Unknown. There are millions of sightings of UFO each year. There could even be trillions of sightings. The ones on earth are at Area 51. Since we don't have access to Area 51, it is still unknown.
Only if there is someone or something to observe them. UFO stand for Unidentified Flying Object. That means no one knows what it is. It does not mean it is a space craft of some kind.
There are 8 planets within our solar system. They are: Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Outside of our Solar System, there are possibly billions, although as of Aug 2010, only...
We have no way to know the answer to that question. No human has ever stepped onto another planet, and no space probe has sent back evidence that suggests that life exists on another planet. That's...