LECTURE 5: CANDIDATE WORLDS FOR ALIEN LIFE
Many bodies in the Solar System have been suggested as being capable of containing conventional organic life. The most commonly suggested ones are listed below; of these, five of the nine are moons, and are thought to have large bodies of underground liquid (streams), where life may have evolved in a similar fashion to deep sea vents.
- Mars - Life on Mars has been long speculated. Liquid water is widely thought to have existed on Mars in the past and there may still be liquid water beneath the surface. Methane was found in the atmosphere of Mars. Recent photographs from Mars Global Surveyor show evidence of recent (within 10 years) flows of a liquid on the Red Planet's frigid surface.[28]
- Europa - Europa may contain liquid water beneath its 100-mile (160 km) thick ice layer. Vents on the bottom of the ocean warm the ice so that 60 miles (97 km) of liquid could exist beneath the ice layer, perhaps capable of supporting microbes and simple plants.[2]
- Jupiter - Possible supporter of floating animals, as hypothesized by Carl Sagan for gas giants in general. This point of view is somewhat controversial because these creatures would not be water-based, but ammonia-based.[29]
- Ganymede - Possible underground ocean (see Europa).
- Callisto - Possible underground ocean (see Europa).
- Saturn - Possible floating creatures (see Jupiter).
- Enceladus - Geothermal activity, watervapour. Possible underice oceans heated by tidal effects.
- Titan (Saturn's largest moon) - The only known moon with a significant atmosphere was recently visited by the Huygens probe. Latest discoveries indicate that there is no global or widespread ocean, but small and/or seasonal liquid hydrocarbon lakes are present on the surface (the first liquid lakes discovered outside of Earth).[30][31][32]
- Venus - Recently, scientists have speculated the existence of microbes in the stable cloud layers 50 km above the surface, evidenced by hospitable climates and chemical disequilibrium.[33]
Numerous other bodies have been suggested as potential hosts for microbial life. Fred Hoyle has proposed that life might exist on comets, as some Earth microbes managed to survive on a lunar probe for many years. However, it is considered highly unlikely that complex multicellular organisms of the conventional chemistry of terrestrial life (animals, plants) could exist under these living conditions.
Astronomers also search for extrasolar planets that they believe would be conducive to life, such as Gliese 581 c and OGLE-2005-BLG-390Lb, which have been found to have Earth-like qualities.[21][22] Current radiodetection methods have been inadequate for such a search, as the resolution afforded by recent technology is inadequate for detailed study of extrasolar planetary objects. Future telescopes should be able to image planets around nearby stars, which may reveal the presence of life (either directly or through spectrography which would reveal key information such as the presence of free oxygen in a planet's atmosphere):
- Darwin is an ESA mission designed to find Earth-like planets, and analyse their atmosphere.
- The COROT mission, initiated by the French Space Agency, was launched in 2006 and is currently looking for extrasolar planets -- it is the first of its kind
- The Terrestrial Planet Finder was supposed to be launched by NASA, but as of 2007 , budget cuts have caused it to be delayed indefinitely
- The Kepler Mission, largely replacing the Terrestrial Planet Finder, to be launched in November 2008
It has been argued that Alpha Centauri, the closest star system to Earth, may contain planets which could be capable of sustaining life.[23]
On April 24, 2007, scientists at the European Southern Observatory in La Silla, Chile said they had found the first Earth-like planet. The planet, known as Gliese 581 c, orbits within the habitable zone of its star Gliese 581, a red dwarf star which is a scant 20.5 light years (194 trillion km) from Earth. It was initially thought that this planet could contain liquid water. However, recent computer simulations of the climate on Gliese 581c by Werner Von Bloh and his team at Germany's Institute for Climate Impact Research suggest carbon dioxide and methane in the atmosphere would create a runaway greenhouse effect. This would warm the planet well above the boiling point of water (100 degrees Celsius/212 degrees Fahrenheit), thus dimming the hopes of finding life. As a result of greenhouse models, scientists are now turning their attention to Gliese 581 d, which lies just outside of the star's traditional habitable zone.[24]
On May 29, 2007, the Associated Press released a report stating that scientists have identified twenty-eight exo-solar planetary bodies. One of these newly discovered planets is said to have many similarities with Neptune.[25]
READING FOR THE NEXT LECTURE
