HD: HD stands for Henry Draper Catalogue, which is a stellar catalog containing information about thousands of stars. It was compiled by Annie Jump Cannon and her colleagues in the late 19th and early 20th centuries.
The number represents the specific star in the Henry Draper Catalogue. In this case, HD 189733 refers to the star around which the exoplanet was discovered. The number of the star has no meaning outside of that it was the 189733rd catalogued one in HD's catalogue
The letter "b" is used to designate the first known planet orbiting the star. (a is for stars) If additional planets were discovered in the same system, they would be labeled with subsequent letters in alphabetical order (e.g., HD 189733 c, HD 189733 d, and so on).

Therefore, HD 189733 b signifies the first planet discovered orbiting the star HD 189733. This naming convention allows astronomers and researchers to identify and differentiate between different planets and their host stars in a systematic manner.

TOI 849 b is TESS Object of Interest's 849th discovery of interest. The numbers are essentially arbitrary outside of that numbers before them have already been taken.

[–] Eheran@lemmy.fmhy.ml 0 points 1 year ago* (last edited 1 year ago) (1 children)

What if it is a 2 star system? A and b etc. is taken. HD 189733 is good the first star, then we find the second... HD189733 AA?

[–] Clairvoidance@kbin.social 0 points 1 year ago (1 children)

I fucked up a little so I understand the confusion, stars are typically in capital letters, A, B, C.
though clear distinctions are also made here. B, C, D implies a hierarchy of orbits, A is the primary gravitational pull in its system
Aa, Ab, (etc.) is used when two (or more) stars have a common center of mass

Real implications of this example:
Kepler-16A and Kepler-16B have a planet that orbits both (but to be clear they do not having a common center of mass), so it is Kepler-16b, and I didn't look it up but I assume it's not 16ABb because there aren't any A or B planets to differentiate it with
Polaris Aa and Ab are orbiting around a common center of mass, and Polaris B orbits them from further away
Castor has 6 stars, here's an image
Aa and Ab are still the main center of mass in this system

as an added bonus, black holes do not have a naming convention yet, Sagittarius A* is just in the Sagittarius A region and * is to indicate an exciting or interesting object

and to my knowledge, it is theoretically possible for a planet to orbit an Aa star without orbiting an Ab star but I don't think we know of any, I assume it would be named like Aa b

[–] Eheran@lemmy.fmhy.ml 0 points 1 year ago (1 children)

When you say common center of mass, so you mean the as in something like... Once it is outside of the object? Otherwise I assume everything has a common center of mass with everything.

Thanks for the info!

[–] Clairvoidance@kbin.social 1 points 1 year ago

Common center of mass means they spin around each other, currently the moon orbits earth, but long long in the future, even though the moon is smaller, it will have waned the pull of the earth to an extent where you both go around in the same circle, this happens quicker with objects the more equal in size they are