"Planets" were public feed aggregators collecting RSS (RDF, Atom) feeds for blogs on a specific topic, so you can go to a "Planet" and find all recent blogs by all people from that community. Thus a place to discover recent things, without following everybody individually.

In this case it is referencing https://planet.gnome.org/ which collects many Gnome people's blogs. And the statements tells us that the subscription was sulfated, so users of "Planet Gnome" don't have to do anything, but may continue consuming from there.