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Nomenclature
The system used in the scientific literature for naming exoplanets is almost the same as the system used for naming binary stars. The only modification is that a lowercase letter is used for the planet instead of the uppercase letter used for stars. A lowercase letter is placed after the star name, starting with "b" for the first planet found in the system (for example, 51 Pegasi b); "a" is skipped to help prevent confusion with the primary star. The next planet found in the system would be labeled with the next letter in the alphabet. For instance, any more planets found around 51 Pegasi would be catalogued as "51 Pegasi c" and then "51 Pegasi d", and so on. If two planets are discovered at about the same time, the closer one to the star gets the next letter, followed by the farther planet. However, in some cases, a smaller planet is found closer to the star than other previously known planets, causing the letter order to not follow the order of the planets from the star. For example, in the Gliese 876 system, the most recently discovered planet is referred to as Gliese 876 d, despite the fact that it is closer to the star than Gliese 876 b and Gliese 876 c. At present, the planet 55 Cancri f, which is the fifth planet found in the 55 Cancri system, is the only planet to have "f" in its name, the highest letter currently in use.
If a planet orbits one member of a multiple-star system, then an uppercase letter for the star will be followed by a lowercase letter for the planet. Examples include the planets 16 Cygni Bb and 83 Leonis Bb. However, if the planet orbits the primary star of the system, and the secondary stars were either discovered after the planet or are relatively far form the primary star and planet, then the uppercase letter is usually omitted. For example, Tau Boötis b orbits in a binary system, but because the secondary star was both discovered after the planet and very far from the primary star and planet, the term "Tau Boötis Ab" is rarely if ever used.
Only two planetary systems have planets that are named unusually. Before the discovery of 51 Pegasi b in 1995, two pulsar planets (PSR B1257+12 B and PSR B1257+12 C) were discovered from pulsar timing of their dead star. Since there was no official way of naming planets at the time, they were called "B" and "C" (similar to how planets are named today). However, uppercase letters were used, most likely because of the way binary stars were named. When a third planet was discovered, it was designated PSR B1257+12 A (simply because the planet was closer than the other two).[20]
An alternate nomenclature, often seen in science fiction, uses Roman numerals in the order of planets' positions from the star. (This is inspired by an old system for naming moons of the outer planets, such as "Jupiter IV" for Callisto.) But for the above reasons, such a system has proven impractical for scientific use. To use our solar system as an example, Jupiter would most likely be the first planet discovered, and Saturn the second; but, as the terrestrial planets would not be easily detected, Jupiter and Saturn would be called "Sol I" and "Sol II" by science-fiction nomenclature, and need to be renamed "Sol V" and "Sol VI" if all four terrestrial planets are discovered later. In contrast, by the current system, even if the terrestrial planets were found, Jupiter and Saturn would remain "Sol b" and "Sol c" and not need renaming.
Finally, some planets have received unofficial names comparable to those of planets in the Solar System. The most noted planets that have been given names include: Osiris (HD 209458 b), Bellerophon (51 Pegasi b), and Methuselah (PSR B1620-26 b). The International Astronomical Union (IAU) currently has no plans to officially assign such names to extrasolar planets, considering it impractical.
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