Artist's impression of Kepler's 3000 light-year target area: Artwork: Jon Lomberg
What is the chance that alien life exists? Nasa's latest mission - the Kepler Space Telescope due to launch on Friday night to survey the heavens for Earth-like planets - could take us a step closer to an answer. Kathryn Westcott asks four experts whether mankind prefers the idea of being alone and unique or whether we long for cosmic cousins.
Robert J Sawyer
"To learn that an extraterrestrial civilisation has survived its technological adolescence would be inspiring"
Science fiction writer
Dr Michael Perryman
"As for intelligent life I'm putting my money on the fact that in
the whole universe, we are pretty much unique."
European Space Agency
Dr Steven J Dick
"Because of popular culture, such as Star Trek, people are already prepared for intelligent life to exist"
Brother Guy Consolmagno
"The idea that there might not only be us, is a wonderful one. It does not question our uniqueness"
Robert J Sawyer is a Hugo Award-winning science fiction writer
Throughout the history of science, there have been a series of developments from Copernicus (who displaced the Earth from the centre of the Universe) to Darwin (who showed we weren't created full-blown by the hand of God) that knocked out our status of being special. The one claim to being special that we have been able to hold on to is the belief that Earth is the only place in the Universe where intelligent life exists.
For many people, there is a psychological need to be special, and so Kepler - which I bet will succeed in its quest - will take away from that. But for those of us who believe that Earth is merely a typical example and that life-bearing planets are common, Kepler's success will be a wonderful thing.
We already know that our galaxy is teeming with planets - that was the first step in dethroning us from being the only abode of intelligent life. Kepler takes us on the next step: determining if many of those planets are Earth-like. After that, we need to determine if such planets have life, and then if that life is intelligent. Still there are only two possible answers to the question of whether other Earth-like worlds exist - and whichever answer we get will be astonishing.
If we were to find intelligent life on an Earth-like planet, that civilisation would almost certainly be more advanced than ours, given that our Universe is 11 billion years old; we are absolute newborns on the cosmic state.
And, because we are so young, we are facing a huge crisis: our civilisation is on the brink of disaster because of our immature use of technology, both through climate change and through weapons of mass destruction. Many people think we won't survive; there's been a resurgence in the belief in Armageddon. To learn that an extraterrestrial civilisation has survived its technological adolescence would be an inspiring object lesson for us, and would help put an end to all the nay-saying and doom-mongering.
At the moment, people's focus is incredibly narrow - there is a lot of navel-gazing. We are not thinking about the big questions. If the Kepler mission is successful, our focus will widen - and that's all to the good.
Robert J Sawyer is the author of Hominids, in which Neanderthals have developed a radically different civilization on a parallel Earth
Dr Michael Perryman is a senior adviser at the European Space Agency
In the past 15 years, the area of exo-planet research has been one of massive progress. Since 1995, more than 300 planets have been discovered orbiting other stars relatively near to us in space.
Earth's circumstances are really far too special to be easily replicated
But when it comes to whether Earth-like systems are common or not, we really are into the realms of pure speculation.
If Kepler finds Earth-like systems, the next question would be whether this is the kind of environment in which one might start looking for life. That next level of detail requires a few steps in inference.
The conditions must be right for life to evolve. These planets would have to be the right distance from their star to have liquid water, and would have to have a similar temperature to Earth. And, in terms of the host star itself, you need very special conditions: it would need to be the right age, mass and luminosity for life as it we know it to develop.
And if the planet is much lighter than the Earth, or much heavier, then the conditions would not be right either.
In terms of what Kepler might find, the best knowledge at the moment is that it might discover some 50-100 Earth-like planets, but we simply don't know.
Astronomers would not be surprised if that many were found. It's an exciting experiment, because it might find many more - or perhaps many less! Whatever it finds, it's going to advance our knowledge.
But is life as we know it common or unique? Earth's circumstances are really far too special to be easily replicated - there are so many coincidences, chances and conspiracies that seem to be needed for life to take hold and thrive.
Perhaps primitive life forms could exist out there amongst the almost infinity of worlds that probably exist, but as for intelligent life I'm putting my money on the fact that in the whole Universe, we are pretty much unique.
Brother Guy Consolmagno studies the nature and evolution of small bodies in the solar system. He is curator of the Vatican meteorite collection - one of the largest in the world - at the Vatican Observatory
We Jesuits are actively involved in the search for Earth-like planets.
The idea that there could be other intelligent creatures made by God in a relationship with God is not contrary to traditional Judeo-Christian thought.
The Bible has many references to, or descriptions of, non-human intelligent beings; after all, that's what angels are.
Our cousins on other planets may even have their own salvation story – including other examples of the incarnation of the second person of the Trinity. We are open to whatever the Universe has for us.
I am, however, sceptical that we will be able to have these conversations with any life form that is discovered... certainly, not in my lifetime!
The idea that there might not only be us is a wonderful one. It does not question our uniqueness or contradict our belief in God. For most people, if new forms of life were to be discovered, it would not mean everything they believed was wrong, it would only reinforce what they believed all along.
