Scientific search for extraterrestrial life

The NASA Kepler Mission for the search of extrasolar planets.

The scientific search for extraterrestrial life is being carried out both directly and indirectly.

Direct search

Scientists are directly searching for evidence of unicellular life within the Solar System, carrying out studies on the surface of Mars and examining meteors which have fallen to Earth.

A mission is also proposed to Europa, one of Jupiter's moons with a possible liquid water layer under its surface, which might contain life.

There is some limited evidence that microbial life might possibly exist (or have existed) on Mars.

An experiment on the Viking Mars lander reported gas emissions from heated Martian soil that some argue are consistent with the presence of microbes.

However, the lack of corroborating evidence from other experiments on the Viking lander indicates that a non-biological reaction is a more likely hypothesis. Independently, in 1996, structures resembling nanobacteria were reportedly discovered in a meteorite, ALH84001, thought to be formed of rock ejected from Mars.

This report is also controversial.

Electron micrograph of martian meteorite ALH84001 showing structures that some scientists think could be fossilized bacteria-like life forms.

In February 2005, NASA scientists reported that they had found strong evidence of present life on Mars.

The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center, based their claims on methane signatures found in Mars' atmosphere resembling the methane production of some forms of primitive life on Earth, as well as on their own study of primitive life near the Rio Tinto river in Spain. 

NASA officials soon denied the scientists' claims, and Stoker herself backed off from her initial assertions.

Though such findings are still very much in debate, support among scientists for the belief in the existence of life on Mars seems to be growing. In an informal survey conducted at the conference at which the European Space Agency presented its findings, 75 percent of the scientists in attendance were reported to believe that life once existed on Mars, and 25 percent reported a belief that life currently exists there.

The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere in chemical disequilibrium, which is relatively easy to determine from a distance by spectroscopy.

However, significant advances in the ability to find and resolve light from smaller rocky worlds near their star are necessary before such spectroscopic methods can be used to analyze extrasolar planets.

On March 5, 2011, Richard B. Hoover, an astrobiologist with the Marshall Space Flight Center, speculated on the finding of alleged microfossils similar to cyanobacteria in CI1 carbonaceous meteorites.

However, NASA formally distanced itself from Hoover's claim.

Indirect search

Terrestrial Planet Finder

A planned Infrared interferometer for finding Earth-like extrasolar planets (as of 2010[update], it has not received the funding from NASA which it needs that funding is going towards the Kepler mission).

It is theorized that any technological society in space will be transmitting information. However, since there are no human systems, in general, intentionally transmitting information randomly into deep space, there is no guarantee that any other species would do so either.

Also, the length of time required for a signal to travel across the vastness of space means that any signal detected, or not detected, would come from the distant past.

Nevertheless, projects such as SETI are conducting an astronomical search for radio activity which would confirm the presence of intelligent life.

A related suggestion is that aliens might broadcast pulsed and continuous laser signals in the optical, as well as infrared, spectrum; laser signals have the advantage of not "smearing" in the interstellar medium, and may prove more conducive to communication between the stars.

While other communication techniques, including laser transmission and interstellar spaceflight, have been discussed seriously and may well be feasible, the measure of effectiveness is the amount of information communicated per unit cost.

This results in radio transmission as the method of choice.