Viewpoint: The obsession with a 'twin Earth'
Discovery of an "Earth-like" planet has generated a wave of excitement, but our fascination with finding other habitable worlds goes back a long way, argues science fiction writer Robert J Sawyer.
The most famous words in all of science fiction are Captain Kirk's opening narration from Star Trek, in which he explains that the Enterprise's mission is "to explore strange new worlds".
But what we really want is familiar new worlds - worlds like good old mother Earth, worlds where we might find "new life and new civilisations."
The notion that "Earth-like" worlds might be abodes of life goes right back to the dawn of science fiction. In HG Wells's 1898 masterpiece The War of the Worlds, Mars is an older world than Earth that has been dying over time.
That our planet reminded the Martians of how wet and verdant their own world had been, led, in Wells's chilling words, "to intellects vast and cool and unsympathetic regarding this earth with envious eyes, and slowly and surely drawing their plans against us".
Wells's novel begins with an epigram from Johannes Kepler, the great German astronomer who died 381 years ago. The orbiting telescope that bears his name recently discovered what scientists were quick to label a twin of Earth orbiting a star some 600 light years distant.
"Twin" is an exaggeration. The newfound world, dubbed Kepler-22b, is much bigger than our planet - but, even so, it's also the smallest world yet discovered outside our Solar System, and finding it does bode well for the eventual detection of a true twin, a planet that really is a second Earth.
Small planets, like the inner four in our own solar system, are rocky - larger ones, like the outer four, aren't much more than balls of gas. Kepler-22b is quite a bit larger than the biggest rocky planet in this Solar System (which happens to be Earth), but much smaller than the smallest of our local gaseous worlds (Neptune).
We really don't know what sort of composition a planet the size of Kepler-22b would have. But if it is a rocky world, its gravity would be about 2.4 times that of Earth - there may be life there, but I wouldn't expect to see anything as gracile as our giraffes or whooping cranes.
It's not just size that matters, though. To be Earthlike, you also need a sun like ours and Kepler-22b has that.
You also need an appropriate orbit - one that puts you not too close to or far from the heat of the star, one that allows liquid water, which most scientists believe is necessary for life, to exist. Kepler-22b satisfies on that score, too. If it has an atmosphere like Earth's, with a corresponding greenhouse effect, its average surface temperature would be a balmy 22C.
Keeping on looking
But that's a mighty big if. Our own neighbouring world of Venus is much more a twin for Earth in size, and it orbits within the range of distances from our sun that could support liquid water - but it has a crushingly dense atmosphere and a runaway greenhouse effect resulting in an average surface temperature of 460C. No life as we know it could exist there.
And, indeed, the chances are pretty slim of Kepler-22b actually being the wet, clement world we hope it is. So be it, we'll keep on looking.
Captain Kirk and crew visited a new planet every week - we're finding new planets at an even greater rate than that. Soon enough, we will detect what we're looking for - not a strange new world, but a familiar one with oceans and land masses and polar ice caps, with a transparent oxygen-rich atmosphere, and with a comfortable surface gravity.
And what then? Captain Kirk said it best - we will want to boldly go where no-one has gone before.
There are serious discussions under way to make that possible. In October 2011, the US Defense Advanced Research Project Agency invited me and nine other science-fiction writers to share our visions with scientists and engineers at the first public symposium for The 100 Year Starship Project, an initiative to get human beings to a planet outside our Solar System within a century.
The consensus of the meeting - held not far from Cape Kennedy - was that this is indeed a realistic goal.
When Neil Armstrong first set foot upon the Moon, commentators worldwide said that science fiction had become science fact.
Eventually that small step will be followed by a giant leap as humans walk on an extrasolar planet. If the planet we visit is indeed a true twin of Earth, perhaps there will be natives on it to greet us when we land.
Let's hope the meeting goes more like the usually friendly encounters of Star Trek rather than the one HG Wells described - but either way, as we move out into the final frontier, science fiction will once more become science fact.