[roywiggins]

They're too busy trying to keep warm on Planet X, which has an extremely eccentric orbit that keeps them well out into the far reaches of the Solar System most of the time. They're kept warm by engineering a blanket of aeresolized gold particles in their atmosphere. They came to Earth the first time and enslaved us to mine for more gold. After the rebellion, they decided it was too hard to keep us under control and withdrew to their planet, which has since retreated into the cold depths. Soon, it will return and they will need more gold...

[This is all based off the nonsense Planet X theories that were going around. There was a good serialized YA novel that mined the ideas for plot, along with other Ancient Aliens nonsense, to quite good effect)