The protein planets were a group of planets that served as a food source for the Nestene Consciousness before they were destroyed in a war. (TV: Rose) After the war, the Nestene targeted early 21st century Earth as the Ninth Doctor explained to Rose Tyler:

You've got such a good planet. Lots of smoke and oil, plenty of toxins and dioxins in the air, perfect. Just what the Nestene Consciousness needs. Its food stock was destroyed in the war, all its protein plants rotted, so Earth, dinner!The Ninth Doctor [src]

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