Why is there no Proxima 'a' planet? I'm confused about the naming convention 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....
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?