Exoplanets are planets that are beyond our solar system. Most of the exoplanets NASA is interested in are Earth sized and orbiting a sun-like star in the habitable zone, though the first few confirmed exoplanets were actually Jupiter sized. The habitable zone is the range of distance from the planet to its parent star where life is possible- not too close, and not too far.
The exoplanets orbiting a sun-like star are the best candidates for becoming definitive planets because the magnetic field of these stars closely resemble the magnetic field of the sun which is strong enough to form a planet. More than 1,000 confirmed exoplanets have been discovered by the Kepler Space Telescope which reached orbit in 2009 and searched for these habitable planets for four years. It discovered these exoplanets by using the transit method which records how much a star's light dims when a planet passes in front of it. This helps determine whether the planet has the exoplanet qualities by showing us how big and how close to its planet star it is. The method used when the first Jupiter sized exoplanets were discovered is the radial velocity technique which detects how much a star "wobbles" when its planet orbits- the closer the planet is to the star, the more the star seems to wobble. Kepler has discovered a variety of planets from gas giants and terrestrial planets, to super-Earths which are planets between the size of Earth and Neptune and are also in the habitable zone.
Has any evidence been found that there could be life on any of these exoplanets?
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