Are we alone?
Just how special is planet earth?
It's a question that's been asked since people first raised their eyes to contemplate the stars, one which touches on the most profound issue of existence: are we alone in the universe?
It's a question NASA's Kepler Space Telescope - which blasts off from Cape Canaveral tomorrow - hopes to answer.
It's the space agency's first mission capable of finding earth-like rocky planets around distant stars. For the next three-and-a-half years Kepler will stare, unblinking, at a small patch of the Galaxy (actually incorporating some 100,000 stars between Cygnus and Lyra) waiting for the tell-tale dip in brightness as orbiting planets pass in front of them.
Bill Borucki, the principal Investigator on Kepler at NASA's Ames Research Centre, hopes to find hundreds of earth-like planets, and dozens in the habitable or "Goldilocks" zone, that's neither too hot nor too cold to support life.
"Although Kepler will not find ET we are hoping to find ET's home," he says.
If that sounds too good to be true, others are even more optimistic. According to the author of Crowded Universe:The Search for Living Planets, Alan Boss argues that earth-like planets may be the norm rather than the exception.
Extrapolating from the data we already have from ground-based telescopes, and combining that with what we know about the processes of planetary formation, Boss comes up with a figure of 10,000 billion billion earth-like rocky planets in the observable universe.
It has to be said that few at NASA are quite as optimistic as Alan Boss. But even if the Kepler Space Telescope only spots one tiny pale blue dot spinning round a distant star, that will still be pretty exciting.