What do we know about extraterrestrial snow? On which (exo)planets or (exo)moons do we have direct hints for its existance? This is indeed a children's question, but I struggle to answer it conclusively.
For exoplanets in the habitable zone, the phase diagram of water may of course allow snow if water existed on an exoplanet. What about non-water snow? Which chemical substances which allow snow have been observed so far on other (exo)planets?
Generalized definition of snow
As snow I would define as precipitation of any (chemical) kind which is not in liquid or gaseous state. The solid objects (most likely crystals) should be moving downward in from the sky and is possibly reaching the ground (if there is any).
This definition excludes ground frost and hail. Hail is excluded since these layered balls of (water ice and/or other solids) are actually moving repeatedly and significantly moving upward (on Earth in Comulonimbus clouds). The definition does include solid clear sky precipitation aka diamond dust.