Kerwan was a planet that was covered in big cites. It was later nuked by Tachyon in an effort to kill Ratchet. Tachyon later regretted it, as he found out the planet was full of Mars Bars.

Galaxy: Solona

Capital: Metropolis

Ratchet & Clank wiki
For those without comedic tastes, the so-called experts at the Ratchet & Clank wiki have an article about Kerwan.