The crew of an alien spacecraft entering the Solar System, maybe looking for planets bearing alien life, would be struck by three things about “planet three,” the third planet out from the Sun (Earth).

Firstly, it is radiating a cacophony of obviously artificial radio emissions. However, perhaps fortunately, the explorers would not be able to interpret them.

The second thing is “planet three” is largely covered by water. That is unique in the Solar System.

Finally, the atmosphere consists of about 20% oxygen. The gas is highly reactive and would rapidly disappear from the atmosphere by reacting with iron in rocks and water and with many other materials, especially carbon, unless continually topped up.

The only process we know of by which that can happen is photosynthesis, taking place in plants. In other words, oxygen is an indicator of life.

There are many other chemicals that should not be there in the atmosphere of a lifeless planet. This provides an effective way to search........

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