Arrival at Prague Airport (Best Option).
Take the Bus Line 119 (departures every couple of minutes) to station Veleslavin.
There is an entrance to the metro. Take the Metro Line A to the station Staromestska (departures every couple of minutes). Leave the metro and go to the tram station Staromestska. It is not far from the exits, go towards the river (you should see the tram track on the right), the station is right there. Take Tram Line 17 (departures every couple of minutes) to station Vystaviste Holesovice. There are exhibition grounds. Left from the entrance to the exhibition grounds is a sideway path. Take it and walk for about 5 minutes and you are at the Planetarium.