It appears to be a fairly common planetary naming scheme in science fiction: Take the common name (or its bayer designation) of star and append the planetary ordinal in the form of a Roman or Arabic numeral.
I found a good discription in a game manual from 1993:
What the star and cluster names mean
To simplify astrogation, places are identified by a series of names. The stars in your ship's patrol region have been grouped into eight different clusters, and the cluster name is the first level you choose. Within each cluster, the stars are named by Greek letters according to how bright they are. The planets of each star are listed by Roman numerals according to their distances from their sun, and the moons of a particular planet are listed by Roman letters according to their distance from the planet. The destination "Codis Alpha IVB" means go to the brightest star (alpha) in the cluster Codis, then look at the fourth planet out and go to its second moon.
Examples
- Earth becomes Sol Ⅲ
- The tenth planet of Pollux becomes Pollux Ⅹ
- The forth Planet of Beta Hydri becomes Beta Hydri Ⅳ
It's not based in reality
I asked an astronomer and he assured me, no astronomer ever proposed such a naming system.
Real exoplanets today are named using an entirely different naming scheme.
Who came up with it?
It is very common in Star Trek, but it exitsted in earlier science fiction novels and short stories. What is the earliest occurrence of this naming scheme? When was it first described? When was it first implied?