If you step onto another planet without a spacesuit, suffocation is the least of your worries. In our solar system alone, you could be crushed, melted, or have your blood literally boil.
Venus offers a crushing atmospheric pressure 90 times that of Earth, temperatures hot enough to melt lead, and rains of sulfuric acid. Mars visually resembles an arid terrestrial desert, but its environment is just as lethal.
Standing on this barren surface without a pressurized suit, a human would face an atmosphere 100 times thinner than Earth's, composed almost entirely of carbon dioxide. They would quickly succumb to ebullism—a condition where the boiling point of bodily fluids drops below the body's internal temperature, causing the water in tissue and blood to vaporize. Asphyxiation would follow in minutes. Gas giants like Jupiter and Saturn lack solid surfaces entirely, offering only a descent into crushing pressure and turbulent storms.
To survive without a spacesuit, a person would need an Earth-like exoplanet located in its star's habitable zone—the orbital region where temperatures allow liquid water to pool. This hypothetical planet must meet strict criteria: surface gravity close to 1G, a magnetic field to deflect stellar radiation, and a breathable atmospheric pressure. Crucially, the atmosphere must contain enough oxygen to sustain human metabolism without being so rich that it becomes highly flammable, mixed with an inert buffer gas like nitrogen.
Even if astronomers identify a planet with the perfect temperature and atmospheric composition, the biological reality of stepping outside remains hazardous. On Earth, free oxygen is continuously replenished by photosynthetic life. If an exoplanet has a breathable oxygen atmosphere, it likely harbors its own biosphere. A human arriving on such a world without a spacesuit might face alien microorganisms, allergens, or protein structures that the human immune system is completely unprepared to handle, making the alien air fatal for entirely biological reasons.