We are likely to be listening for a long time, even if there are many worlds
Intelligent civilisations are out there and there could be thousands of them, according to an Edinburgh scientist.
The discovery of more than 330 planets outside our solar system in recent years has helped refine the number of life forms that are likely to exist.
The current research estimates that there are at least 361 intelligent civilisations in our Galaxy and possibly as many as 38,000.
The work is reported in the International Journal of Astrobiology.
Even with the higher of the two estimates, however, it is not very likely that contact could be established with alien worlds.
While researchers often come up with overall estimates of the likelihood of intelligent life in the universe, it is a process fraught with guesswork; recent guesses put the number anywhere between a million and less than one.
"It's a process of quantifying our ignorance," said Duncan Forgan, the University of Edinburgh researcher who carried out the work.
In his new approach, Mr Forgan simulated a galaxy much like our own, allowing it to develop solar systems based on what is now known from the existence of so-called exoplanets in our galactic neighbourhood.
These simulated alien worlds were then subjected to a number of different scenarios.
The first assumed that it is difficult for life to be formed but easy for it to evolve, and suggested there were 361 intelligent civilisations in the galaxy.
A second scenario assumed life was easily formed but struggled to develop intelligence. Under these conditions, 31,513 other forms of life were estimated to exist.
The final scenario examined the possibility that life could be passed from one planet to another during asteroid collisions - a popular theory for how life arose here on Earth.
That approach gave a result of some 37,964 intelligent civilisations in existence.
Form and function
While far-flung planets may reduce uncertainty in how many Earth-like planets there are, some variables in the estimate will remain guesses.
For example, the time from a planet's formation to the first sparks of life, or from there to the first intelligent civilisations, are large variables in the overall estimate.
For those, Mr Forgan says, we will have to continue to assume Earth is an average case.
"It is important to realise that the picture we've built up is still incomplete," said Mr Forgan.
"Even if alien life forms do exist, we may not necessarily be able to make contact with them, and we have no idea what form they would take.
"Life on other planets may be as varied as life on Earth and we cannot predict what intelligent life on other planets would look like or how they might behave."
Do you believe that there is intelligent life out there and what form do you think it may take? Do you think there has been or ever will be any form of contact between earth and extra-terrestrial life?
Here is a selection of just some of the comments you've made on this story.
Why would an intelligent (extra terrestrial) life form want to contact this war,conflict,disease and starvation ridden mess we call planet earth?
It's hit and miss. Like billiards. But one day they shall find us or we shall find them. Hope I live long enough.
John, Ukiah, CA. USA
No question of belief. It's a fact that aliens have been visiting Earth for last several thousand years. Many of our technologies are gifts from them. It is a different matter that our Governments and some our scientists are in constant denial. Maybe this is their policy or perhaps the aliens have asked them to keep their identity secret in lieu of technical inputs given.
Shailendra Singh, Mumbai
There will be most certainly be vast amounts of intelligent civilizations out there. The only problem is the vastness of space itself. Light itself takes hundreds of thousands of years to travel across our own galaxy and travel it doesn't get any faster than that, as Einstein taught us. The idea that extra terrestrial life would be anything like us, like 95% of the extraterrestrials in science fiction books and films are bipeds, is far flung. They might just be 'intelligent shades of the colour blue' for all we can tell at this time. Or around an inch tall. Or experiencing time 50 times as slow as us. Communication could be quite a challenge. If we recognize them for what they are in the first place.
Dirk Jan, Rotterdam, Netherlands
There is life out there, that's for sure. In the same space, if earth can have life why not in other planets in different solar systems? I don't think we'll ever be able to contact them.
The probability seems high to me given that conditions seem possible for intelligent life in a large number of planets in our own galaxy. Multiply this by the number of galaxies in the universe(s) and I guess we are talking about certainties, but communicating with them seems most unlikely given time differences and lack of technology. Fascinating!
David, Hong Kong
I love the rich speculation here. By the articles own admittance, there is no idea of how many Earth-like planets exist. And yet, scientists say there could be 361 intelligent civilisations in our Galaxy alone. Silly math. Brilliant! Upon further investigation out of this fairy tale you'll discover the only planets that have been verified are Jupiter-like planets. Huge gas planets. Want to live there, folks?
Sven, CA, USA
The vastness of space is in itself a question. Taking the Earth as a standard case in this expieriment is showing of how small our perception truely is. Intelligent civilization in the universe is a definite. Whether or not contact can and will be made is only matter of infinite variables. Perhaps the greatest extra-terrestrial contact will manifest itself in ruins.
Daniel Franco, United States
My friend spoke of a B47 pilot who in 1963 described a disc-shaped object that approached his aircraft, flew in formation, then accelerated at a speed he could not match. The pilot did not make an official report because he did not want people to think he was crazy and lose security clearance. The co-pilot independently verified the incident. Reports like that can be multiplied by the thousands. They fill the literature and those who follow them up often verify that witnesses are credible and intelligent. The characteristics of the vehicles have been documented; they are consistently described from one experience to the next. In short, yes, and yes there has been contact.
Richard Thieme, Milwaukee, WI, USA