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Why is there no exoplanet around Proxima Centauri designated with the letter 'a'? There is, famously, a 'b' and now a newly confirmed 'c'....

I have read that exoplanets around a star are designated from 'a' to z or whatever in alphabetical order, starting from the closest to the star OR beginning with the first one discovered, then the second....

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Usually, the primary host star reserves the right to have the 'a' designation. Therefore 'a' never gets assigned to a planet.

Please check the following post for a detailed explanation and some examples. Where did TRAPPIST-1a go?

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    $\begingroup$ Technically th eprimary star should not be labeled "a", and I don't know of any example. If the star is single it is know by its proper name of catalog designation. If there are two or more stars in the star system The brightest one has the suffix A, the next brightest one the suffix B, and so on. So the primary is never labeled "a", but the planets start with "b", and are labeled in order of discovery. $\endgroup$
    – M. A. Golding
    Commented Jun 16, 2020 at 15:25

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