Hi,

Earth's atmosphere is 20% oxygen, mostly in the form of the molecule O₂. It is the only planet with anywhere near that amount. Mars's atmosphere is only 0.13% oxygen and it has a much thinner atmosphere than Earth's to boot! Other atmospheres have oxygen in other molecules. For instance, the main gas making up the atmospheres of Venus and Mars is carbon dioxide (CO₂) but we can't breath that. That means Earth is the only place in the solar system where a human can breath (and live) without a whole lot of equipment.

Terry