Mercury

Mercury
Inhabitants	None
Moons	None
Notable Facts	The Butt of Mercury
First Appearance	"Battle of the Planets"
[Source]

Mercury is a real-life planet existing in the Solar System in the Milky Way Galaxy. It is the closest planet to the Sun and has boiling hot days and ice cold nights. In Invader Zim, Mercury is a planet featured in the episode "Battle of the Planets". The planet contained a strange formation called the "Butt of Mercury".

This planet had been converted into a spacecraft by the aliens that lived on Mars, like they also did with Mars before they worked themselves into extinction. It was done to test if the process could work on Mars. Dib used the planet to stop Zim from crushing the human race by rolling Mars (also a spacecraft) over Earth's surface, which the Almighty Tallest found hilarious.

It is unknown what happened to it after Dib defeated Zim in the Battle of the Planets. It's possibly it was put back into orbit by Dib, or it might have been broken loose from its orbit to drift across the universe, the same as Mars.

Trivia

• There is no 'butt' on Mercury. Rather, this is a parody of the "face" on Mars, an actual natural formation.
• Zim said to the Tallest that he sent probes to the surrounding planets to find something useful for his mission but never went through the information that the probes got back from Mercury, had he seen the "butt" on Mercury, Zim may have gone to the planet after visiting Mars.
• Despite being the planet closest to the sun, Mercury is not the hottest place in the Solar System as that honor goes to Venus.
• Jhonen Vasquez only had two options to turn planets into vessels for the episode as the Martians may have respected life on Earth and chose not to test their technology on the planet, Venus is too hot a planet and its unlikely any life actually can survive there for long if at all, and the rest of the planets are gas giants and Pluto (which is no longer a planet) may not have been an option for the Martians to even visit.