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Selnia Prime was a small Delta Quadrant planetoid in the Wyanti system.

It was visited by Tom Paris and Neelix in 2376 to collect mold for The Doctor to grow antiviral proteins with. In its caves they encountered con artists Dala and Mobar. (VOY: "Live Fast and Prosper")

Paris claimed his scans didn't detect anyone "down here". It is unclear if that referred to the planetoid or just the caves.
The Star Trek Encyclopedia (4th ed., vol. 2, p. 263) identified Selnia Prime as a planet.