"JackPeters" wrote in message
...
I was under the impression that Pluto has a tenuous atmosphere. At
least, I remember back in the 90s when it was inside the orbit of
Neptune, there were Hubble photos of apparent cloud patterns. I
believe the explanation was it was an atmosphere primarily of
sublimation and not in equilibrium as Pluto does not have enough
gravity to sustain it.

It just seems logical to me that a "planet" should have an atmosphere.

1) Round by self-gravity
2) In orbit around the sun
3) Maintains an atmosphere

That would give us nine planets. Perhaps a few more KBOs would
qualify eventually?

But that would probably exclude Mercury
http://sci.esa.int/science-e/www/obj...objectid=31275 as none of
its atmospheric components are 'trapped' as such by its gravity.