UFOs, alien abductios, lost time ... It might sound like the latest series of the X-Files, but many people believe that these things really exist!

We asked Jack Smith, a top UFO researcher, what makes him believe that we're not alone. 'Well,' he told us, 'just picture the scene ... I am riding my bike home one evening with my sister when suddenly a strange bright light appears in the sky in front of us. I think it's a star until it starts to change colour and move up and down and from side to side. I'm afraid, but then I feel strangely calm. After a few minutes, it disappears behind the hills and my sister and I ride home as fast as we can.'

Astronauts, pilots, scientists, even presidents have had similar experiences. Some witnesses have even found debris and burn marks on the ground from UFO crashes. One UFO society, however, collected a list of 70,000 sightings from all over the word and found explanations for 80% of them: comets, meteors, bright planets like Venus, aircraft lights or even birds - anything but UFOs! But what about the other 20%? 'Well, there are thousands of solar systems, or planets that go around a central star like our sun,' Jack told us. 'If the right conditions exist on just one of these planets, it is possible that life exists there. Personally, I'm convinced that aliens are here and that they walk among us!'

There is just one problem with this. So far no one has ever found any real aliens. So, is there a logical explanation for UFO and alien sightings, or is the truth still out there? You decide!