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/
No atmosphere no helicopter. Thats why mercury is not listed. There are other objects with an atmosphere in the silar system, but the atmosphere is way to thin on them to fly anything. So the 7 planets and titan are actually well chosen, those are the only objects in the solar system a helicopter could ever fly on.