An exoplanet, or extrasolar planet, is a planet that orbits a star other than the Sun, a stellar remnant, or a brown dwarf.
There are already more than 4,000 confirmed exoplanets in 3,000 systems. At least 670 systems having more than one planet.
There are also rogue planets, which do not orbit any star and which tend to be considered separately, especially if they are gas giants, in which case they are often counted, like WISE 0855−0714, as sub-brown dwarfs.
The Kepler space telescope has also detected a few thousand candidate planets, of which about 11% may be false positives.
There is at least one planet on average per star. Around 1 in 5 Sun-like stars have an “Earth-sized” planet in the habitable zone. Assuming 200 billion stars in the Milky Way, that would be 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if red dwarfs are included. The rogue planets in the Milky Way possibly number in the trillions.
There are planets that are so near to their star that they take only a few hours to orbit and there are others so far away that they take thousands of years to orbit. Some are so far out that it is difficult to tell if they are gravitationally bound to the star.
Almost all of the planets detected so far are within the Milky Way, but there have also been a few possible detections of extragalactic planets.
The discovery of exoplanets has intensified interest in the search for extraterrestrial life, particularly for those that orbit in the host star’s habitable zone where it is possible for liquid water (and therefore life) to exist on the surface. The study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.
– adapted from “Exoplanet.” Wikipedia, The Free Encyclopedia. 2 Jul. 2015
Where are expolanets located?
Orrery of exoplanets in the Kepler systems (as of June 2014). Kepler orrery by Steven Rieder
Check out this great article from the Smithsonian
As a planet orbits a star, the star also moves in its own small orbit around the system’s center of mass. Variations in the star’s radial velocity—that is, the speed with which it moves towards or away from Earth—can be detected from displacements in the star’s spectral lines due to the Doppler effect.
A super-Earth is an extrasolar planet with a mass higher than Earth’s, but substantially below the mass of the Solar System’s smaller gas giants Uranus and Neptune, which are 15 and 17 Earth masses respectively.
See more here: Superearths!