"Is it not just a trifle arrogant to believe that our planet is the only one with any form of life on it?   I think so..."

Very true, but this observation has nothing to do with UFO's or visitations by aliens. Anyway, as I've said on other threads they're already here (a boon for the seeding theorists); have a look for these papers online:

THE RED RAIN PHENOMENON OF KERALA AND ITS POSSIBLE
EXTRATERRESTRIAL ORIGIN
Godfrey Louis and A. Santhosh Kumar

Growth and replication of red rain cells at 121oC and their red fluorescence
Rajkumar Gangappa1,2, Chandra Wickramasinghe2* , Milton Wainwright3 , A. Santhosh Kumar4 and Godfrey Louis4

I've posted these as clickys on a thread on ETL before but seeing as they were ignored then (obviously got in the way of a good debate) I can't be arsed to root the links out again. Here's my prediction: One day terrestrial ecosystems will be seen as subsets of much vaster solar-system or galaxy-wide ecosystems.

As for life . . . there's a distinct possibility we don't recognise all forms of life on this planet; there might be lifeforms in our back gardens we don't recognise as being 'alive' in the familiar sense, but might be sentient and ancient creatures.

How utterly disappointing if the aliens look anything like mankind has the wit to dream up. The reality will be far more wonderful and inspiring.