In Django create a model method which determines an id for all objects that share the same foreign key

Kind of new to Django and while trying to create my first own project, i stumbled upon a problem. I have two different model classes: Star and Planet. Each planet has a foreignkey which belongs to a Star. Now I would like to have a field for Planet, which is basically the id/pk for its star system that is given on creation/discovery. All known planets of the star system already have their star system id and thus the newly created/discovered planet would require an id that is incremented on the last id that was given.

My models are looking like this

models.py:

class Star(models.Model):
   name = models.CharField(max_length=200)

class Planet(models.Model):
   name = models.CharField(max_length=200)
   star = models.ForeignKey(Star, on_delete=models.CASCADE)

   def starSystemPlanetId(self):
      What could work in here?

This new Id should be available to be called in a template like this:

{% for planets in object_list %}
    <li> 
         <p>Planet {{planets.name}} is the Nr. {{planets.customer_id}} planet of the star system</p>
    </li>
{% endfor %}

I have learned already, that business logic should be implemented inside models.py via a model method.

def starSystemPlanetId(self, star_var):
   starSystemPlanetId = Planet.objects.filter(star = star_var).filter(pk >> earlierPlanetsPk).count()
   return starSystemPlanetId

Maybe there is a standard query function available for this kind of problem already.

Any help and thought about it is very welcome!

class Planet(models.Model):
    name = models.CharField(max_length=200)
    star = models.ForeignKey(Star, on_delete=models.CASCADE, related_name='planets')

    # Use property since it's more pythonic
    @property
    def star_system_planet_id(self):
       """0-based display identifier for a planet in the start system."""
        # No need for the 'star_var' variable since you already have it in the 
        # model instance
        return self.star.planets.filter(pk__lt=self.pk).count()