Discovered exoplanets each year as of 26 November 2017
Size comparison of Jupiter and the exoplanet TrES-3b. TrES-3b has an orbital period of only 31 hours and is classified as a Hot Jupiter for being large and close to its star, making it one of the easiest planets to detect by the transit method.
NASA histogram chart of confirmed exoplanets by distance
An exoplanet (UK: t/, US: t/) or extrasolar planet is a planet outside our solar system. The first evidence of an exoplanet was noted as early as 1917, but was not recognized as such. However, the planets in 2,853 systems, with 633 systems having more than one planet.
The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, 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.
Besides exoplanets, 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 rogue planets in the Milky Way possibly number in the billions (or more).
The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the designation is normally formed by taking the name or, more commonly, designation of its parent star and adding a lower case letter. The first planet discovered in a system is given the designation "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist.