The naming convention has been in common usage forever in science fiction. E.E. Smith from the first Galactic Patrol serial in 1937 had Kimball Kinnison visiting planets such as Velantia III, Rigel IV, and Palain VII, and Earth was specifically referred to as Sol Three from time to time in the series.
Addendum
Because apparently Examples are necessary, the following are from Galactic Patrol:
"For instance, Kinnison here once had a highly adventurous interview with a lady of Aldebaran II and her friends."
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"Kimball Kinnison of Sol Three calling Mentor of Arisia. Is it permitted that I approach your planet?"
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"Kinnison of Tellus, greetings. Tregonsee of Rigel IV calling from Trenco space-port. Have you ever landed on this planet before?"
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"Lensman of Trenco Space-port--Tregonsee or his relief? Lensman Kinnison of Sol III asking permission to land."
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"....the fifth dove into the deepest ocean of Corvina II, in the depths of which all rays are useless."
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Out from Radelix and into deep space shot the speedster, bearing the Gray Lensman toward Boyssia II, where the Boskonian base was situated.
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"He was Lageston of Mercator V--a good man, too. What is your pressure now?"
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