No planet we’ve discovered so far is ready for humans to walk outside and breathe freely. But Mars is the closest thing we have, and it’s the only world where scientists and engineers are actively planning for human settlement. Beyond our solar system, a handful of exoplanets sit in the right zone around their stars to theoretically support liquid water, though reaching any of them is far beyond current technology.
Why Mars Is the Top Candidate
Mars checks more boxes than any other planet in our solar system. It has a 24.6-hour day, seasonal cycles, polar ice caps, and evidence of ancient riverbeds. Its surface gravity is about 38% of Earth’s, meaning a 150-pound person would weigh roughly 57 pounds there. That’s low, but it’s enough to keep your feet on the ground and may be sufficient to prevent the worst bone and muscle loss that astronauts experience in zero gravity.
The atmosphere, however, is a dealbreaker for unprotected humans. It’s over 95% carbon dioxide with almost no oxygen, and surface pressure is less than 1% of Earth’s. Average surface temperature hovers around minus 31°C (minus 24°F) at ground level and drops further with altitude. You’d need a pressurized habitat, a reliable oxygen supply, and serious radiation shielding to survive even a single day.
What It Would Take to Live on Mars
Water is the foundation of any Mars settlement, and the planet actually has some. Regions of Martian soil contain 8 to 10 percent water by mass in the top meter of ground. Engineers have already tested a method for extracting it: heating the soil to bake off the water while blowing Martian air through the material as a sweep gas to collect the vapor. A rotating tiller churns up fresh moist soil from below the surface to keep the process going. It’s not elegant, but early experiments show it works.
Oxygen is the other critical piece. NASA’s MOXIE instrument, which rode aboard the Perseverance rover, proved that you can split carbon dioxide from the Martian atmosphere to produce breathable oxygen. At its best, MOXIE generated about 10.56 grams of oxygen per hour, sustained production of nearly 10 grams per hour for 40 minutes at a stretch. That’s a tiny amount (a person needs roughly 840 grams of oxygen per day), but MOXIE was a small technology demonstration. A scaled-up version could, in theory, produce enough oxygen for breathing and rocket fuel.
Radiation remains one of the hardest problems. Mars has no global magnetic field and only a wisp of atmosphere, so cosmic rays and solar particle events reach the surface largely unblocked. A 650-day Mars mission scenario (including transit time) would expose astronauts to radiation doses approaching or exceeding the 600 millisievert career limit NASA recently adopted. That limit exists because higher exposure meaningfully increases lifetime cancer risk. Any permanent settlement would need habitats buried under regolith or built with heavy shielding to keep residents safe over years and decades.
Venus: Livable Only in the Clouds
Venus is often called Earth’s twin because of its similar size, but its surface is one of the most hostile places in the solar system. Temperatures exceed 450°C (850°F), and the pressure would crush you like being nearly a kilometer underwater. However, about 50 kilometers above the surface, conditions become surprisingly Earth-like. At that altitude, temperatures range from 30 to 70°C (86 to 158°F), and atmospheric pressure is close to what you’d feel at sea level on Earth.
Some researchers have proposed floating habitats, essentially airships suspended in the Venusian atmosphere at that sweet spot. The concept is fascinating but wildly impractical with current engineering. The atmosphere is still mostly carbon dioxide with clouds of sulfuric acid, so you’d need enclosed, acid-resistant structures. No space agency has serious plans for this, but it remains an intriguing thought experiment.
Titan: Thick Air, Brutal Cold
Saturn’s moon Titan is the only other body in our solar system with a thick atmosphere and stable liquid on its surface. Its atmosphere is about 95% nitrogen (Earth’s is 78% nitrogen), and the surface pressure is roughly 60% higher than Earth’s, comparable to being 15 meters underwater. You wouldn’t need a pressure suit, just protection from the cold and a way to breathe.
The catch is temperature. Titan’s surface sits around minus 179°C (minus 290°F). The lakes and seas on its surface aren’t water but liquid methane and ethane. Generating enough energy to keep a habitat warm in that environment would be an enormous challenge, and Titan is about 1.2 billion kilometers from Earth, making resupply missions far more difficult than trips to Mars.
Exoplanets in the Habitable Zone
Beyond our solar system, astronomers have identified several planets orbiting within their star’s habitable zone, the distance range where liquid water could exist on the surface. These are the most exciting long-term candidates, but they come with a major caveat: even the closest one is unreachably far with any propulsion system that exists or is under development.
Proxima Centauri b orbits the nearest star to our sun, about 4.2 light-years away. It has a mass of roughly 1.07 Earths and sits in its star’s habitable zone. But it orbits a red dwarf star at just 0.049 AU (far closer than Mercury is to our sun), completing a full orbit every 11.2 days. Red dwarfs are prone to intense flares that could strip away a planet’s atmosphere over time. Whether Proxima Centauri b has retained an atmosphere thick enough to support life is unknown.
TRAPPIST-1e, about 40 light-years away, is considered the most promising of the seven planets in the TRAPPIST-1 system. Modeling suggests it could have Earth-like surface temperatures and possibly liquid water oceans. Like Proxima Centauri b, it orbits a red dwarf, so stellar radiation remains a concern.
Kepler-452b is sometimes called Earth’s “cousin” because it orbits a sun-like star with an orbital period of 384.8 days, remarkably close to our own year. It sits in the habitable zone, and its star is similar in type and age to ours. The problem is distance: Kepler-452b is roughly 1,400 light-years from Earth. Even traveling at the speed of light, the trip would take over a millennium.
The Realistic Timeline
For any destination beyond Mars, the travel time alone puts human settlement centuries or more into the future. Mars, by contrast, is reachable in about six to nine months with current rocket technology. NASA, SpaceX, and other agencies are actively developing the vehicles, life support systems, and resource extraction tools needed for crewed missions in the 2030s or 2040s.
Living on Mars won’t look like living on Earth. Early settlers would spend most of their time inside pressurized habitats, growing food hydroponically, recycling every drop of water, and limiting their time on the surface to reduce radiation exposure. It would be more like living on a submarine than in a new country. But it’s the one place where the engineering problems, while enormous, are at least solvable with technology we can see on the horizon.