An unmanned NASA mission to search the sky for Earth-like planets with the potential to host life has launched from Cape Canaveral in Florida.
The Kepler telescope will orbit the Sun to watch a patch of space thought to contain about 100,000 stars like ours. It will look for the slight dimming of light from these "suns" as planets pass between them and the spacecraft.
Kepler blasted off atop a Delta II rocket from Cape Canaveral Air Force Station.
"This is a historical mission, it's not just a science mission," said Dr Edward Weiler, associate administrator for the Science Mission Directorate at Nasa.
"I maintain that it really attacks some very basic human questions that have been part of our genetic code since that first man or woman looked up into the sky and asked the question: 'Are we alone?'"
Equipped with the largest camera ever launched into space, it is the first mission designed to find rocky worlds orbiting sun-like stars. Planets located in a warm zone - known as the habitable zone - might host liquid water on their surfaces. Where there is liquid water, scientists argue, there is at least the potential for life.
Of the 300-plus extra-solar planets discovered to date, only a handful are thought to be rocky like the Earth, and none are Earth-mass. The vast majority are gas giants like Jupiter or so-called ice giants like Neptune.