From Wikitroid, the Metroid wiki

Earth is a planet mentioned in the Metroid series but never seen. It is where Humans originated from. It is sometimes speculated by fans to be the capital of the Galactic Federation. Earth is located in the Solar system. One of Earth's colonies is K-2L.

Planets in the Metroid Series
Note: Bolded Planets were visited by Samus Aran
Tetra Galaxy
Alinos | Arcterra
Ooromine System
Bilium | Ooromine II | Twin Tabula | Zebes | Tallon IV
Federation Solar System
Bryyo | Norion | Elysia
SR 388 System
SR227 | SR388
Other
Aether | Aliehs III | Bes III | Cylosis | Earth | Jovia XII | Earth Colony K-2L | Mondreus | Phaaze | Phrygis | Pirate Homeworld | Tivus | Vho | Wotan VII