Before seeking life in the Univers in a relevant way, we have to know the limits of life on Earth. Bacteria (but only procaryotes, cells without nucleus) can survive at temperatures going from -12oC (psychrophilous) (6) up to 113oC (hypertermophiles) with a temperature range that doesn't exceed 40oC.
The acidity of the environment varies from a pH near 0 (basophiles) to 11 (acidophiles). These data concern only certain families of bacteria known as extremophiles that can sometimes develop under 85oC with a pH of 2,5 (5). A bacteria can even resist an irradiation by gamma ray of 10000 grays (10 grays kills a human, 100 grays kills a bacteria in our intestine) (7). Other bacteria known as halophilous live in water where the salt concentration is ten times higher than in sea water (8). Barophilous bacteria thrive with 10 000 meters of depth with pressure equivalent to 7 atmopsheres (69 megapascal).

All these extremophiles bacteries would represent a supplement of 10% to be added to the terrestrial biomass, they cannot be considered highly unusual.

These extremophiles enable us to scientifically consider whether life might exist on planets with conditions similar but not identical to those on Earth.

Symbiosys makes it possible for living beings to survive when the conditions are mortal for the majority of animals. A worm from the abyssal pits nourishes itself with generaly poisonous compound, hydrogen sulphide, thanks to the bacteria it shelters (9). The lichen (symbiosis of a fungus and an alga) colonizes almost all the terrestrial biotopes (10).

We can thus find life forms in all terrestrial biotopes, no matter how hostile they may be.