Conditions for Life: The Environment In order to develop and survive, Life needs an environment that provides it with protection and stability over long times. The Earth is such a place and our existence here would not have been possible without the very special - and mostly quite favourable - conditions for life on its surface, over periods of billions of years. Protection What sorts of planetary environments are likely to provide access to energy useful for life to develop? Obviously radiation from stars is a free source of energy in the universe, but it has drawbacks. Although life needs energy, some parts of the radiation spectrum (X- and gamma-rays; ultraviolet radiation) are simply too energetic for life, in fact, they produce photochemical reactions that are lethal for life systems. An adequate distance from the source and protection devices are necessary. On our planet, the protection from solar winds is provided by the magnetic shield that is a consequence of the composition of the planet. The protection from ultraviolet light is given by the ozone layer, which is a biological byproduct that results from the production of oxygen during the oxygenic photosynthesis. Geothermal energy is also considered a good source of energy for life. And obviously these environmental conditions which are all interlocked should be stable enough to facilitate the development of life. Stability We know that the environmental conditions on our planet have changed radically many times throughout its history. These radical changes were in some cases exterior to the system, like meteoritic impacts, but others were the consequence of the evolution of the planet, like the different glaciation episodes. Some may even have been produced by life itself, like the radical transformation of a reductive atmosphere to a an oxygenic one.