Dying stars could make frozen planets habitable

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...when the star begins to burn its hydrogen in a shell around a growing helium core, it brightens and expands and becomes a "sub-giant". The habitable zone sweeps outward, extending from 2 to 9 AU. The inner edge of this zone remains habitable for several billion years while the outer extreme, where Saturn currently orbits, is habitable for a few hundred million years. The star then fluctuates in brightness for about 20 million years as it switches to burning helium almost exclusively, before becoming a red giant and swelling to 10 times the diameter of the Sun. For about a billion years afterwards, the habitable zone around the red giant extends from 7 to 22 AU, the outer edge of which lies beyond the orbit of Uranus.