Sirius is the home planet of S.P.D. It was destroyed by Emperor Gruumm.
Unknown, though images of the planet show an overcast, slightly sepia-tinted world with a greyish desert. The primary inhabitants are Sirians, humanoid doglike creatures with scaly skin.
Sirius is the planet Space Patrol Delta originates from, and contained an academy belonging to it. Emperor Gruumm attacked it, destroying S.P.D.'s current headquarters along with most of the planet's inhabitants, and the original A-Squad, with the exception of Anubis Cruger. Given Gruumm's onscreen treatment of planets he's destroyed, it is likely there is nothing left of Sirius.