Why does exoplanets' naming start from "b" and not from "a"? An exoplanet named after its host star receives the suffix letter "b" if it was the first planet discovered to orbit that star, or "c" if it was the second, or "d" if it was the third, and so on.

Why this naming system starts from "b" and not from "a"? Is the letter "a" reserved for another object in the planetary system? If yes, for what is "a" reserved?

As an example, here is a list of the major objects in the Gliese 667 triple-star system:


Gliese 667A (a star)
Gliese 667B (a star)
Gliese 667C (another star)
Gliese 667Cb (a planet orbiting star C)
Gliese 667Cc (another planet orbiting star C)
A  quick lmwtfy reveals the answer:


  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.[2] 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.