isn’t as simple as pointing a telescope and looking. Life doesn’t announce itself with a single unmistakable signal. Instead, scientists look for indirect clues called biosignatures: combinations of gases in an atmosphere, organic molecules on a surface, or chemical energy sources that could power living organisms. The challenge is that many of these clues can also be produced by non-biological processes like volcanic activity or chemical reactions between rock and water.
The strongest theoretical sign of life would be an atmosphere in “chemical disequilibrium,” meaning it contains gases that shouldn’t coexist unless something is actively producing them. On Earth, the combination of oxygen and methane is exactly this kind of signal. Methane breaks down within roughly a decade in an oxygen-rich atmosphere, so it only persists because living things keep replenishing it. If astronomers spotted that same pairing on a distant world, it would be a powerful hint of biology at work.
Mars: Organic Molecules but No Smoking Gun
Mars is the most thoroughly explored candidate. NASA’s Perseverance rover, working inside Jezero crater (a dried-up lake bed), has detected several types of aromatic organic molecules in rock formations. These carbon-containing compounds were found embedded within minerals linked to ancient water processes, suggesting that liquid water may have played a role in creating, transporting, or preserving them.
Organic molecules are necessary building blocks for life, but they aren’t proof of it. They can form through non-biological chemistry too. What makes the Jezero findings notable is their diversity and the fact that they’ve survived millions of years of harsh radiation on the Martian surface. Perseverance has been collecting and sealing rock samples that a future mission could return to Earth, where lab equipment far more powerful than anything a rover can carry would analyze them for clearer signs of ancient biology.
Europa and Enceladus: Oceans Under Ice
Two moons in the outer solar system may be better candidates for present-day life than Mars. Jupiter’s moon Europa and Saturn’s moon Enceladus both have liquid saltwater oceans hidden beneath thick shells of ice. Enceladus has already revealed some of its secrets: NASA’s Cassini spacecraft flew through geysers erupting from its south pole and detected molecular hydrogen, carbon dioxide, methane, and organic compounds in the spray. That mix suggests hydrothermal vents on the moon’s ocean floor are generating chemical energy, the same kind of energy that supports thriving ecosystems around deep-sea vents on Earth.
Europa is thought to have an even larger ocean, possibly containing twice as much water as all of Earth’s oceans combined. NASA’s Europa Clipper spacecraft, now on its way to Jupiter, will investigate whether that ocean has the right chemistry for life. The mission’s instruments will look for sulfates, hydrogen, organic compounds, and key ions like sodium, magnesium, and chloride in any material that reaches the surface or escapes as plumes. Detecting hydrogen alongside carbon species like carbonates or carbon dioxide would point to available energy for methane-producing microbes, one of the simplest forms of life scientists expect to find.
K2-18 b: A Hint From a Distant World
Beyond our solar system, the most intriguing lead comes from K2-18 b, a planet about 8.6 times Earth’s mass orbiting a star roughly 120 light-years away. In 2023, the James Webb Space Telescope detected methane and carbon dioxide in its atmosphere. More provocatively, the data showed a possible trace of dimethyl sulfide, a molecule that on Earth is produced exclusively by life, primarily by phytoplankton in the ocean.
That detection is preliminary. The research team described it as “less robust” and requiring further validation. Additional Webb observations are planned to determine whether dimethyl sulfide is genuinely present at significant levels. If confirmed, it wouldn’t be absolute proof of life (the chemistry of this type of planet is poorly understood), but it would be the strongest biosignature candidate ever found outside our solar system.
The TRAPPIST-1 System
The TRAPPIST-1 system, about 40 light-years from Earth, contains seven Earth-sized rocky planets, three of which orbit in the habitable zone where liquid water could theoretically exist on the surface. Planet e has drawn the most attention because its distance from the star is just right for surface water, provided it has an atmosphere to maintain warmth and pressure.
So far, Webb has found no definitive signs of an atmosphere around any of the TRAPPIST-1 worlds. The innermost planet, TRAPPIST-1 b, appears to be bare rock. For planet e, researchers have ruled out a thick hydrogen-helium atmosphere (the star’s intense flares likely stripped that away long ago) and consider a Venus-like carbon dioxide atmosphere unlikely. Whether the planet managed to build a secondary atmosphere through volcanic outgassing or other processes remains an open question. Only four transits have been analyzed so far, so more data could change the picture.
What “Habitable” Actually Means
When scientists call a planet “habitable,” they don’t mean it’s comfortable or Earth-like. They mean it meets a minimum checklist: surface temperatures between roughly negative 15°C and 122°C, atmospheric pressure above about 0.01 atmospheres, and conditions that allow liquid water to exist. Life on Earth has been found growing at both extremes of that temperature range. Microbes in Arctic permafrost can grow and divide at negative 15°C, while a methane-producing organism discovered near deep-sea hydrothermal vents survives at 122°C under extreme pressure.
The habitable zone (sometimes called the Goldilocks zone) refers to the orbital distance where a planet receives enough starlight for liquid water without boiling it away. But distance from a star is only one factor. A planet needs an atmosphere to trap heat and maintain pressure. Moons like Europa show that even worlds far outside the traditional habitable zone can harbor liquid water, heated instead by gravitational forces that flex and warm their interiors.
Radio Silence From Intelligent Life
Alongside the search for microbial biosignatures, projects like SETI continue scanning the sky for artificial radio signals that would indicate technological civilizations. A recent survey using the Sardinia Radio Telescope searched for narrowband drifting signals (the kind an alien transmitter might produce) aimed at the Galactic Center and 72 nearby star systems identified by the TESS planet-hunting mission. The result: no definitive evidence of signals with an extraterrestrial origin. This outcome is consistent with every major radio survey conducted over the past six decades. The silence doesn’t rule out intelligent life, but it hasn’t confirmed it either.
The Search Is Closer Than Ever
The next decade will bring the most sensitive tests yet. Europa Clipper will reach Jupiter’s system and begin detailed flybys of Europa’s icy surface, sniffing for ocean chemistry that could support organisms. Webb will continue probing the atmospheres of K2-18 b and the TRAPPIST-1 planets with increasingly precise measurements. NASA is also developing concepts for a Habitable Worlds Observatory, a future space telescope specifically designed to image Earth-like planets around sun-like stars and analyze their atmospheres for oxygen, methane, water vapor, and other biosignature gases.
For now, Earth remains the only known planet with life. But the list of places where life could plausibly exist, from Martian rock to ice-covered ocean moons to distant exoplanet atmospheres, is longer and better supported by evidence than at any point in human history.