Hydrus Four
Hydrus Four, or Hydrus IV, is the only planet on which the miracle restorative Nucleon can be found. Now, this WOULD be plenty of cause to journey to the planet except for a few meddlesome problems.
Specifically, the fact that the planet is inhabited by creepy obsessive zombie mechanoids who guard the Nucleon. Also: the only inhabitants of the planet who are NOT Robo-Zombies live only to warn people of the zombies.
Hydrus Four is well known for the high quality of its medical professionals, so presumably there is more to this planet than has been seen.
Fiction
Marvel Comics continuity
Bumblebee, thankful to Ratchet for fixing him up, told Ratchet that he was the best surgeon "this side of Hydrus Four". Skin Deep
Ravage made the claim that with technology he could procure, Wheeljack could become the best engineer "this side of Hydrus Four". ...Perchance to Dream
The AllSpark Almanac
Hydrus IV was a robotic planet owned by the Vestial Imperium. Swindle noted it had fascinating medicinal research. The AllSpark Almanac II
Wings Universe
A chunk of Hydrus Four somehow broke off and ended up tumbling through space as an asteroid. It was spotted by Gutcruncher and his Decepticon gang, who believed it held unimaginable power and hired a ship to take them to it. Little did they know that the Darklon was actually crewed by the Elite Guard's Stealth Team. After landing on the asteroid, they found a crystal monolith that irradiated them with strange energy, giving them increased speed and strength but taking away their ability to transform. During the course of affairs, the monolith was destroyed, shattering the asteroid and causing pieces of it to rain down on Cybertron. A Team Effort
IDW Generation 1 continuity
Chromedome mistook Hydrus IV for Hydrus V. Rules of Disengagement