John Herschel, son of the discoverer of Uranus and a founder of the Royal Astronomical Society, argued that it would have been a waste of a Universe if God had only created one place where there were people He loved. This is not an argument based on logic, science or philosophy, but an aesthetic one.
The important thing is to keep in mind that the Universe is the deliberate creation of a loving God. Catholics should not be afraid to embrace such speculations, but we should always remember that they are just speculations. We don't know. But reflecting on these possibilities lets us appreciate in a deeper way what God's redemption actually does mean for us.
My science tells me how God created the Universe and that he loves that Universe.
We shouldn't be afraid of the truth.
I would be delighted if other Earths harbouring intelligent life were discovered. For most people, however, it would be nothing more than a nine-day wonder. I think that we've lived with the idea so much, from speculations by scientists to creatures in science fiction movies, that the human race is already well used to the idea that we are not alone.
We need to look beyond ourselves – that's what religion does when it's done right and what astronomy does when it's done right.
Dr Steven J Dick is an astronomer and chief historian at Nasa
The Kepler mission is definitely a landmark one, and finding an Earth-sized planet will raise the debate about whether we are alone or not.
Because of popular culture, such as Star Trek, a lot of people would be expecting intelligent life to exist
We have known for a long time that we are not the centre of the Universe, the question now is whether biologically we are central. It's all we have left.
Even if intelligent life were discovered, we would remain unique in terms of morphology and form.
The chances are another civilisation would be more advanced than us because of the age of the Universe and the fact that our species is comparatively young.
For the human mind, this is a natural question going back to ancient Greeks. Because of popular culture, such as Star Trek, a lot of people would be expecting intelligent life to exist, and are already prepared.
The whole idea of life on another world would certainly raise a lot of debate, particularly in terms of how unique our religion and our philosophy is. This would be good, we have got into a rut of looking at everything from a terrestrial point of view.
Is it scarier to learn that we are alone or not? Well, there are those who would warn us to be careful but most people would be open-minded.
This telescope could have quite a profound impact on the human race. Hubble opened our eyes and imagination to the wonders of the galaxy & universe so will Kepler spark the imagination of what could inhabit an Earth like planet within the 3000 l/y region.
Taking in images of continents and land masses will truly inspire generations of thinkers and leaders.
Perhaps it will give mankind the boost of inspired curiosity it needs to reach for the stars once again.
We can only speculate for the moment what pictures of Earth did to those civilizations in our stellar neighbourhood.
Allen Array/SETI, get ready for some hot leads!
I am sure that there is or has been life in the universe, but nobody appears to have mentioned that because of the vast distances measured in Light Years, between other Star systems and our own, any radio or other form of communication that we might receive would have had to have been sent hundreds or thousands of years ago.
Therefore these civilisations may have become extinct themselves by the time their signals reach us. Likewise any signals that we send today will not be received by other civilisations until thousands of years into the future by which time we will no doubt be extinct as a species ourselves.
John Grinham, Weston super Mare, UK
It may well be that we are currently the only sentient species in the universe, but there's no way of knowing that of course.
There is no doubt in my mind however that sentient species arise, exist for a time, then die out as suns expire. Chances of two such species being around at the same time and close enough to communicate ? Virtually nil - but that doesn't mean it's impossible, merely improbable.
The only way for any life to escape destruction when their parent star dies is to become space-faring and find a habitable planet to colonise - again a highly improbable scenario to my mind.
Barry, Burton-in-Kendal, UK
It's amazing that the discussion seems to be centred around the search for 'intelligent' life. Surely even the discovery of alien bacteria would be astonishing. Given the tens of thousands (even millions) of variations in ‘life’ on earth - from microbes to plant to animal - logic would say that if life is found anywhere other than Earth, intelligent life MUST also exist elsewhere. If the universe is infinite (if it isn't then what's on the outside?) - and we know that life evolves into many forms to cope with even the most extreme conditions, then even bacterial evidence would be enough to know that somewhere out there must be every form of life we can possibly imagine? A one in infinity chance is still infinite.
Pete Vox, London
While your writers suggest that alien life may be more advanced than ours, there's no guarantee that such life would have advanced morality, at least not from our point of view. In order to survive for so long, alien life may have habitually removed any competition, and when we signal our presence perhaps that's how we will be regarded. There could be a short interlude before, in line with their own impeccable morals, we are simply squashed like bugs.
Stormerne Hunt, Glasgow, UK
It's sheer arrogance to believe our life form is special - or correct. There are assuredly other life forms well-suited to their environment that exist in the wide universe that may be more capable of survival than we fragile humans. The apparently hard-wired tendency for humans to invade new territory and destroy existing cultures and beings (European invasions of the Americas, for example) may be our downfall when we go to space.
Dorey, Washington DC
Let's hope we don't find an intelligent species out there.Our history on Earth shows that we would try and dominate and exploit them.And if they are more advanced than us...oh dear.
paul taylor, monte carlo,monaco
Brother Guy Consolmagno, why would you say that mentioning a god makes any sense in this discussion? Just a question, why do you question and search for known planets while you blindly believe in a deity for which there is no evidence?
