Planning space search

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... 

Further, an advanced search model is presented which, by introducing the powerful concept of a Planning Space, allows the search for syntheses to go forward, backward and to leap into the middle under controlled conditions. 

The distinction between the Selection of transforms by Applicability and by Relevance is an important one when considering the strategies one might employ to search for synthesis sequences. The search in a Planning Space has the characteristic that the search "leaps" into some intermediate point in the synthesis sequence and establishes an "island" and the solution search could then proceed from the island to the target molecule in the forward direction or from the island backward in the retrosynthetic direction toward available molecules. The significance of this ability to leap has been explored in other task areas than synthesis search and has been found to be a powerful tool in converging on solutions rapidly. Its utility for synthesis search remains to be shown and for now can be illustrated only in terms of examples. 

The criterion of applicability would require that the target structure contain the -C(OH)-C-CO-substructure and would not be applicable in the State Space search. In search conducted in the Planning Space, if the transform indicated is considered relevant to the target structure (e.g., if the presence of the alcohol and an unsubstituted 1,4 carbon is sufficient) then this transform may be used. The original synthesis problem is replaced with two subproblems shown in Scheme II. 

One of the projects planned for the next decade is Darwin, to be organised by ESA. Darwin will be a flotilla of four or five spacecraft that will search for Earth-like planets around other stars and analyse their atmospheres for the chemical signature of life. Three of the spacecraft will carry 3 1 m space telescopes , which will form the Infrared Space Interferometer IRSI they will be stationed 1.5 million kilometres from Earth, in the opposite direction from the Sun, at the Lagrangian Point L2 (a libration point at which the gravitational forces of the Earth and the sun cancel out). 

For the optimization of, for instance, a tablet formulation, two strategies are available a sequential or a simultaneous approach. The sequential approach consists of a series of measurements where each new measurement is performed after the response of the previous one is knovm. The new experiment is planned according to a direction in the search space that looks promising with respect to the quality criterion which has to be optimized. Such a strategy is also called a hill-climbing method. The Simplex method is a well known example of such a strategy. Textbooks are available that describe the Simplex methods [20]. 

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