The convention for naming exoplanets is an extension of the one used by the Washington Multiplicity Catalog (WMC) for multiple-
star systems, and adopted by the
International Astronomical Union. ::In the WMC naming system, the brightest member of a star system receives the letter "A". Distinct components not contained within "A" are labeled "B", "C", etc. Subcomponents are designated by one or more suffixes with the primary label, starting with lowercase letters for the second hierarchical level and then numbers for the third. For example, if there is a triple star system in which two stars orbit each other closely with a third star in a more distant orbit, the two closely orbiting stars would be named Aa and Ab, whereas the distant star would be named B. For historical reasons, this standard is not always followed: for example
Alpha Centauri A, B and C are
not labelled Alpha Centauri Aa, Ab and B. Following an extension of the above standard, an exoplanet's name is normally formed by taking the name of its parent star and adding a lowercase 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. For instance, in the
55 Cancri system the first planet –
55 Cancri b – was discovered in 1996; two additional farther planets were simultaneously discovered in 2002 with the nearest to the star being named
55 Cancri c and the other
55 Cancri d; a fourth planet was claimed (its existence was later disputed) in 2004 and named
55 Cancri e despite lying closer to the star than 55 Cancri b; and the most recently discovered planet, in 2007, was named
55 Cancri f despite lying between 55 Cancri c and 55 Cancri d. As of April 2012 the highest letter in use is "j", for the unconfirmed planet
HD 10180 j, and, as of July 2020, with "i" being the highest letter for a confirmed planet (
Kepler-90i). If a planet orbits one member of a
binary star system, then an uppercase letter for the star will be followed by a lowercase letter for the planet. Examples are
16 Cygni Bb and
HD 178911 Bb. Planets orbiting the primary or "A" star should have 'Ab' after the name of the system, as in
HD 41004 Ab. However, the "A" is sometimes omitted; for example the first planet discovered around the primary star of the
Tau Boötis binary system is usually called simply
Tau Boötis b. The star designation is necessary when more than one star in the system has its own planetary system such as in case of WASP-94 A and WASP-94 B. If the parent star is a single star, then it may still be regarded as having an "A" designation, though the "A" is not normally written. The first exoplanet found to be orbiting such a star could then be regarded as a secondary subcomponent that should be given the suffix "Ab". For example, 51 Peg Aa is the host star in the system
51 Peg; and the first exoplanet is then 51 Peg Ab. Because most exoplanets are in single-star systems, the implicit "A" designation was simply dropped, leaving the exoplanet name with the lower-case letter only:
51 Peg b. A few exoplanets have been given names that do not conform to the above standard. For example, the planets that orbit the pulsar
PSR 1257 are often referred to with capital rather than lowercase letters. Also, the underlying name of the star system itself can follow several different systems. In fact, some stars (such as
Kepler-11) have only received their names due to their inclusion in planet-search programs, previously only being referred to by their
celestial coordinates. == Circumbinary planets and 2010 proposal ==