Tim B., Sasebo, Japan
If you feel you need an explanation for why the human race exists then this search provides as good an answer as any. We are here to understand the Universe. This works just as well for people of no religion as it does for people of any religion. On this basis, this is one of the most wonderful efforts of human civilization and we are fortunate to be around as it happens.
jeremy slawson, Plymouth UK
Humans finding a similar planet: ..........you are joking? Do you really need to ask what we would do? With the track record our species has. This has to be the most facile comment to date. THE VERY SAME WE ARE DOING TO THIS ONE! We can only hope that any species we may find will be less developed than our own. l could be terminal for our very existence to come across a truly advanced, objective species.
Mark Webb, B C Canada
The sheer size of the universe indicates that even the most improbable event - Intelligent life - is likely to occur many many times over. However the age and size of the Universe suggests that for two intelligent lifeforms to exist simultaneously and close enough to see/hear each other the chances are tiny.
'Humans' have been around for a few 10,000s of years yet only in the last couple of hundred have we been at an advanced stage to be seen or heard by anyone so for us to be found there would need to be an intelligent lifeform present in our backyard.
The question is similar to putting 5 people randomly across the Saraha over a period of a year each with only 2 days food and water and working out how likely it is any of them would meet another living person.
Karl, Cambridge UK
I love the idea of there being other planets with life, but I think we need to focus on sorting ours out first. Its the equivalent of having guests round to your house without tidying it up first... imagine they came to visit... what they will think when they see how untidy we are...
John Wroath, London
Here we go again ! "to Darwin (who showed we weren't created full-blown by the hand of God) that knocked out our status of being special."
First of all.. Darwin NEVER proved or showed that we were not created "full-blown" by the hand of God. It was and continues to be a theory. A "Theory people... a theory !!!! Not proof, not truth, but simply a theory. One that has never been proven, but many times over has been shown to be majorly flawed even in its very basic concept.
As for there being life outside of the Earth.. I have no problem with that idea or concept. As a creationist, I am not appalled by the idea that if God created us, and this earth and system.. He could have very well of created another. Does that make me any less special? No it doesn't. Does it make me nervous .. not at all.
Dr. Phillip D. Murray, Martinsburg, WV
I agree with Dorey, the arrogance of Dr Perryman to assume that intelligent life could only evolve under the exact circumstances seen on Earth is unbelievable. On Earth there are examples of life in what we as humans would consider extreme environments such as undersea thermal vents at huge temperatures and pressures. Who is to say that given another 3 billion years, these would not evolve into a completely different (but still intelligent) form of life.
Jamie, Leeds, UK
We are probably unique. Our sun is not 'average' as many like to think. It is not large, but is quite dense and white - not yellow. Our Earth is in the 'life' zone and shielded magnetically from much of the radiation. It has a high water content. In a statistical sense, we are 'off the curve'. Mathematicians use a set of averages to try to prove we are one of many, but their arguments are flawed. (If Sapiens existed out there, we would see blue glowing areas within the galaxy showing us where 'Star Wars' were happening.)
Wungulai, Toowoomba Australia
I hope there are other intelligent life forms out there, it would be fantastic to find some! Lets hope the powers that be don't try and destroy them like we seem to do so well with things on our own planet!
Julian Wilson, Godalming
if there was an intelligence species, we would have found one by now.
victor, dubai, UAE
It seems likely that we will come across other intelligent life. And, I believe that by the time we do and have the ability to communicate with them us humans will be evolved enough to respect and understand our own and others cultures.
Simon, Yambol, Bulgaria
I believe that, chances are, there is intelligent alien life out there. Considering the philosophical and scientific advances mankind has made over the past (relatively short) few thousand years, it seems to me that if there is intelligent life, it could easily be a few thousand years more advanced than us.
I just hope they aren't hungry, and consider us a delicacy. Nor that they ruined their planet's environment and decide that our planet would suit them very nicely thank you very much, and consider us to be nothing more than cockroaches to be exterminated from their new home.
Scott, Amsterdam, Netherlands
It would be good for all of us ,as a race as we would be forced to think of ourselves as humans not black, white, Muslim, Christian etc etc.
Nigel Bratt, Stafford,England
I'm still looking for intelligent life on Earth.
Kathleen, New Orleans, LA
If the universe is 12 billions years old and our earth only 4.5, this means that during 7.5 billions years no intelligent life developed elsewhere, for the simple reason that THEY would have encountered us, or given us clear traces of them. The fact that no credible trace was found is a sufficient sign we are alone, although I cannot exclude the possibility of the contrary.
It's generally accepted that, for all practical intents and purposes, the universe is infinite. Mathematically, therefore, in an infinite universe there must be an infinite number of planets with intelligent life forms. Or at least one...
Jan Vingerhoets, Johannesburg, South Africa
The search for life on other "worlds" is the most ridiculous quest that humanity has ever entertained. Despite all our faults, we are the only intelligent beings in our space/time. The Universe we are aware of is a miracle for us alone to wonder at, and even explore, so that we could see the power of the Creator we try to deny.
Add your comments on this story, using the form below.
The BBC may edit your comments and not all emails will be published. Your comments may be published on any BBC media worldwide.