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It's one of the great unanswered questions......are we alone or is there life elsewhere in the universe?
Well it's been a big day today for the many people who think aliens DO exist - as the Government released its files on reports of UFOs for the final time.
The files contain a letter from a child in Altrincham asking the Prime Minister if he believes in life beyond earth.
Our correspondent Andy Bonner's been looking at the files and talking to an expert.
Newly released files on reports of UFO sightings include a plea from a schoolchild from Greater Manchester for answers on whether "living things" exist outside Earth.
The letter, sent to the Ministry of Defence in January 2009, said the child and his or her father had seen "little air crafts" in the sky.
The MoD, which closed down its UFO desk later that year, replied to the writer, as well as sending "RAF goodies".
The letter reads: "To Mr/Mrs. Please tell me if living things live out side our planet.
"I would like to now because me and my father have seen little air crafted in the sky. And my father have seen 2 little light dancing around each other.
"Pleas send me a letter Telling me the answer."
The letter, received from a child in Altrincham, Greater Manchester, includes a picture of a flying saucer with an alien waving.
MoD staff replied in February that year, saying: "Thank you for your letter asking if there is life outside our planet.
"It's an interesting question and we remain totally open-minded about it, but we don't know of any evidence to prove life exists in outer space.
"We do look at reports of 'unidentified flying objects' but only to see if the country's airspace might have been affected but we haven't had any evidence of this so far.''
The number of UFO sightings reported to the Ministry of Defence (MoD) trebled in the year its hotline was closed, newly released files show.
The MoD closed its desk in 2009, saying it served "no defence purpose" and was taking staff away from more valuable "defence-related" work.
Its decision came in a year when officials had received a surge in the number of reported sightings.
According to a briefing in the files, during the years 2000-07 the MoD received an average of 150 reports per year.
But by November 2009, it had already received 520 reports that year, as well as 97 freedom of information requests on UFOs.
Possible reasons for the increase included the craze for releasing Chinese lanterns at weddings and public holidays, appearing as floating lights in the sky.
One example included a sighting by soldiers in Shropshire which hit the headlines in June 2008.
The Ministry of Defence declined an offer of a weapon that could "shoot down UFOs", newly released files reveal.
One suggestion was sent to the MoD by a "UFO researcher" who wrote:
In a response, an MoD official politely declined the offer, writing:
Reported UFO sightings to the Ministry of Defence's UFO desk and hotline trebled in 2009, the year the department was closed.
Daybreak's Tiffany Royce reports:
UFOs were spotted near several major UK landmarks, including the Houses of Parliament, Stonehenge and Blackpool Pier, according to files released today.
The files, released by the National Archives, include sightings reported to the MoD's UFO Desk between 2007 and 2009, when it was closed.
The sightings, which saw a surge in 2009, came from across the British Isles, and even further afield, including the Canary Islands.
One person reported seeing a UFO over the Houses of Parliament in London, leaving a message on the UFO hotline's answerphone.
The report, from February 12, 2008, described "green, red and white lights", static in the sky and quite far in the distance, seen for about an hour and a half.
The report said the person "said there were many aircraft passing the UFO, but it stayed static in the air. He said that he had seen it on Sunday, February 10, 2008 too".
Another sighting described "discoid" shapes noticed in photographs of Stonehenge.