Arrakis! Vulcan! Tatooine! Alderaan! Barsoom! Akir! Zeist! Veridian III! Hoth! Mustafar! Coruscant! Trantor! Krypton!
All hail the Planet!
Planets are everything. Without planets, we've got nothing, in SF. Without planets, we've got no place for stories to happen. Planets are where it's at. Without a nifty planet, your space story isn't going anywhere.
Some planets are Earthlike in most ways. Others are inhospitable almost to the point where humans can't live there...but only just. Some planets are gas giants, leaving us to hope for livable environments on the moons; other planets are tiny, cold, airless rocks.
Planets give rise to amazing life forms. There are enormous worms that slither through the sands of the worldwide desert, catching entire towns in their gaping maws. Or there are scavenging little dwarfs, who wander around picking up bits of technology to sell at the settlements. There are planets whose inhabitants are governed by their fiercest passions, and there are planets whose cultures are rigidly devoted to logic and reason. Planets, planets, planets. Planets everywhere.
I speak here more of fictional planets, but it's always worth remembering that planets are very real in our universe, and not just in our Solar System: planets are places, and reasoning out their motions is one of the greatest achievements of all time. Here's Carl Sagan on Johannes Kepler and Tycho Brahe:
It's interesting to note that for a time, in Carl Sagan's early years in the spotlight, planetary astronomy wasn't much of a going concern.
(I keep forgetting to include the A-to-Z graphic on these posts. Apologies to the event organizers!)