HAVE you ever looked up at the night sky and wondered if somebody, or something, is looking back? If perhaps somewhere out there, the mysterious spark we call life has flickered into existence?
Intuitively, it feels as if we can't be alone. For every one of the 2000 stars you can see with your naked eye, there are another 50 million in our galaxy, which is one of 100 billion galaxies. In other words, the star we orbit is just one of 10,000 billion billion in the cosmos. Surely there is another blue dot out there - a home to intelligent life like us? The simple fact is, we don't know.
One way to estimate the number of intelligent civilisations was devised by astronomer Frank Drake. His equation takes into account the rate of star formation, the fraction of those stars with planets and the likelihood that life, intelligent life, and intelligent creatures capable of communicating with us, will arise.
It is now possible to put numbers on some of those factors. We know that about 20 stars are born in the Milky Way every year and we have spotted more than 560 planets around stars other than the sun. About a quarter of stars harbour a planet similar in mass to Earth
But estimating the biological factors is little more than guesswork. We know that life is incredibly adaptable once it emerges, but not how good it is at getting started in the first place.
Unique planet
Some astronomers believe life is almost inevitable on any habitable planet. Others suspect simple life is common, but intelligent life exceedingly rare. A few believe that our planet is unique. "Life may or may not form easily," says physicist Paul Davies of Arizona State University in Tempe. "We're completely in the dark."
So much for equations. What about evidence? Finding life on Mars probably won't help, as it would very likely share its origin with Earthlings. "Impacts have undoubtedly conveyed microorganisms back and forth," says Davies. "Mars and Earth are not independent ecosystems."
Discovering life on Titan would be more revealing. Titan is the only other place in the solar system with liquid on its surface - albeit lakes of ethane. "We are starting to think that if there is life on Titan it would have a separate origin," says Dirk Schulze-Makuch at Washington State University in Pullman. "If we can find a separate origin we can say 'OK, there's a lot of life in the universe'."
Discovering alien microbes in our solar system would be some sort of proof that we are not alone, but what we really want to know is whether there is another intelligence out there. For 50 years astronomers have swept the skies with radio telescopes for any hint of a message. So far, nothing.
But that doesn't mean ET isn't there. It just might not know we're here. The only evidence of our existence that reaches beyond the solar system are radio signals and light from our cities. "We've only been broadcasting powerful radio signals since the second world war," says Seth Shostak of the SETI Institute in Mountain View, California. So our calling card has leaked just 70 light years into space, a drop in the ocean. If the Milky Way was the size of London and Earth was at the base of Nelson's Column, our radio signals would still not have left Trafalgar Square.
"It's probably safe to say that even if the local galaxy is choc-a-bloc with aliens, none of them know that Homo sapiens is here," says Shostak. That also works in reverse. Given the size of the universe and the speed of light, most stars and planets are simply out of range.
It is also possible that intelligent life is separated from us by time. After all, human intelligence has only existed for a minuscule fraction of Earth's history and may just be a fleeting phase (see "Existence: Will we die out? "). It may be too much of a stretch to hope that a nearby planet not only harbours intelligent life, but that it does so right now.
But let's say we did make contact with aliens. How would we react? NASA has plans, and most religions claim they would be able to absorb the idea, but the bottom line is we won't know until it happens.
Most likely we'll never find out. Even if Earth is not the only planet with intelligent life, we appear destined to live out our entire existence as if it were - but with a nagging feeling that it can't be. How's that for existential
uncertainty?
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