input: def planets_between(planet_a, planet_b):
      '''
      There are eight planets in our solar system: closest to the Sun is Mercury, followed by Venus,
      Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
      Implement a function that takes two planet names as strings, planet_a and planet_b.
      The function should return a tuple containing all planets situated between the orbits of planet_a and planet_b,
      arranged by their distance from the sun.
      The function should return an empty tuple if either planet_a or planet_b
      are not valid planet names.
      Examples
      planets_between("Jupiter", "Neptune") ==> ("Saturn", "Uranus")
      planets_between("Earth", "Mercury") ==> ("Venus")
      planets_between("Mercury", "Uranus") ==> ("Venus", "Earth", "Mars", "Jupiter", "Saturn")
      '''
output:     
      planets = ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
      
      if planet_a not in planets or planet_b not in planets:
          return ()
      
      index_a = planets.index(planet_a)
      index_b = planets.index(planet_b)
      
      if index_a < index_b:
          return tuple(planets[index_a + 1:index_b])
      else:
          return tuple(planets[index_b + 1:index_a])
  