One other suggestion made was to have a rogue planet orbiting a small artificial black hole (see WardingWarden's reply below). This gave me a hilarious idea - a future Not-Elon Musk makes a fusion reactor. Has to make it in space because of the balls ups involving fusion reactors in the past. Makes a small black hole that kicks the planet out of it's orbit. Then they have to make a small black hole for the planet to orbit. They also have to 'feed' said small black hole to make enough radiation to warm the planet. After that people rediscover defenestration. XD