The Extra-Solar Star 55 Cancri

Only two planetary systems have planets that are named "unusual". 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. Being that 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). Some nomenclatures (generally in science fiction) use Roman numerals in the order of planets' positions from the star, but for the above reason, this is not practical. If the planet orbits in a non-circumbinary system, the letter of the star is added to the name.


The star 55 Cancri is the star with the most confirmed planets found around any star known (excluding the Sun) and may contain more planets. The planet 55 Cancri f (artist's conception pictured) is currently the only planet with the designation "f"



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, the name 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 to never used. However (in the cases of 16 Cygni Bb and 83 Leonis Bb), if the planet orbits a secondary star of the system, the star's name is always used. Some planets have received unofficial (informal) names that can be compared to the planets of 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 name extrasolar planets, considering it impractical.