[tomr_stargazer]

That's the naming convention for exoplanets. It descends from the naming conventions for multiple star systems defined by the International Astronomical Union [0], in which the brightest (and usually gravitationally dominant) member of a multiple system receives designation "A", the second-brightest gets "B", and so on. Note the capitals. Planets, being considered a lower-mass extension of stars, get lower-case letters accordingly; so the first planet gets "b", the next planet gets "c" and so on. There is no lowercase "a" in a star-planet system. (I guess you might have that if you found binary planets without a star system; that would be very unusual).

Source: my PhD in astrophysics, and: [0] https://en.wikipedia.org/wiki/Exoplanet_naming_convention

[sp332]

So in this case the binary system together is 1A? Or is the brighter star 1A, the planet is 1b, and the black hole is 1C because it's darker?

[tomr_stargazer]

I had to look at the paper itself to figure this out, but - in this paper, they designate the binary (star + stellar remnant) together as M51-ULS-1, and the planet actually orbits the two members together (a so-called circumbinary planet [0]). The logic of the nomenclature system actually breaks down in such cases - here's a quote from the Wikipedia article on naming conventions [1] stating as much:

'According to Hessman et al., the implicit system for exoplanet names "utterly failed with the discovery of circumbinary planets", and they state that it is unhelpful. '

So, yeah, take that for what you will.

[0] https://en.wikipedia.org/wiki/Circumbinary_planet

[1] https://en.wikipedia.org/wiki/Exoplanet_naming_convention#Ci...