Currently, Earth is the only known planet capable of sustaining life (Goldilocks planets), however, a number of other planets, both within and outside of the Solar system have been seen as candidates;
- Mars - a common favourite among astrobiologists, it is believed the planet was once covered in liquid water, and could have been able to support life. However, today, due to a lack of magnetosphere, life on the surface is deemed impossible, however, life under the surface, perhaps in subterranean caverns, is a possibility;
- Gliese 581 system - Considered to be a prime candidate, Gliese 581 is a star system comprised of 1 red dwarf star, and 6 extrasolar planets, each of which having varying degrees of existence. Gliese 581 e, d and g have all been noted for being within the habitable zone, and are the most likely candidates for life outside of the Solar system.
- Kepler 22b - A planet located within the Kepler 22 system, it orbits a class G star, similar to the Solar system's Sun. Despite being 35 times the mass of Earth, it is still a key candidate for extrasolar life, as it is located well within the stars habitable zone, with a 95% certainty rate.
- See also: Habitable solar systems
While looking for life on other planets, a number of assumptions should be/are made whilst searching for extrasolar planets that are capable of sustaining life. One of these assumptions should be predicated around the notion of "carbon-based life"; life in which the chemical structure of life is bound to carbon molecules. Whilst is may not always be the case, carbon, one of the most abundant elements on Earth, is essential for "producing" life (as we know it), and may be a major element on planets capable of sustaining life.
Any major notion for which many astrobiologists base their assumptions on is the presence of liquid water. A major chemical compound on Earth, it is often noted that water is one of the to most major indicators of life outside of Earth, and can provide an excellent enviroment for the development and evolution of life.