Uchuu Shounen Bulge

Long ago Industria was a peaceful planet, ruled by a single royal family. Since then, contact with alien races has led to fifty years of invasion, turmoil, violence, and war, leaving the Royal Family in control of the planetary capitol and little more. Astro the Destitute lives in the capitols slums, working a miserable job to support his makeshift family. One day he is approached by Bulge, the runaway heir to the planetary throne. Shockingly identical in voice and appearance, Prince Bulge proposes Astro take his place on the throne so that Bulge can escape his life as Prince. Moments later the Prince's guard approaches, mistaking Astro for Bulge, they drag him back to the planet. Thus begins Astro's adventure of mistaken identity, thrust into the role of heir to the throne, and savior from the warring alien hordes.

(Source: Fallen Angels)