For UFOs to be extraterrestrial visitors, there must be habitable planets elsewhere in the universe for them to come from. Up until now there has been no evidence that such planets exist.
An article in the Australian Sky and Telescope magazine, volume 7 number 1, dated January 2011 (page 12) reports on the finding of the first "Potentially habitable" exoplanet.
Gliese 581 is a red-dwarf star in the constellation of Libra. It is 20 light years from Earth.
Exoplanet Gliese 581g is a world of 3-4 Earth masses, which orbits its sun at a distance which makes it have a temperature "for life as we know it." Its diameter is calculated to be 1.2 to 1.5 times Earth's and a surface gravity of 1.5 to 2 times Earth's.
""This is both an incremental and a monumental detection" said exoplanet specialist Sara Seager (MIT.)"
For more on this exoplanet click here.