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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)
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  • $\begingroup$ @zibadawatimmy. I hadn't found this question, since it is more specific (where is TRAPPIST-1a), and mine is more generic (where is "a"). Thank you, for the comment, I hadn't thought it was already answered. $\endgroup$
    – Seninha
    Aug 24, 2017 at 13:43
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    $\begingroup$ @Seninha It is outwardly more specific, but I believe the answers in particular address the general concern. I'll note that duplicate questions are often a good thing, as it improves site searchability, which basically resolves your problem: the answer apparently exists on the site but it was unlikely you'd find it because your question wasn't phrased in the same way. Duplicate questions increase the odds that people will get the answers they seek to the question they first ask. $\endgroup$
    – zibadawa timmy
    Aug 25, 2017 at 0:32
  • $\begingroup$ So the title of your question might actually bring more future readers to the answer there than the title of that question. I remember the first time or two that I had a question closed as duplicate I wanted to do anything possible to stop it. More recently I've even been the first one to vote to close my own question as duplicate a few times. (Out of ~1000 SE questions I've asked just about anything can happen). $\endgroup$
    – uhoh
    Aug 26, 2017 at 6:35

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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.

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