Don't know if it's a hobby, but I'm fascinated with exoplanets, specifically exoplanets that might be inhabited with intelligent life (as we know it).

Every year I will spend several hours catching up on the latest exoplanet discoveries/news, and try my hand at crunching numbers to predict the number of intelligent species in the Milky Way right now.

The cool thing about this is that each time I play catch-up the numbers get a bit clearer. When I started about 15 years ago many of the nested filters to whittle down the final count were very fuzzy, to the point of just being very vague guesses (I think I usually go with 0.1% of planets from the prior filter in that situation). But it seemed like each time I review the latest data the next filter down gets clear. When I say filter I'm taking about things like: planet exists in the habitable zone of its parent star; planet is far enough away to not be in a locked orbit; planet (or large moon) is big enough to have a molten core (and thus a magnetosphere), etc, etc).

Of course, none of this means we'll ever necessarily meet another species (space is so absurdly vast). But it's fun to ponder nonetheless.