To begin searching for life the basic components that determine what life is need to be understood.
A definition for life: The two main requirements for life are:-
- The storage and replication of molecular information with the properties of a nucleic acid
- The presence of enzyme catalysis. Without enzyme catalysis a system is inert, not alive.
The non metallic chemical elements that exist in all life are oxygen, hydrogen, carbon, nitrogen phosphorous and sulphur and account for more than 97% of the weight of most organisms. All these elements can form stable covalent bonds. However the relative amounts of these elements vary between organisms. There are 29 elements that are commonly found in living organisms.
Planets that can support life will be found in an area around their stars’ known as the habitable zone. This is a distance where the energy received from the star results in an average surface temperature where liquid water can exist, between 0 and 100°C.
Other factors affect the area of the habitable zone; these include radiation levels, asteroid impacts and neighbouring bodies. For example, on Earth life was unable to develop until after the late heavy bombardment as the impacts provided enough energy to vaporise any liquid water present on the surface. If our star was of a different mass or a different stage of life, the radiation output may have been too high for life to develop. This is not to say that life cannot exist in high radiation levels, as microbes were recently discovered living inside nuclear reactors. However, high radiation levels may break down complex molecules required for life to develop. If a large neighbouring body was close by to a planet, its orbit may be too unstable for stable environments to be achieved, which would be detrimental to the formation of complex life.
There are some places in the universe where it is highly unlikely that life would exist. These places are known as dead zones and include the surface of stars, globular clusters, the vacuum of space, gas giants and close to the centres of galaxies.
In order to detect life remotely, infra red spectroscopy can be used to analyze the planets atmosphere. When light is incident on the planets surface it loses energy and is re-emitted as infra red. A spectroscope is used to detect this infra red spectrum. Certain chemicals in the planets atmosphere absorb specific wavelengths of the emitted radiation. These show up as black bands on the continuous spectrum. The following wavelengths are absorbed by water (7 μm), ozone (10 μm) and carbon dioxide (15 μm). These are possible markers for the existence of life. A high concentration of these molecules may indicate the presence of life, as without life the planets atmosphere would be in chemical equilibrium; oxygen is highly reactive and would only be detected in trace amounts. Oxygen and nitrogen would combine with water to form nitric acid. The presence of life on Earth prevents large amounts of nitric acid from forming and allows nitrogen, oxygen and water to exist separately.
Another method of detecting possible life is using the albedo of the planet, which is the ratio of radiation reflected from the planets surface to the total amount falling on it. This ratio would be affected by photosynthetic activity as some of the energy would be absorbed.
One of the best but least practical methods of detecting life would be direct observation. This could include manned missions and unmanned probes. Current technology is a limiting factor due to the large distances involved.