The European Darwin Mission will be a flotilla of four or five free-flying spacecraft that will search for Earth-like planets around other stars and analyse their atmospheres for the chemical signature of life in the mid of the next century.
The search after extrasolar planets is a hard business because even by the nearest stars it is like trying to see the difference between a candle light next to a lighthouse from a point 1000 kilometres away. 
Darwin will observe in the mid-infrared because at these wavelenghts is the detection of a litte, nonluminous planet a litte easier. Similar to the American Terrestrial Planet Finder this mission will search after traces of life on a planet. The lifeforms on Earth expel carbon dioxide and methane and these gases mingle with our atmosphere. This can detect from a far away viewpoint how the Venus Express space probe demonstrated .
Every gas and also other substances, such as water, leave their unique fingerprints by absorbing certain colours (wavelengths) of infrared light. Darwin will split the light from an extrasolar planet into its individual colours, using an instrument called a spectrometer. So the specific elements can identified.
The telescope is designed to work at just 40K (–233°C) while the actual detector has to be reduced in temperature further to just 8K (–265°C). This stops the telescope radiating its own infrared signal and allows it to search for the faint light of distant worlds. 
Three of the spacecraft will carry 3-4 metre space telescopes based on the Herschel design and these will deflect light to the central hub spacecraft. All telescopes together will work like a single large telescope, with a diameter of up to several 100 metres. The spacecraft will probably be equipped with tiny ion engines that need just five kilograms of fuel to last the entire five-year mission. The ion engines expel small particles at very high velocity such that the spacecraft moves slightly in the opposite direction.
For the launch, ESA will use two launches with Soyuz-Fregat rockets. Instead of an orbit around the Earth, Darwin will be placed far away, beyond the Moon. At a distance of 1.5 million kilometres from Earth, in the opposite direction from the Sun, Darwin will operate from a special location known as Lagrangian Point L2. 
The Darwin mission is named after British naturalist Charles Darwin who wrote the ground-breaking book The Origin of Species and the idea for this mission was proposed in 1993.
   http://www.esa.int/esaSC/120382_index_0_m.html