Back in the day, there was a website, Planet Quake, which was a hand-curated collection of all the news about the game Quake. This led to a bunch of other gaming-related “planet” sites (such as Planet Dreamcast), and then the company behind it, CriticalMass Communications, eventually got into other areas of reporting. Eventually they sold to GameSpy, which in turn eventually got bought out by IGN1.
At some point, a couple of other sites emerged with the name “planet” as what I believe was a tongue-in-cheek reference to the “planet” gaming sites. Planet Debian is the first one I remember seeing but I have no idea if it was the first to exist. Many of these sites were built using auto-aggregation from the then-new RSS protocol. This joke ended up spreading pretty far and wide and at one point there was even a “planet planet” to keep track of all the planets2 (although it seems to have gone down sometime in 2017).
A fun side note, Something Awful was originally a spinoff of Planet Quake; at the time Lowtax claimed it was because of a “falling out” but that may have been an attempt at satire. In retrospect, he might have named it “Planet Awful!”