What should I do if I want a planet to get bonus based on the population of the whole star system?

I looked at the documentation of simulation. It says:

## Simulation modifiers - Simple modifier

| Attribute name      | Default value     | Description
|---------------------|-------------------|-----------------------------------------------------------------------------------------------------------------------------------------
| TargetProperty      | `<Required>`      | The name of the property which will receives the result.
| Operation           | `<Required>`      | The modifier's operation. Available operations: Force, Addition, Subtraction, Multiplication, Division, Power, Percent.
| Value               | `<Required>`      | The value needed to compute the operation. (It can be a float value or another property value using $(PropertyName) syntax)
| Path                | `""`              | Dertermine where to find the target property (By default the source object).
| Priority            | `0`               | The computation priority value of the modifier (used to change the default operation order).
| SearchValueFromPath | `false`           | If the value attribute reference a property, it will be retrieved from the same path as the target property.

The "Value" can reference another property, and it seems that the properties are found from the class of "Type". There is an option to find property from the class of "Path". But it seems to have no way to find the property from an arbitary location. How can I access the population of star system in a planet's modifier?