In response to a recent Dr. Who episode: I supose ET could build a machine that froze a planets atmosphere into large spheres of frozen gas, and launched the blocks/spheres into space at high speed, even 0.9999 c. It would be an effective weapon with a range of perhaps one light year. A year or two after launch the blocks would have evaporated and the gas would scatter in another year or so making the widely scattered gas little hazard to friendly space craft and planets, as compared to launched boulders, which would continue to be a hazard for trillions of years. I can't imagine a natural process that can do this, so I have to vote no cold emitting stars. Neil