In the past two decades over a thousand planets have been discovered orbiting stars other than our own, and significant progress has been made in understanding planetary systems and processes that do not occur in our own Solar System. One of the most fascinating fields in this new science is the search for life within other star systems, though as yet no hint of such life has been found, and all methods for searching for it are entirely theoretical in how they would interpret any results apparently indicative of life.
The search for life in other star systems holds a great deal of fascination not just for astronomers but for the movie-going public. James Cameron/Avatar/20th Century Fox.
Other than a direct and ambivalent message from an alien civilization, any life on another planet would have to be detected by subtle affects upon the planet’s detectable properties. It has often been suggested that the presence of liquid water would at least indicate life to be possible on another planet, though such water would not in itself be evidence of life, so additional data would be needed before the possibility of life could be asserted.
For example it is known that the Earth’s albedo (the light reflected by the Earth) is shifted into the infra-red by the presence of substantial plant cover on the planet. This is because plants tend to absorb visible light and emit infra-red light. In the event that another planet was found showing a similar shift in albedo into the infra-red part of the spectrum, compared to what would be predicted based upon other known properties of the planet, then it might be inferred that such a planet has an extensive coverage of a photosynthetic organism similar to plants.
The Earth viewed at infrared wavelengths. The planet radiates considerably more light at these wavelengths than would be expected, due to its extensive vegetation cover. European Space Agency/Rosetta.
Another potential method of detecting life would be the presence of ‘incompatible’ gasses in the atmosphere of a planet. For example the Earth’s atmosphere contains both substantial amounts of oxygen (O2) and trace methane (CH4). On Earth both of these have a biological origin, though both could potentially be produced by non-biological means. However the two gasses are unlikely to co-exist in the atmosphere of a planet for very long, since they react to form water (H2O) and carbon dioxide (CO2), so unless both were being constantly added to the atmosphere, one of these gasses would be quickly used up be the reaction. Since the most likely explanation (that we are aware of) for the constant adding of these gasses to the atmosphere would be the presence of life, then the presence of both together could be taken as evidence for the presence of life as well.
In a paper published in the Proceedings of the National Academy of Sciences of the USA on 28 April 2014, Hanno Rein of the Department of Environmental and Physical Sciences at the University of Toronto, Yuka Fujii of the Earth-Life Science Institute at the Tokyo Institute of Technology and David Spiegel of the Astrophysics Department at the Institute for Advanced Study in Princeton New Jersey, point out a potential flaw in this methodology.
Rein et al. observe that at the current time we have no means of resolving moons about planets in other star systems, much less differentiating such bodies from their parent planets while carrying out spectral analyses of their atmospheres. While the Earth’s Moon lacks an atmosphere in any meaningful sense, this is not automatically the case, as we know that Saturn’s moon Titan has a methane based atmosphere denser than that of the Earth. They reason that were such a moon to be orbiting a planet in another star system being studied with current technology, we would have no means of differentiating the atmospheric signature of the moon from that of the planet, and that we might therefore misinterpret the data to infer the possibility of life where none exists.
The Earth and the Moon would be impossible to differentiate from one-another in a different star system. NASA/Johns Hopkins University Applied Physics Laboratory/NEAR Spacecraft.
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