i think that scientists believe the planet with the best bet for life is Gliese 581 c. its about 20 light years away. in fact we sent a signal towards that planet in 2008
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There are many planetary systems in the universe, but only 3% of the stars in the Milky Way have planets orbiting around them. Only a couple hundred exoplanets have been discovered, so we would have know idea where they would come from if they visited Earth or something...
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If you mean this UFO is of extraterrestrial origin we probably wouldn't even know the star system they came from. Our knowledge of the galaxy is extremely limited.
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Our own. The planet Venus accounts for the lion's share of UFO sightings. Human-made aircraft/balloons/satellites/etc would account for the next largest segment.
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Ours considering that they are from Earth and are mostly military experiments.
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Ours, duh!