Jupiter, also known as "Dairy Planet" by its natives, is the fifth planet of the solar system and the largest. It is composed mostly of hydrogen.
ALF also mentions that Jupertonians advertise Jupiter as the "dairy planet" on their license plates. This implies some creature or thing on Jupiter produces some sort of milk like substance that can be used in making of dairy products, but ALF doesn't go into any more detail.
- The Gambler (mentioned only)