“Free oxygen is present on which planet?”

"Earth is the only known planet with abundant free oxygen in its atmosphere comprising about 21% of its composition. This oxygen supports aerobic life forms and is produced primarily through photosynthesis by plants and algae. Its presence distinguishes Earth’s habitability in the solar system. Understanding Earth’s oxygen-rich environment highlights its unique ecological balance and the role of biological processes in sustaining life a key topic in planetary science and environmental studies."