Seirei no Himegimi Stea ~Stoom Core no Densetsu~

The Fillol. Though being a small star, it was a rich planet full of resources and known as a hidden treasure of the space.
Long ago, a space bandit DEGDAL tried to invade this planet and failed. This tainted their glorious history of having known no defeat.
Negdal, the protagonist, takes the revenge of his ancestry. Now he knows where the strong power that the planet once defeated his ancestor lies: the sacred core, the treasure the princess of the Fillol carries in her very womb, is the source of their strength!

[From DLsite English]