Title text:
I'm excited for Dragonfly, because on the list of places in the solar system that I desperately want more pictures from, 'Titan's surface' is right up there at the top with 'Jupiter's atmosphere.'
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3322/
I think it's saying those are the eight places a helicopter could work, and we are checking them off as we accomplish them.
Helicopters require atmosphere (obviously), so this is a list of the eight bodies in the solar system with atmosphere sufficient for heavier-than-air flight.
Mercury basically doesn't have an atmosphere for instance, which is probably why it's not on the list.