input: def find_planets_between(planet_start, planet_end):
      '''
      In our solar system, the sequence of planets from the Sun is: Mercury, 
      Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
      Write a function that accepts two planet names as strings: planet_start 
      and planet_end. The function should return a tuple of planets that lie 
      between the orbits of planet_start and planet_end, ordered by their 
      distance from the sun. The tuple should be empty if either 
      planet_start or planet_end are invalid planet names.
      Examples
      find_planets_between("Jupiter", "Neptune") ==> ("Saturn", "Uranus")
      find_planets_between("Earth", "Mercury") ==> ("Venus")
      find_planets_between("Mercury", "Uranus") ==> ("Venus", "Earth", "Mars", "Jupiter", "Saturn")
      '''
output:     
      planets = ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
  
      if planet_start not in planets or planet_end not in planets:
          return ()
  
      start_index = planets.index(planet_start)
      end_index = planets.index(planet_end)
  
      if start_index < end_index:
          return tuple(planets[start_index + 1:end_index])
      else:
          return tuple(planets[end_index + 1:start_index])
  