In discussions about the search for life beyond Earth two basic possibilities have been proposed: either we will discover signs that biology exists on other planets through atmospheric analysis or we will detect an unambiguous radio (or maybe laser) signal from outer space which does not have a natural source and hence can be assumed to originate from an exoplanet. The former discovery would be via a biosignature, such as abundant oxygen in the atmosphere of an exoplanet. The reason why this is a good bio-indicator is that free oxygen (as a diatomic gas, O2) is highly reactive and if it existed in the atmosphere of an exoplanet it would disappear quickly by forming molecules like carbon dioxide or water, or rust with iron, unless it were continuously replenished by some biological source, like trees and plants on Earth, which release oxygen into the atmosphere during photosynthesis.