An exoplanet, also known as an extrasolar planet, is a planet outside the Solar System. Most are giant planets thought to resemble Jupiter or Saturn. They are unlikely to support life as we know it, but some of these planetary systems might also contain smaller, terrestrial planets like Mars and Earth.
A large number of the exoplanets found so far are known as 'Hot Jupiters' - gas giant planets similar in size to Jupiter that orbit extremely close to their parent star. They are easy to detect because of their large size and short orbital period.
A substantial fraction of stars have planetary systems, including at least around 10% of sun-like stars. There also exist planets that orbit brown dwarfs and free floating planets that do not orbit any parent body at all.
The first confirmed detection was made in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmed detection of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet, 51 Pegasi b, was found in a four-day orbit around the nearby G-type star 51 Pegasi.
The discovery of extrasolar planets has intensified interest in the possibility of extraterrestrial life. It is thought the most likely location for life to exist is on small 'Earth-like' rocky planets within a region of the stellar system known as the 'Habitable zone' where temperatures are suitable for liquid water to form.
NASA's Kepler Mission has discovered the most planet-rich planetary system yet, dubbed Kepler-11. The Kepler 11 system is a planetary system containing six Earth-sized planets orbiting around a sun-like star dubbed Kepler-11, approximately 2000 light years away, in the constellation of Cygnus. The six planets orbiting Kepler-11 are all larger than Earth, with the largest ones comparable in size to Uranus and Neptune. All of the Kepler-11 planets orbit their parent star in roughly the same plane, which is similar to the arrangement of our solar system.
Any planet is an extremely faint light source compared to its parent star. Astronomers have had to resort to indirect methods to detect extrasolar planets, with one of the most popular methods being the Transit Method. During the transit method, If a planet crosses (or transits) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors. To verify the existence of a planet the space telescope needs to observe the planet at least three times.