There's actually a good reason not to use Arabic numerals for the orbits - they're sometimes used in star catalogues when designating the star themselves, so this allows for the star to be (for example) HD 209458 (which lies in Pegasus), and to then designate the orbits by distance, using Roman numerals.
So you might have HD 209458 III for the third planet in the system.
Siimilarly you have stars that are known by a name and number (in Arabic numerals), like
https://en.wikipedia.org/wiki/Kepler-11
The problem with that exoplanet naming convention is that they're not done by distance, but by order of discovery, so b might actually be a gas giant somewhere in the middle of the orbits, whilst c is an external iceball, and d is an internal rocky planet. If they're discovered at the same time, then they're numbered inside to out, but you could still end up with, say, Jupiter and Saturn being b and c, with Earth, and Venus being d and e, then the others coming in in a fairly patchwork system.