Life - Extraterrestrial Life

Extraterrestrial Life

Earth is the only planet known to harbor life. Other locations within the Solar System that may host life include subsurface Mars, the atmosphere of Venus, and subsurface oceans on some of the moons of the gas giant planets. The Drake equation, which predicts the number of extraterrestrial civilizations in our galaxy with which we might come in contact, has been used to discuss the probability of life elsewhere, but many of the variables in this equation are difficult to estimate.

The region around a main sequence star that could support Earth-like life on an Earth-like planet is known as the habitable zone. The inner and outer radii of this zone vary with the luminosity of the star, as does the time interval during which the zone survives. Stars more massive than the Sun have a larger habitable zone, but remain on the main sequence for a shorter time interval. Small red dwarf stars have the opposite problem, with a smaller habitable zone that is subject to higher levels of magnetic activity and the effects of tidal locking from close orbits. Hence, stars in the intermediate mass range such as the Sun may have a greater likelihood for Earth-like life to develop. The location of the star within a galaxy may also have an impact on the likelihood of life forming. Stars in regions with a greater abundance of heavier elements that can form planets, in combination with a low rate of potentially habitat-damaging supernova events, are predicted to have a higher probability of hosting planets with complex life.

Panspermia, also called exogenesis, is a hypothesis that life originated elsewhere in the universe and subsequently transferred to Earth in the form of spores via meteorites, comets, or cosmic dust. Conversely, terrestrial life may be seeded in other solar systems through directed panspermia, to secure and expand some terrestrial life forms. Astroecology experiments with meteorites show that Martian asteroids and cometary materials are rich in inorganic elements and may be fertile soils for microbial, algal and plant life, for past and future life in our and other solar systems.