Terrestrial Planet Finder (TPF)
The Terrestrial Planet Finder is a mission which primary target is to search after Earth-like planets and currently under study by NASA. At the moment are two complementary concepts for this mission. The visible-light Coronograph and a formation-flying infrared Interferometer.
"There are countless suns and countless earths all rotating around their suns in exactly the same way as the seven planets of our system. We see only the suns because they are the largest bodies and are luminous, but their planets remain invisible to us because they are smaller and non-luminous. The countless worlds in the universe are no worse and no less inhabited than our Earth." Giordano Bruno (1584) 
The mission of the TPF is to study all aspects of extrasolar planets, from their formation and development in protoplanetary disks of dust and gas around newly forming stars to fully-fledged planets orbiting stars within a radius of nearly 50 light years (15 parsec (pc)) . The TPF will measure the size, temperature, and placement of planets as small as the Earth in the habitable zones of distant solar systems.
Furthermore, TPF's spectroscopy will allow scientists to analyse the atmospheric chemistry from far away worlds in our galactic neighborhood to search after traces of life, especially after water vapor or methane.
Although there are some similar missions today (like Kepler and COROT) is the TPF mission necessary and unique. Because only the TPF will have the capability to analyse the spectral characterization of distance worlds. In addition, we know almost nothing about the inner regions of protostellar disks where planet formation and migration is thought to occur.
"The discovery of life on another planet is potentially one of the most important scientific advances of this century, let alone this decade, and it would have enormous philosophical implications." 
During almost 20 years of study the scientists discussed a lot of concepts for the TPF mission but in May of 2004 the NASA announced it would fly two separate missions with distinct and complementary architectures to fully realize the scientific goals.  But unfortunately is until today no launch date available and despite the big benefit of this mission is not clear whether the mission will realise.
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