Published: 7:30AM BST 29 Sep 2009
SETI was founded in response to a September 1959 Nature journal article, “Searching for Interstellar Communication”, which suggested that a systematic search for alien life was worthwhile.
Since then, it has spent 50 years listening to the stars with radio telescopes, and at times trying to send messages of its own to other planets.
Here are 10 of the most significant events in mankind’s search for other life.
Little Green Men: Pulsars
In 1967, astronomers in the UK spotted a radio signal from another star. Nothing too remarkable in that, as all stars emit radio waves.
What was unexpected, though, was that this one turned itself on and off with perfect regularity: one 0.04 second pulse every 1.3373 seconds.
The metronomic beat sparked speculation that it might be a signal from an alien life form, and the signal was given the name LGM-1, standing for Little Green Men.
In fact, the astronomers had discovered a new type of star – a pulsar. A form of neutron star (an incredibly dense dead star: one just 12 miles across would weigh more than our Sun), they rotate at a huge velocity, giving off powerful beams of radiation as they spin.
Martian bacteria?
In 1984, a rock was found in the Allan Hills, Antarctica. It is believed to have been formed on Mars around 4.5 billion years ago, before being blasted into space by meteorite impacts around 15 million years ago.
It then wandered through space for millions of years, before crashing into our planet about 11,000 years before the birth of Christ.
An interesting enough history, but the meteorite – known as ALH84001 – really made the headlines in 1996. Structures that resembled tiny fossilised bacteria were found, as were organic molecules, sparking theories of ancient life on Mars. The excitement grew so great that US President Bill Clinton made a televised announcement about the find.
Arguments over whether these really are evidence of Martian life, or whether the sample has become contaminated during its time on Earth, are still raging today.
Earth-like planets
‘Terrestrial’, or rocky, planets, are considered the most likely place for life to be found. There are four in our own solar system – Mercury, Venus, Earth and Mars.
But in the last few years, the first possible examples of Earth-like planets outside our system have been found. In 2007 European scientists said they had found two – the third and fourth planets around the red dwarf Gliese 581 – which might be habitable.
Water on Mars
This month the news broke that the Mars Reconnaissance Orbiter has spotted evidence of ice in five locations on Mars. It is the latest in a series of discoveries of water on the planet, indicating that the planet may have had a more humid climate at some stage in the last few thousand years.
Water is required for all known forms of life, so the existence of water on other planets increases the likelihood that life exists elsewhere.
Drake equation
The first serious attempt to quantify the likelihood of humanity coming into contact with alien life, the Drake equation reads: N=fp ne fl fi fc L.
It states that if we know the rate of star formation, the percentage of stars that have habitable planets, and the percentage of those planets that are likely at any given time to support life that signals its existence to the universe, we could estimate how many civilisations in our galaxy we might communicate with.
Unfortunately, none of those factors are known, so we are still guessing.
The Aricebo Message
Not part of the search for life per se, but rather helping anyone else who may be looking for us. In November 1974, a message was beamed from the Aricebo radio telescope towards the M13 star cluster, about 25,000 light years away. The message, created by Drake equation author Frank Drake and physicist Carl Sagan, included an image of a human and information about the makeup of DNA.
Sadly, M13 will have moved by the time it arrives.
A later broadcast - beaming the music of the Beatles at the Pole Star, 431 light years away - was, in all apparent seriousness, condemned as inviting an interstellar attack from warlike aliens.
Voyager
Three years later, in 1977, the two Voyager space probes were launched. As well as their scientific equipment, they contain gold discs containing information about Earth, including Mozart’s music and greetings in 54 human languages. It is currently around 10 billion miles from Earth; its radio signals take 15 hours to reach us, even at the speed of light.
In May 2005, NASA scientists said that Voyager 1 had reached the heliosheath, the boundary that marks the edge of the solar system. It is expected to pass through into interstellar space in 2015, becoming the first man-made object ever to leave the Sun’s orbit.
Kepler telescope
In March, the Kepler Mission was launched with the express goal of finding Earth-like planets around other stars.
It is designed to find small planets by watching stars closely to see if they suddenly get dimmer, indicating that something has passed in front of them. For an Earth-sized planet, that would mean the star becoming dimmer by just 0.01 per cent.
The Wow! signal and Radio source SHGb02+14a
In 1977, an astronomer with SETI spotted an unusual radio signal while working on a radio telescope at Ohio State University. He was so excited by it that he wrote “Wow!” in the margin of the printout.
However, it was never spotted again. After he had calmed down, the discoverer Dr Jerry R Ehman said he believed it was “an Earth-sourced signal that simply got reflected off a piece of space debris.”
Another candidate for evidence of alien life was SHGb02+14a, a signal spotted three times in March 2003. It was at a frequency expected to be used by extraterrestrials – the frequency at which hydrogen absorbs and emits photons.
However, again there were reasons to be sceptical, not least that it came from a direction in which there are no stars for 1000 light years. While impossible to rule out extraterrestrial origin, it may have been a distorted signal from a pulsar, or just random noise.
SETI@home
In the 21st century the search for extraterrestrial life continues in hundreds of thousands of bedrooms and offices around the world.
Launched in 1999, SETI@home is the name given to a virtual supercomputer made of huge numbers of internet-linked home computers. Anybody can download the software and allow SETI to use their spare processing power to scour data from radio telescopes for evidence of alien intelligence.
In terms of power it is the fourth most powerful computer on the planet. It was this network of home computers that spotted SHGb02+14a, and one NASA scientist has predicted that it will find an alien signal by 2025.
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