I understand that exoplanets are named by adding a lowercase letter to the a designation of the planet's parent star or stellar system, beginning with 'b' (the star itself is 'a') in order of discovery (and orbital distance from star where discoveries of more than one planet around a star are announced together). My question is whether there is an established standard for using a space between the stellar designation and the planet's letter. Is it Kepler-22 b or Kepler-22b? Journals seem to have different editorial policies that determine how they handle this, but the comprehensive databases available online (e.g., the Extrasolar Planets Encyclopaedia, the Visual Exoplanet Catalogue, and NASA's Planet Quest) seem to all use a space.

  • $\begingroup$ As an example of how confused this can get, note the Economist's use of a space in some names and not in others (along with their use of 'Kepler 22' rather than 'Kepler-22'). $\endgroup$
    – orome
    Dec 13, 2011 at 18:02

1 Answer 1


You are correct that the standard for naming exoplanets is normally the lower-case letter after the star name in the order of discovery. So in our system, Earth would be Sol b. If there are multiple stars in the system, like 16 Cyg (which has 16 Cyg A and B), then the planet's lower-case letter would be appended to the star's, such as 16 Cyg Bb.

So the space is normally there unless it's a binary star system in which case the space is not there because there already is one between the star's A/B/C/etc. designation and the name. (Different systems have been proposed, such as by Hessman et al. (2010.)

However, as you note, it appears as though the Kepler team, does not include a space. Their papers, for example, would list a planet as Kepler-22b. Or Kepler-16b (which is confusingly short for Kepler-16 (AB)-b). Newspaper/internet editors often in these cases will go to an established grammar guide or set of rules for their own publications instead of following what the team does. Just looking at the External Links on the Wikipedia page for Kepler-22b shows the BBC using "Kepler 22b," NASA properly using "Kepler-22b," and Planetary Habitability Lab using "Kepler-22 b."


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