See also:

Life on Mars

Detecting Terrtestrial Planets

SETI

External Links:

NASA Mars Exploration Program

Europa Orbiter

Europa Cryobot Hydrobot mission (artists impression)

TPF project

Darwin/IRSI project
 

Extraterrestrial Life

Why Search for Extraterrestrial Life?

At present we don't know whether the occurrence of life is inevitable given a planet with the right conditions, or is the result of a rare chance event. Finding one other incidence of life in the neighbourhood of Earth, be it only microbial life, would show that life forms easily given a suitable planet, and would indicate that we live in a universe teeming with life.

Where to Search?

Life on Earth depends on liquid water, with all the important chemical reactions which form the basis of life occurring in solution in water. Extremophilic microorganisms can be found which thrive at temperatures from -5 to 110 degrees C provided water remains liquid. Thus anywhere where liquid water can exist is a promising place to search for life.

1. Mars

Mars appears to have had conditions similar to those on Earth early in its history with evidence for liquid water and perhaps an ocean. So it seems plausible that life could have been present at one time. Life on Mars today seems less likely as currently the atmospheric pressure is too low for liquid water. A series of Mars probes are planned leading up to missions which will return samples to Earth to search for fossil life.

If we do find life on Mars it doesn't necessarily mean that life originated independently on both planets. We can't exclude the possibility that life jumped between planets (most likely from Mars to Earth) carried by meteorites. We have a number of examples of meteorites which have come from Mars.

2. Europa

Jupiter's satellite Europa has been studied extensively by NASA's Galileo probe. There is evidence that below its icy surface there is an ocean of water kept liquid by the heating due to the tidal effect of Jupiter. If this ocean exists it could be a promising location to search for life.

NASA's planned Europa Orbiter mission will put a probe into orbit around Europa to determine conclusively whether or not an ocean exists, and the thickness of ice above it. This can be achieved through detailed analysis of the probe's orbit as well as by means of a radar system that can penetrate the ice. If an ocean is confirmed then we will want to pentrate the ice layer to search for life within the ocean. Scientists envisage a probe called a cryobot, which would slowly melt its way through the ice and then release a remote controlled submarine (or hydrobot) to explore the ocean.

3. Extrasolar Planets

The most ambitious of the space missions now planned to search for planets of other stars aim to do more than just find such planets - they should be able to determine whether the planets have developed life. The missions are the infrared space interferometers, NASA's Terrestrial Planet Finder (TPF) project and ESA's Darwin/IRSI project. These systems consist of an array of space telescopes arranged such that they can use the destructive interference of light to suppress the light of the star, and reveal the much fainter planet.

When these missions find an earth-like planet they can then measure its spectrum which will reveal the mix of gases in its atmosphere. The detection of ozone in the spectrum would indicate that the planet has oxygen in its atmosphere. Oxygen indicates that living organisms undergoing photosynthesis are present. On Earth it is believed that oxygen only began to build up in the atmosphere about 2 billion years ago due to action of photosynthetic cyanobacteria.

4. The Search for Extraterrestrial Intelligence (SETI)

SETI projects aim to detect life by means of radio or other signals sent by advanced civilizations. SETI can potentially detect life at much greater distances than other techniques if their transmitters are powerful enough. However, unlike the other techniques which can detect microbial life, SETI can only detect intelligent life, and then only if they are sending signals in our direction. This raises the question of whether life inevitably evolves towards complex organisms and to the eventual development of intelligence. If so, SETI could be a very powerful technique. However, some scientists argue that there is no such inevitability, and that microbial life could be common in the universe while advanced intelligent life could still be very rare.