The Search for Extra-Terrestrial Intelligence (SETI) is potentially one of the most important scientific endeavours ever undertaken. As the late Arthur C Clarke put it "Two possibilities exist: Either we are alone in the Universe or we are not. Both are equally terrifying."
I don't suppose 24 hours goes by without someone, somewhere claiming to have had a close encounter with ET, and no year goes by without a film about aliens. However claims by scientists to have found intelligent alien life are much, much rarer.
Here is a run down of the top five occasions scientists have claimed they might have found ET.
5. "I had been the first to hear the greeting of one planet to another"
lation. The astronomer Percival Lowell thought he could see canals on Mars, but although this caught the public's imagination he was always a bit of a lone voice. Other astronomers looked and saw nothing.
A rather more serious claim to have heard ET calling though was made by the scientist Nikola Tesla. Tesla, who was played by David Bowie in the film The Prestige, is one of the most important characters in the story of electricity. However he also had enough wacky ideas, and made enough grandiose statements, that he has a place at the heart of many Conspiracy Theories. His claim in 1899 to have picked up signals from space, which he attributed to intelligent life on Mars, generally feature in quite a few.
Tesla was certainly ahead of his time in realising that radio could be used to send signals between planets. What he picked up though is somewhat more uncertain. Possibly it was nothing, just interference caused by his own equipment. Possibly it was the radio signals given off by astronomical objects themselves, something that wouldn't be seriously studied until after the Second World War.
Or maybe ET really did drop by to give the great man a personal message.
4. "A civilization in possession of energy on the scale of its own galaxy"
In 1963 though astronomers at the California Institute of Technology discovered something that took their breath away. CTA-102 was both more distant and more active than anything ever found before.
At about this time the Soviet Union was just starting its own SETI program, and the Russian astrophysicist Nikolai Semenovich Kardashev became particularly interested in CTA-102. He postulated that this was an extremely advanced alien civilisation. Kardeshev even came up with a handy scale to rate how advanced alien civilisations were. We are Level I. These signals, he thought, came from a Level II or III.
But it wasn't so. Quasars turned out to be formed by violent events at the heart of ancient galaxies. They were monsters from the dawn of the known universe, but they were natural.
With more exciting quasars being discovered all the time, CTA-102 soon became yesterday's quasar and old news. However is not entirely forgotten, as The Byrds paid a folk-rock tribute to it with a song on their 1967 album Younger Than Yesterday, complete with unintelligible alien dialogue.
3."Little green men"
Jocelyn Bell was working at the Cavendish laboratory on a project to map Quasars. However Bell soon noticed what she called "scruff" on the radio telescope's recordings.
Eventually she worked out where exactly this "scruff" was coming from and when she was able to record it properly, the "scruff" turned out to be discrete pulses of signal exactly 1.3 second apart and as regular as a metronome.
The duration of the pulse was so short that it must have come from something small, no bigger than a planet. The team nicknamed the new phenomena LGM - for 'little green men'. Bell wasn't impressed:
"Here was I trying to get a Ph.D. out of a new technique, and some silly lot of little green men had to choose my aerial and my frequency to communicate with us"Bell was wondering how to break this news to the astronomical community without being laughed at when she found another, similar radio source in the sky. Then another. Then another. It now seemed clear these pulsars were natural, and shortly afterwards a convincing explanation was found. They were neutron stars, the super-dense collapsed cores of old stars that had died in massive explosions called supernova.
The mystery had been explained and Bell was the most famous female astronomer in the world. She had designed and built the telescope, analysed the data and discovered something completely new.
However the Nobel Prize committee decided to give its award to her, initially sceptical, supervisor and not to her. It was an injustice, and a poor reward for what has been called "the greatest astronomical discovery of the twentieth century".
At 3:30PM on 16 August 1977 Elvis Presley was pronounced dead at the Baptist Memorial Hospital in Memphis. A few days later volunteer astronomer Jerry Ehman was checking the logs of the Big Ear when he noticed something unusual recorded at 22:16 EST on the night before Elvis passed on.
The signal was narrow and focussed and in the 1420MHz bracket which SETI researchers had identified as being the most likely frequency to be used for interstellar communication, as it has the least amount of cosmic background noice. It is also not used by terrestrial radios for this reason. The signal had lasted for 72 seconds. An excited Ehman scribbled "Wow!" on the printout.
Big Ear could not be pointed like a normal telescope and just used the rotation of the earth to track across the sky. A single point in space is audible to its detectors for 72 seconds. At the time of the Wow! Signal it was pointing to the constellation of Sagittarius.
The signal contained no data and nobody has ever heard anything else from Sagittarius again. It has been analysed and analysed, and no explanations, either terrestrial or extra-terrestrial have been forthcoming.
It remains a tantalising mystery.
One of the most amazing advances in astronomy since I graduated with my rather pathetic B.Sc. (no hons) in 1991, is the ability to discover planets outside of our solar system.
This is done either by watching for very tiny wobbles in a star's orbit, or by spotting the tiny dip in a star's brightness as a planet passes in front.
The Kepler space telescope has been using the second technique since 2009 and has discovered dozens of exo-planets. Then, on 14 September this year, the team announced they had found something extremely odd.
When a planet passes between it's sun and us the light of the star it orbits drops by less than 1%. When Kepler looked into the constellation of Cygnus, at star KIC 8462852, it found something was passing between us and it every 750 days that blotted out up to 22% of the starlight and took up to 80 days to go past.
Whatever the thing was, it was huge. If this was a newly formed star the explanation would be a cloud of dust, but KIC 8462852 - now known as 'Tabby's Star' after astronomer Tabetha S Boyajian - was a venerable old star. There are possible natural explanations; a massive cosmic pileup of asteroids or comets or something, but no natural structure this size could survive for long in star's gravity field. Either we are lucky enough to see this event just after it has happened or ... the structure is not natural.
Dr Boyajain's paper, entitled Where's The Flux? (abbreviated to WTF) makes no such claim.
The most likely explanation, she says, is a very large clump of comets, although she admitted to journalists she was also exploring "other scenarios".
What that might mean was spelt out by Jason Wright, a SETI searcher from Penn State University who told The Atlantic magazine that the finding were consistent with "a swarm of mega-structures" built by an advanced civilisation. The sort of thing he had in mind was a Dyson sphere like the one on Star Trek, or possibly something like the titular object in Larry Niven's Ringworld.
It is far too early to tell if this will be another false alarm like pulsars and quasars. However unlike Tesla's messages or the Wow! Signal we do at least know where to look for more evidence, and no doubt soon other telescopes will be being pointed at Tabby's Star to find out more. This mystery will almost certainly have a resolution.
Either it's alien or it isn't and, as Arthur C Clarke might have said, either answer will be interesting.