One of the central research areas within astrobiology is finding out the extent of conditions that terrestrial life can survive. Extremophiles are organisms that can tolerate what are considered to be extreme environments.

For example, some organisms thrive in the extremely salty water of the so-called Dead Sea, others survive the highly acidic conditions within volcanic pools such as at Yellowstone Park, and other specialised cells tolerate temperatures over 100ºC and the crushing pressure around deep-sea ‘black smoker’ hydrothermal vents. However, the concept of ‘extremity’ is obviously in the eye of the beholder – many forms of life would consider the cold and low-pressure of Earth’s surface, awash with toxic oxygen ands bathed in lethal ultra-violet radiation from the sun, to be an extreme environment. Complex life, including land plants and animals like us, tends to be much more fragile than simpler cells like bacteria, and so many habitats on Earth are populated only by microscopic single-celled organisms. These hardy kinds of life are also the most likely to be discovered in extraterrestrial settings.