Irk

"We are now in Irk. It's a planet filled with purple and magenta buidlings, and those little green bastards, the Irkens. It seems they are now highly appreciated to the Coalition of the Red Star to enhance themselves a strong alliance for China, Russia, North Korea, and their allies, including the Irken Empire itself."

--Su Ji-Hoon, Irk

Irk is the homeworld of the Irken race, and capital planet of the Irken Empire. Irk's appearance from space is unknown to humans, but the capital city could be seen when Zim was rampaging through it in an Invader Zim episode, "The Nightmare Begins," having mistaken it for an enemy planet. A screen displaying a map of the Irken Empire in the same episode depicted a large, dark blue planet in the middle, identified as Irk. This coloring was likely meant to highlight Irk for easier identification by the Irken military audience. Irk is believed to be red, magenta or pink in color.

Physical Characteristics
Irk's climate, geography, flora, fauna and topography are unknown officially, though it is theorized that with the dominance of the planet by the Irken species, the entire planet has become covered in sprawling cities and metropolises, possibly even completely converted into a 100% mechanical/industrial world. There is even civilization deep under Irk's surface. The buildings in these cities are largely purple or magenta in color, reinforcing that it is an Irken custom to use shades of purple or magenta. The sky on Irk is light pink in color, much like on Foodcourtia and Conventia. The idea of rain puzzled Zim when he first saw it on Earth, indicating that there was no rain on Irk. In addition, Zim was unaware that staring at the sun from Earth causes blindness, implying that the star that Irk orbits is much dimmer than the Earth's sun or that Irk has a thick atmosphere or cloud cover that blocks much of the star's light.

Life
The only known organisms of the planet are the Irkens themselves, the Digestor and the Control Brains, who dominate the planet in sprawling cities and metropolises. However, if Irk is or was anything like Earth, it will of course have other species native to it. Zim has made references to other species (Slorbees and Gasquiggasplorches, canonically) when speaking in metaphors, but it is not known if any of these creatures are native to his planet. Also, Irken Smeets are born presumably in a large "birthing facility" (known to be called the "Hatcheries", or, less commonly, "Smeeteries") beneath the surface of Irk. Very few Irkens are taller than five feet (or 1.5 meters), except the Tallest and a few exceptions. Canonically, Jhonen has not documented any flora native to Irk.

Orbit
The idea of Irk being distant from its star would suggest the planet has a very low temperature. However, this does not increase the likelihood of water in the forms of snow, ice and rain, as water requires hydrogen and oxygen to be present on the planet, which Irk may be devoid of. However, this is again highly unlikely, since hydrogen is the single most common element in the universe, with oxygen being similarly common. Additionally, Irk's sun is presumed to have a very thick ozone layer, meaning that Irk must have oxygen in some form.

Air
It has been confirmed that Irk has an atmosphere, since it has clouds and a pink sky, but the make-up of its air is unknown. It is presumed that it has contains a breathable gas also found on Earth, as Zim does not need his space suit to breathe on Earth. If this is true, it is conceivable that, conversely, humans can breathe unassisted on Irk.

It is suggested that Irk is rich in oxygen for it to be able to let the Digestor grow to such a tremendous size, similar to dinosaurs in Earth's distant past, it is also implied in an Invader Zim episode "Walk of Doom" that Irk's sun does not emit ultraviolet rays, or that Irk's ozone layer is unusually thick, as Zim was shocked to discover that staring at the sun will blind you. However, in another Invader Zim episode, "Door to Door," Zim is plagued by the sun again, this time its intense heat assaulting him rather than his brightness. Zim complains that "even Sirius Minor didn't orbit such a horrible star" suggesting that the sun itself is more powerful, not that the atmosphere is thinner on earth.