Final answer:
The presence of methane and oxygen simultaneously in an exoplanet's atmosphere is a good indication of life because methane, an easily oxidized gas, would typically be absent in an oxygen-rich atmosphere unless continuously replenished by biological processes.
Step-by-step explanation:
The simultaneous detection of methane and oxygen in an atmosphere is a significant indicator of the existence of a biosphere on that planet. This is because methane is an easily oxidized gas, and its presence alongside significant amounts of oxygen suggests an active source replenishing it—likely biological processes.
Photosynthesis on Earth has significantly altered our atmosphere, where over 20% of it is oxygen, a byproduct of this process. Furthermore, Earth's atmosphere contains a mix of gases like nitrous oxide and methane that, when found with oxygen, are potential biosignatures indicative of life. Detecting such gases in exoplanet atmospheres, especially in the habitable zone, could therefore imply biological activity. If future space instruments uncover an exoplanet with substantial oxygen and methane, this could be compelling evidence of a habitable and inhabited world, prompting further investigation.