[geoschmo]

Originally posted by Baron Munchausen:
You can presume that these 'nice' aliens would have found a world fairly close to earth in major characteristics but they cannot have had time to think of all of the details like light wavelengths.

True, for this to work I think we'd have to make the basic assumption that earth type worlds are fairly common. Plentiful enough that the aliens could find one suitable for us in their planetary database that is far enough off the beaten track that the refugees weren't likely to be discovered in a short time.

Of course, since this is a story our assumption doesn't have to be likely for it to be realistic. It only has to be plausible.