How are planets named?

Extra-solar planets take the name of the star and then have a letter (b, c, d, etc.) appended in the order of discovery.

Many stars are known by an identification number from a particular catalog.

Gliese's catalog is a list of nearby stars (within 20 to 25 parsecs, i.e. max of 80 light years) and the stars are numbered in the order of their position in right ascension (the sky equivalent of longitude).

http://en.wikipedia.org/wiki/Gliese_Catalogue_of_N...

Thus Gliese 581 is star number 581 in Gliese's catalog (happens to be a red dwarf in the constellation Libra approximately 20 light years from Earth) and planet Gliese 581 b is the first planet found orbiting Gliese 581.

You'll see stars from the Henry Draper catalog (HD xxxxx).

You'll also see stars labelled by the Flamsteed number within the constellation (such as 51 Pegasi and 55 Cancri)

http://en.wikipedia.org/wiki/Extrasolar_planet#Nom...