In the current continuity, III (orbiting A) and VII (orbiting B) are the two class-M worlds in the Alpha Centauri system, the planet-numbering system applied to Alpha Centauri A starting at I with the several worlds orbiting B starting at V.
Actually, as Deranged Nasat
mentioned, The Persistence of Memory
establishes that Alpha Centauri IV is also settled in the 24th century -- perhaps a callback to Crisis on Centaurus
Actually I'm a little embarrassed. I went with Centauri III as the inhabited planet, and justified Centauri VII as a planet around Alf Cen B, because I was going from computer simulations of planet formation in the Alf Cen system done by Elisa Quintana et al. which typically showed only 4-5 planets around the A star (due to the gravity of the B star disrupting wider orbits) and the second and/or third being in the habitable zone. Which was fine as far as it went; the simulations were based on the dynamics of planet formation in our own system, which seemed a reasonable model at the time. But now we've discovered an Earth-sized planet only 0.04 AU from Alf Cen B, far closer than we ever would've expected. It now looks like our system may be anomalous and other systems could often have planets forming much closer to their stars than we thought. So the idea of Alf Cen A having at least seven planets with the fourth in the habitable zone is more credible now than I assumed when I wrote The Buried Age
. If I were writing it now, I'd go with a different set of assumptions.
But that's the occupational hazard of writing science fiction. You never know when new discoveries or theories will supersede something you've written.
"Centaurus" is obviously of Earth origin, taken from the name of the constellation Alpha Centauri is located in. "Al Rijil" seems to refer to an alternative Earth name for Alpha Centauri, "Rigel Kent".
Actually "al Rijil" is Arabic for "the foot" -- in the same way that Rigel is the foot of Orion. (There's also Mu Virginis, aka Rijl al Awwa, "foot of the barking dog.") The full Arabic name for the star is Ar-Rijl al-Qanṭūris
, "the foot of the centaur." This was Romanized to "Rigil Kentaurus," which is often abbreviated to "Rijil Kent" or "Rigel Kent."
In medieval times it was also called Toliman, whose etymology is unclear.