It’s the only fair way to do the comparison, really.
But planets have atmospheres, and even though Pluto is only a dwarf planet right now, it’s no different.
Who’s tiny now, eh? Ok fine, it’s still Pluto. But look how far its atmosphere stretches.
The “outer limit” of Pluto’s atmosphere is very difficult to define, although we know that it is very far from the surface. If one defines it similar to the way we define the exobase of Earth’s atmosphere, then Pluto’s atmosphere has an outer limit of at least seven times Pluto’s radius above it surface. This means that the volume of Pluto’s atmosphere is over 350 times the volume of Pluto itself! This illustrates what a strange and wonderful new kind of world we are about to visit and explore.
And this is a conservative estimate: Pluto’s atmosphere could actually be even bigger than it’s shown in the image above. We’ll have to wait for New Horizons to get there later this year to find out for sure.