In an early episode of the TV show The X Files, a government informer known as “Deep Throat” poses a question to FBI agent Fox Mulder:

“Mr. Mulder, why are those like yourself, who believe in the existence of extraterrestrial life on this Earth, not dissuaded by all the evidence to the contrary?”

Mulder answers: “Because, all the evidence to the contrary is not entirely dissuasive.”

The exchange neatly sums up the problem with conspiracy theories, notably that it is far harder to prove something is definitely not true than to show that it may be true. The door of possibility for the incredible, always remains slightly ajar.

This week Barack Obama poured more fuel on the alien conspiracy fire, admitting that extraterrestrials are “real”, although insisting none are housed at Area 51 – a top secret air force base in the Nevada desert where ufologists believe alien technology from the 1947 “Roswell Incident” is stored.

The Roswell Incident involved a supposed extraterrestrial spacecraft crashing into the New Mexico desert, with the debris – and possibly alien bodies – recovered by the US government.

Mr Obama is arguably the most sober and well-informed public figure to entertain the existence of alien life in recent years, although he is not the first president to acknowledge that aliens could be real.

Jimmy Carter reported seeing a UFO when he was governor of Georgia in 1969, while Ronald Reagan had a similar experience while flying his Cessna in California in 1974. He later asked Mikhail Gorbachev whether Russia would help, should the US be attacked by “someone from outer space”.

Most scientists believe that alien life is possible – even if it is just microbial – but many also think that intelligent life is likely. After all, the universe is huge and has been around for a very, very long time.

There are billions of stars similar to the Sun in our galaxy – the Milky Way – with a high probability that some have planets like Earth, orbiting in the habitable zone, where life could have evolved.

If Earth-like planets are typical then some have had the time to develop intelligent life, and interstellar travel.

After all, it took 67 years from the first flight of the Wright brothers for humans to reach the Moon, and Nasa’s Voyager probes have now left our Solar System.

Read More: Are aliens real?