No other planet in our solar system is confirmed habitable, but several worlds, including a handful of moons and distant exoplanets, have conditions that make life at least plausible. The key ingredient scientists look for is liquid water, and it turns out water may exist in more places than you’d expect: buried under ice on Mars, sloshing in vast underground oceans on moons of Jupiter and Saturn, and potentially covering the surface of planets orbiting other stars entirely.
What Makes a Planet “Habitable”
Habitability starts with one concept: the habitable zone, sometimes called the Goldilocks zone. This is the range of distances from a star where temperatures allow liquid water to pool on a planet’s surface. Too close and water boils off. Too far and it freezes solid. Earth sits comfortably in our sun’s habitable zone, and that’s the baseline scientists use when evaluating other worlds.
But the habitable zone is really just a starting point. A planet also needs an atmosphere thick enough to maintain stable surface pressure and trap some heat. Without one, even a perfectly positioned world would lose its water to space. Mars is a good example: it sits near the outer edge of our sun’s habitable zone, but its thin atmosphere means liquid water can’t persist on the surface today. The habitable zone tells you where to look, not what you’ll find.
Mars: The Closest Candidate
Mars once had rivers, lakes, and possibly a shallow ocean. Today its surface is cold and dry, with atmospheric pressure so low that exposed water would either freeze or evaporate almost instantly. But that doesn’t mean all the water is gone.
In 2018, radar data from the Mars Express spacecraft revealed evidence of liquid water trapped beneath the southern polar ice cap. The water appears to stay liquid because of extremely high salt content and the pressure of the ice above it, surviving at temperatures around minus 15 to minus 18 degrees Celsius. Salty, frigid, and buried under ice isn’t exactly inviting, but on Earth, microbes thrive in similarly harsh conditions: deep in Antarctic ice, in ultra-salty lakes, and in rock formations kilometers underground. If life ever got started on Mars during its warmer, wetter past, pockets of briny subsurface water are the most likely place it could still hang on.
Europa: An Ocean Beneath the Ice
Jupiter’s moon Europa is roughly the size of Earth’s moon, but it may hold more liquid water than all of Earth’s oceans combined. A thick shell of ice covers its surface, and beneath that shell sits a global saltwater ocean estimated to be 60 to 150 kilometers deep. The ocean stays liquid because Jupiter’s enormous gravity flexes Europa’s interior, generating heat through tidal forces.
What makes Europa especially interesting is that its rocky seafloor likely interacts directly with the ocean water. Scientists expect high-temperature hydrothermal vents on that seafloor, similar to the deep-sea vents on Earth that support entire ecosystems without sunlight. Chemical reactions between rock and water could supply the energy that life needs. NASA’s Europa Clipper spacecraft, which launched in October 2024, will arrive at Jupiter in 2030 and conduct nearly 50 flybys to map Europa’s ice shell and search for signs of habitability beneath it.
Enceladus: Organic Molecules in the Spray
Saturn’s moon Enceladus is tiny, only about 500 kilometers across, but it punches well above its weight in habitability discussions. Geysers at its south pole shoot plumes of water vapor and ice particles hundreds of kilometers into space, and NASA’s Cassini spacecraft flew directly through those plumes multiple times before the mission ended in 2017.
What Cassini found was striking: saltwater, hydrogen gas (a potential food source for microbes), and complex organic molecules, including large, heavy compounds that had never been detected there before. The hydrogen strongly suggests hydrothermal activity on the ocean floor, where hot water reacts with rock. On Earth, that exact chemistry powers microbial communities in the deep ocean. Enceladus essentially advertises the ingredients for life by spraying them into space, making it one of the most compelling targets for future missions.
Titan: A Different Kind of Habitability
Saturn’s largest moon, Titan, is unlike anything else in the solar system. It has a thick, hazy atmosphere denser than Earth’s, weather systems with rain and storms, and vast lakes and seas on its surface. The catch: those lakes aren’t filled with water. They’re liquid methane and ethane, pooled across Titan’s subpolar regions at surface temperatures around minus 180 degrees Celsius.
Titan’s atmosphere is rich in nitrogen and carbon, and complex chemical reactions driven by sunlight produce a steady rain of organic compounds, giving the moon its characteristic orange haze. Laboratory experiments simulating Titan’s conditions have shown that these reactions can produce nitrogen-containing molecules that are important precursors to amino acids. If a large asteroid impact were to puncture Titan’s icy crust and expose subsurface liquid water mixed with ammonia, that temporary warm environment could persist for thousands of years, potentially long enough for interesting chemistry to occur. Titan stretches the definition of habitability: it’s not a place where Earth-like life would be comfortable, but it might host chemistry we haven’t imagined yet.
Exoplanets in the Habitable Zone
Beyond our solar system, thousands of exoplanets have been discovered, and a growing number sit in the habitable zones of their stars. Three systems stand out.
TRAPPIST-1
This small, cool star about 40 light-years away hosts seven Earth-sized planets, three of which (designated e, f, and g) orbit within the habitable zone. Any of the seven could potentially have water, but the three in the habitable zone are the most likely to support liquid water on their surfaces. The TRAPPIST-1 system is the largest known collection of roughly Earth-sized worlds in a single star’s habitable zone, which makes it a priority target for atmospheric studies.
Proxima Centauri b
The nearest known exoplanet in a habitable zone orbits Proxima Centauri, just 4.2 light-years from Earth. But proximity to its star creates problems. Proxima Centauri b orbits at only 0.0485 AU (far closer than Mercury is to our sun), and its host star is moderately active, with a magnetic field roughly 600 times stronger than the sun’s and frequent powerful flares. Those flares blast the planet with ultraviolet radiation and energetic particles that could strip away an ozone layer and make the surface hostile to life. Still, modeling suggests that if the planet has oceans, life could survive at water depths of about 9 meters or more, deep enough to be shielded from radiation while still receiving enough light for photosynthesis.
K2-18b
This is currently the most intriguing exoplanet for biosignature detection. K2-18b is 2.6 times the size of Earth and 8.6 times as massive, orbiting in the habitable zone of its star 124 light-years away in the constellation Leo. Using the James Webb Space Telescope, astronomers at the University of Cambridge detected methane and carbon dioxide in its atmosphere, the first time carbon-based molecules were identified on a habitable-zone exoplanet. Those findings are consistent with a “Hycean” world: a planet covered by ocean beneath a hydrogen-rich atmosphere.
Even more compelling, follow-up observations detected chemical signatures of dimethyl sulfide and dimethyl disulfide at a three-sigma level of statistical significance, meaning there’s only a 0.3% chance the signal occurred by random noise. On Earth, these sulfur compounds are produced almost exclusively by living organisms, primarily marine microbes. The concentrations on K2-18b appear to be thousands of times higher than on Earth, exceeding ten parts per million compared to less than one part per billion here. Theoretical models had predicted that Hycean worlds could sustain such high levels, and the observations match those predictions. This isn’t proof of life. Other explanations remain possible. But it’s the strongest chemical hint of biology ever found outside our solar system.
Why “Habitable” Doesn’t Mean “Inhabited”
Every world on this list has at least one feature that would make life possible: liquid water, chemical energy, organic molecules, or a protective atmosphere. None has confirmed life. The gap between “conditions that could support biology” and “biology actually exists there” is enormous, and closing it requires either sending spacecraft to collect samples or building telescopes powerful enough to read atmospheric chemistry from light-years away. Both efforts are underway. Europa Clipper will survey one of the most promising ocean worlds starting in 2030, and JWST continues to dissect exoplanet atmospheres with unprecedented precision. The question isn’t really whether other habitable places exist. It’s whether anything is living in them.