Galaxoid and Nebular

In the original Calvin and Hobbes, two short aliens named Galaxoid and Nebular gave Calvin 50 leaves from their planet. Calvin and his neighborhood once helped saved Galaxoid and Nebular's home planet Ankor from Rupert, Earl, and Dopey Alien in Calvin and Hobbes 2. Galaxoid is the leader of the planet and named it after his last name. Simon Pegg plays Galaxoid, and Nick Frost plays Nebular. 

Characteristics

They are short aliens, about the same height as Calvin. They have one large eye, and at least four tentacles at the bottom. Their spacesuit consists of a short robe with a ringed collar and a "wizard-style" cap, both with a symbol on the front. In canon, it is unknown what colour they are, but in The "Dimension" Saga they're just black and white.

Galaxoid and Nebular are black and white (along with the inside of their ship) in The "Dimension" Saga because they have never been seen in a canon Sunday strip.

Homeworld

Galaxoid and Nebular claimed that at light speed, their ship could "just make it" to a short stop on their planet and back to Earth within an afternoon. Although their estimate proved somewhat liberal, the fact remains that their planet must have been only a few light-hours away.

They have been claimed to live in the Star-Moon Homeworld in The "Dimension" Saga, but in Calvin and Hobbes 2, it's Ankor.

Organization

Nebular claimed that "[their] master will be very pleased", possibly meaning they have an organization. This was revealed in The "Dimension" Saga, where they had the Star-Moon Alliance. The aliens of this organization live in the Star-Moon Homeworld (see above). The "master" that Nebular is referring to has never been seen.