A biosignature is a chemical or physical process that can be detected at a distance and indicates the presence of living organisms in a particular region. The concept is often used in the field of astrobiology, which is a branch of biology that searches for life outside of the land, air, and sea environments of the Earth. The search for a biomarker to indicate the past or present existence of life on the planet Mars has gained increasing focus in astrobiology since the US Viking I and II missions were sent there to look for life in the mid-1970s to early 1980s, and probes to other regions of the Solar System have continued the search. The field has begun to broaden as of 2011, however, due to the fact that dozens of extrasolar planets have been discovered outside of Earth's Solar System. A small minority of these planets are theorized to be Earth-like in size and structure, and they may have biosignature astrochemistry that indicates a capability for supporting life.
The understanding of the necessary conditions for planetary habitability by at least primitive life forms such as bacteria has been evolving in the 20th and 21st centuries. This is because science has discovered the biosignature of organisms on Earth in regions such as deep underwater volcanic vents that were previously thought to be completely inhospitable to all forms of life. The hardiness of such organisms to live in conditions absent of light and oxygen, and under extreme levels of temperature and pressure, suggest that the biosignature for life on other worlds may be broader than previously assumed.
The presence of liquid water is still considered to be essential for any life to exist outside the confines of the Earth. While liquid water was once thought to be rare in the Solar System existing only on Earth itself, this view has changed in the 21st century. Both Europa and Callisto, moons of the planet Jupiter, may possess liquid water sub-surface oceans, and Enceladus, which is a moon of Saturn, is now known to have water-based volcanoes that may support basic living organisms as well. The US Phoenix Mars Lander also found evidence of water-based ice in a region distant from the polar caps on Mars in 2008, which may indicate a biosignature for bacterial activity that once existed or still does barely underneath the surface of the red planet.
Detecting a bioindicator for distant worlds is more challenging for current science as of 2011, since finding the worlds themselves is a challenge. The focus of the research may start by narrowing down the range of star systems to those of red dwarfs. These are both the most common types of stars, making up about 75% of all stars in the Milky Way galaxy, and the most likely type to possess planetary systems that may be habitable due to their age and presence in the galaxy's main sequence of stars.
M class dwarf stars are significantly smaller and cooler than the Earth's Sun on average, so the planets orbiting them would need thick atmospheres to capture more light from their parent suns than the Earth does. Probability suggests that, if life exists outside of Earth, it would be found on planets around red dwarfs more likely than elsewhere. Star classes such as F, G, and K, which are hotter and brighter like the Sun, are also relatively rare compared to red dwarfs, so research is being focused on examining the M class stellar regions for planets with biosignature activity.
Certain gasses individually or together would be a clear biosignature for the presence of potential life forms. These gasses would also be longer-lived in atmospheres on planets orbiting red dwarfs, and easier to detect than in planets orbiting around hotter stars. These biosignature compounds include methane — CH4, nitrous oxide — N2O, chloromethane — CH3Cl, and ozone in the form of O2 or O3.
The detection of organisms on Earth living in sulfur environments near volcanic vents has also suggested that life may thrive on anoxic planets that are low in or entirely devoid of oxygen. Organic sulfur compounds would also therefore be a strong indicator of life if they were detected in extraterrestrial atmospheres, including methanethiol — CH3SH, and carbon disulfide — CS2. The presence of sulfur-based compounds would reflect biosignature theories about early life on Earth that existed before oxygen was widespread, and was a dominant living condition on Earth for at least 1,500,000,000 years.