A former Lib Dem MP, and now a lecturer for the Association for the Scientific Study of Anomalous Phenomena, said it "beggars belief" there was no life on other planets.
Lembit Opik said it was a "crying shame" the government closed the Ministry of Defence's UFO desk and telephone hotline, as he claimed a small proportion of sightings could never be explained.
He spoke to Andrew Neil and Dr David Clarke, an expert on UFO history at the National Archives, on the day files were released files relating to reports of aliens and lights in the sky, made between 2007 and 2009.