input: def planet_between(planet1, planet2):
      '''
      Our solar system contains eight major planets: starting from the one closest 
      to the Sun, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, 
      Neptune.
      Write a python function that accepts two strings planet1 and planet2, signifying 
      planet names. The function should return a tuple featuring all planets positioned 
      between the orbits of planet1 and planet2, ordered by their distance from the Sun.
      If either planet1 or planet2 are invalid planet names, the function should output 
      an empty tuple.
      Examples:
      planet_between("Jupiter", "Neptune") ==> ("Saturn", "Uranus")
      planet_between("Earth", "Mercury") ==> ("Venus")
      planet_between("Mercury", "Uranus") ==> ("Venus", "Earth", "Mars", "Jupiter", "Saturn")
      '''
output:     
      planets = ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
      
      if planet1 not in planets or planet2 not in planets:
          return ()
      
      idx1 = planets.index(planet1)
      idx2 = planets.index(planet2)
  
      if idx1 > idx2:
          idx1, idx2 = idx2, idx1
      
      return tuple(planets[idx1 + 1:idx2])
  