Simon: Hi Zebeast! Great question!
In the Solar System, the celestial body that has an atmosphere closest to ours in Titan (the largest moon of Saturn). It has a nitrogen atmosphere but no oxygen. Scientists are also starting to discover the first Earth sized “rocky planets” around other solar systems. I think we will see within the next 5 to 10 years the first exoplanets with similar atmospheres to Earth. And if we find oxygen in any of them in significant quantities….
Watch Out!: Oxygen is a very reactive substance and so it doesn’t usually hang around long before reacting with something else. In order to have an atmosphere with sustained levels of oxgen you need to have a process to create oxygen. There are not too many naturally occurring “chemical processes” that do that this but there is one significant organic process “Photosynthesis” that does. If we detect oxygen around an exoplanet then this could well be a marker for life elsewhere in our galaxy!
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