from pydantic import BaseModel, Field

# Base Schema
class PlanetBase(BaseModel):
    name: str = Field(..., description="Name of the planet")
    description: str | None = Field(None, description="Description of the planet")
    diameter: int = Field(..., gt=0, description="Diameter of the planet in kilometers")
    mass: int = Field(..., gt=0, description="Mass of the planet in kilograms")
    distance_from_sun: int = Field(..., gt=0, description="Distance from the sun in kilometers")

# Schema for creating a new Planet
class PlanetCreate(PlanetBase):
    pass

    class Config:
        schema_extra = {
            "example": {
                "name": "Earth",
                "description": "The planet we live on",
                "diameter": 12742,
                "mass": 5972000000000000000000000,
                "distance_from_sun": 149600000
            }
        }

# Schema for Planet responses
class Planet(PlanetBase):
    id: int

    class Config:
        orm_mode = True
        schema_extra = {
            "example": {
                "id": 1,
                "name": "Earth",
                "description": "The planet we live on",
                "diameter": 12742,
                "mass": 5972000000000000000000000,
                "distance_from_sun": 149600000
            }
        }
```

This code defines three Pydantic schemas for the Planet entity:

1. `PlanetBase`: A base schema that defines the common fields for a planet.
2. `PlanetCreate`: A schema for creating a new planet, inheriting from `PlanetBase`. It includes an example configuration for request validation.
3. `Planet`: A schema for planet responses, inheriting from `PlanetBase` and adding an `id` field. It includes an example configuration and sets `orm_mode=True` for compatibility with ORMs like SQLAlchemy.

The schemas use appropriate Pydantic field types and include validation rules using `Field()`. The `description` field is marked as nullable (`None`), while other fields are required. The `gt=0` validation ensures that numeric fields are positive.