The author believes the most exciting substance to find in the atmosphere of an Earth-sized planet would be:

• A. Oxygen
• B. Methane
• C. Water vapor
• D. Carbon dioxide

Answer: (A)

Oxygen in an atmosphere of an Earth-sized world would be especially exciting because it implies ongoing replenishment by active processes, most plausibly photosynthesis. Oxygen is highly reactive and would be removed from the atmosphere over time by reactions with rocks and other gases, so a substantial, persistent amount suggests a source continually producing it. Other common atmospheric gases like water vapor, carbon dioxide, or methane can be present for non-biological reasons and don’t by themselves point to life. Water vapor is widespread and not a unique indicator, carbon dioxide is a common greenhouse gas, and methane can arise from both biological and geological processes, making its presence less definitive on its own. Therefore, detecting significant oxygen would be the strongest hint of a life-related process on an Earth-sized planet.