Extrasolar life

The search for extrasolar life requires us to find planets outside the solar system. But since stars are only visible as small dots of light, it will be very difficult to find planets in outer space, since these do not emit light. This is true even for the nearest star, Alpha Centauri, which is about 4.3 light years away (roughly 7,000 times the distance from the sun to Pluto). 

How could extrasolar planets be detected In recent years, astronomers have developed four different methods of finding non-emitting objects (planets) and even, to some extent, characterizing them. These are  

The Doppler shift or radial velocity method The transit method 

The microgravitation lens (microlens) method Astronomical measurements 

The transit method requires that the central star, the planet and the observer are connected by a line of sight. The dark planet passes across the light source and thus diminishes its light intensity to some extent. Observation is only possible when observer, star and planet are in a favourable position, i.e., the planet lies between the star and the observer. In spite of this requirement, the method permits the discovery of planets of about the size of the Earth information is also available on the size, mass and density of the planet as well as on its orbit. Because of its limits of applicability, this method is not often used. In the case of the star OGLE-TR-56, it was possible to detect an extrasolar planet, the orbit of which is very close to its sun only a twentieth of the distance of Mercury away from it. The temperature of the planet was determined to be around 1,900 K its diameter is about 1.3 times larger than that of Jupiter, its density about 500 kg/m3 (Brown, 2003 Konacki, 2003). 

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The detection of small extrasolar planets (of around the size of the Earth) will be done by registering the infrared light which they emit. Interference filters will blot out the light emitted by the star in question. Because of the huge distance from the Earth, effects due to its atmosphere and its IR radiation will not interfere. Darwin is intended not only to discover planets but to analyse their atmospheres for possible signs of life. 

In the search for extraterrestrial signatures of life, we need to distinguish among Solar System sites, extrasolar system sites, and the search for intelligent life. It is likely that within the 21st century most potential life-bearing... 

The search for signs of life, present or past, is an important goal of NASA s robotic solar system exploration programs and, ultimately, for its astronomical programs designed to probe the gross characteristics of extrasolar planetary systems. To date, that search has been governed by a model of life that is based on the life that we know on Earth—terran life. Several features of terran life have attracted particular focus ... 

To evaluate the possibility that nonstandard biochemistry (i.e., biochemistry different from what we find as the universal biochemistry on Earth) might support life in known solar system environments and conceivable extrasolar environments and... 

Finally, the committee calls attention to the importance of using remote sensing to detect and characterize extrasolar planets that could support alternative carbon-based life. In addition to looking for evidence of water in... 

HZ is defined as the orbital area around a star where a planet can sustain liquid water at the surface. Several studies have attempted to define the HZ as a function of stellar type (e.g. F, G, K, M dwarf stars) and time (see e.g. 46, 47, and references therein). The HZ is of interest because it is widely believed that liquid water is necessary for the genesis of (recognizable) life. The particular emphasis of the planned space missions is to search for signs of life on extrasolar Earth-like planets via spectroscopy. Atmospheric compounds such as O2, O3, N2O, CH4, and CH3CI are considered biomarkers, and their spectroscopic detection in a terrestrial-type atmosphere, particularly O2 or O3 found together with a reduced gas such as CH4, would suggest life (48, 49). Detection of CO2 would indicate that the planet is indeed a terrestrial-type planet... 

Oxygen absorption lines can be observed e.g. at 687 and 760 nm (see also Fig. 1.7). It has been proposed to use such measurements as biomarkers for life on extrasolar planets. 

Water is one of the basic elements for life. It is even assumed that the evolution of life is only possible if there is liquid water present. A water molecule has some remarkable properties that make it quite unique in the universe. In the first chapter of this book we will review these basic properties of water and the role of water on Earth. All ancient civilizations realized the importance of water and their cities were constructed near great reservoirs of water. But is water unique on Earth Do we find water elsewhere in the solar system, on extrasolar planetary systems or in distant galaxies We will start the search for the presence of extraterrestrial water in our solar system. Surprisingly enough it seems that water in some form and sometimes in only minute quantities is found on any object in the solar system. Even on the planet nearest to the Sun, Mercury, there may be some water in the form of ice near its poles where never the light of Sun heats the surface. And there are objects in the solar system that are made up of a large quantity of water in terms of their mass, like comets and several satellites of the giant planets. 

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