I've had the idea of writing a short story about a highly developed race somewhere in the universe. They are on the brink of extinction. There is no way to save them, so a group of scientists want to put a very basic life form on a comet and manipulate that comet so it will eventually crash on a planet that can support life. The story will show that it's difficult to find a suitable planet however when they find one, it should be clear (based upon description) that the planet they found, is Earth. In order to make is somewhat realistic I need to find an event that would cause that race to be extinct. Considering they are an advanced race as they can manipulate the trajectory of comets, asteroids and put life forms on it so it will survive the trip. I came up with a few ideas, but I would like to hear your input on how realistic this would be and if it would fit the setting. 1) Because it's an advanced race, we can safely assume they have been living on the planet for a while. Their sun is starting to expand and will destroy the planet. Although they are advanced, they do not have sufficient technology to colonize other planets or travel vast distances into space. I'm not entirely sure if this is realistic... 2) They are at the losing at of a war with an even more advanced race. Most of them are already dead and the other race is about to make their final move to kill all remaining survivors. This would be nice for the plot, as the scientist would also need to find a way of executing their plan without being noticed. 3) Gamma Ray Burst. A planet is doomed if a GRB is on its way to it. There is not much you can do about it... I'm not 100% sure if this is accurate. If I am correct, the earth has already been hit by such a ray and the creatures living deep down in the ocean were saved. Other ideas are also more than welcome!