I'd like to make a post as a Flat Earther who does believe in life on other planets.
My own personal conception of the universe is not geocentric per-se, it is my belief that the other planets visible to us (and the many that are not) exist similarly to the Earth. They are flatish circles of matter "floating" on the universally accelerating plane of dark matter, and I believe that certain of them are likely to have their own rotational (or stationary) satellites, depending on the specifics of their magnetic fields (the Earth has two natural satellites - the Sun and the Moon - which rotate in accordance with Earth's spinning magnetic field).
Assuming that another planet had a heat-producing satellite like our Sun and a sufficient portion of water or other substances with similar points of melting and freezing, it is likely that a similar Ice-wall situation could develop, providing a natural trap for gases and creating an atmosphere. In this instance, I see no reason why plant and eventually animal life could not be a possibility.
I do not believe that UFOs are visitors from other planets. While interplanetary travel is actually possible (though has never been achieved by mankind), it would involve "riding" the universal accelerator rather than flying freely through space (which is simply impossible unless one can accelerate more than the UA itself). Any aliens visiting this planet would not appear in the sky, they would climb over the Antarctic wall.
UFOs, in my estimation, are by and large just conspiracy surveillance ships - nothing more than souped up aeroplanes. I believe that past conspiracies (the Roman religious conspiracy is a prime example) may have lauched such craft as well, explaining UFO sightings (as well as many "comet" and "meteor" occurances) in historical literature.
In semi-concurence with Dionysios's demon hypothesis, I think it's possible that our conspiracy, or past conspiracies, have unleashed their foul genetically-modified spawn into the skies. What's to stop the conspiracy growing demon-like animals in labs and releasing them into the air, to cause possible havoc, confusion and/or to help the conspiracy dominate the air?
Anyway, if actual aliens do choose to visit Earth, I imagine they're in for a nasty surprise. The conspiracy is between them and the skies of the Earth, on the ice wall, and I doubt those money-hungry Earth-destroyers will be too pleased to see them.