Space Interferometry Mission
The Space Interferometry Mission also known as SIM Lite and formerly known as SIM PlanetQuest is equipped with a 6-meter Michelson stellar interferometer, operating in the visible waveband, and will be launched into an Earth-trailing solar orbit where it will operate for a mission lifetime of five years.
SIM will use two separated mirrors and combine their light with a technique known as interferometry and will observe stars after the typical "wobble" that occurs when a planet is pulling with his gravitational force on a star.
At the moment we know over 300 extrasolar planets whereas the most planets are gas giants like Jupiter. SIM will search after Earth-like planets, the "holy grail" for planet hunters, around 60 nearby stars. A second planet search program will search after Neptune-like planets around 2000 stars and the scientists hope that SIM can improve the understanding how “hot Jupiters” formed around young stars.
Furthermore, SIM will study the birth, evolution and architectures of young planetary systems and will deliver ultra-precise measurements of stellar masses, luminosities and ages. Distances to stars can be determined to an accuracy of 1% out to about 1000 light years, and to 10% across the Milky Way galaxy. 
The scientists hope also that SIM can be helpful by the detection of the nature of dark matter, a non-baryonic material of unknown character whose presence can be inferred only from its gravitational effect on visible matter.  In addition SIM will peer into the hearts of galactic nuclei and maybe deliver new informations over the understanding of the accretion disks that surround the core, which is the source of the powerful relativistic jets.
The spacecraft provides a X-band telecom system with a fixed high gain antenna and will communicate over NASA’s Deep Space Network. 
The instrument consists of two visible-wavelength Michelson stellar interferometer sensors, one 30 cm Guide telescope, and one external metrology sensor, all supported by a Precision Structure subsystem (PSS).