# Star K2-18 - why planets b and c, but not a?

**URL:** <https://boards.straightdope.com/t/star-k2-18-why-planets-b-and-c-but-not-a/985964>\
**Category:** Factual Questions\
**Created:** [June 23, 2023, 4:06am UTC](https://boards.straightdope.com/t/star-k2-18-why-planets-b-and-c-but-not-a/985964 "2023-06-23T04:06:16Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\
**Post date:** [June 23, 2023, 4:06am UTC](https://boards.straightdope.com/t/star-k2-18-why-planets-b-and-c-but-not-a/985964/1 "2023-06-23T04:06:16Z")

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The wikipedia article on Star K2-18 indicates that it has two planets, named K2-18b and K2-18c.

Why b and c, for a system with only two planets? Shouldn’t they be K2-18a and K2-18b?

> **[K2-18](https://en.wikipedia.org/wiki/K2-18)**
>
> K2-18, also known as EPIC 201912552, is a red dwarf star located 124 light-years (38 parsecs) from Earth, in the constellation of Leo.
> The star has an exoplanet, called K2-18b, a super-Earth located within the habitable zone of K2-18. It is the first exoplanet in the habitable zone, albeit a hydrogen-rich sub-neptune, to have water discovered in its atmosphere. The star also has a second planet K2-18c, which is proven by system tidal simulation to be a small gas giant.

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**Author:** ![Elmer\_J.Fudd](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/elmer_j.fudd/32/1211_2.png) [@Elmer\_J.Fudd](https://boards.straightdope.com/u/Elmer_J.Fudd)\
**Post date:** [June 23, 2023, 4:25am UTC](https://boards.straightdope.com/t/star-k2-18-why-planets-b-and-c-but-not-a/985964/2 "2023-06-23T04:25:01Z")

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K2-18 is the whole star system. K2-18-a is just the star; the primary subcomponent of the system.

“If the parent star is a single star, then it may still be regarded as having an “A” designation, though the “A” is not normally written. The first exoplanet found to be orbiting such a star could then be regarded as a secondary subcomponent that should be given the suffix “Ab”. For example, 51 Peg Aa is the host star in the system [51 Peg](https://en.wikipedia.org/api/rest_v1/page/mobile-html/51_Pegasi); and the first exoplanet is then 51 Peg Ab. Because most exoplanets are in single-star systems, the implicit “A” designation was simply dropped, leaving the exoplanet name with the lower-case letter only: [51 Peg b](https://en.wikipedia.org/api/rest_v1/page/mobile-html/51_Pegasi_b).”

> **[Exoplanet naming convention](https://en.wikipedia.org/wiki/Exoplanet_naming_convention?wprov=sfti1)**
>
> The exoplanet naming convention is an extension of the system used for naming multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the name is normally formed by taking the name of its parent star and adding a lowercase letter. A provisional IAU-sanctioned standard exists to accommodate the naming of planets that orbit two stars, which are known as circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper name...