Kepler will measure the minuscule changes in stars' overall brightness when planets pass across their face Nasa

The most extensive search for Earth-like planets that could harbour life beyond the solar system is due to get under way in the early hours of tomorrow morning with the launch of a one-tonne spacecraft from Cape Canaveral in Florida.

The Kepler probe is the first Nasa mission capable of finding habitable planets like our own in faraway regions of the galaxy. If all goes to plan, the probe will be blasted into space at 03.48 GMT on Saturday atop a Delta-2 rocket, which will put the spacecraft into a solar orbit that lags behind the Earth as it circles the Sun.

From this vantage point, the spacecraft will spend three-and-a-half years gazing at a star-rich region of the Milky Way in the hope of spotting planets like our own that are in their solar systems' "Goldilocks zone", or just the right distance from their suns for liquid water to exist. To find life as we know it, Nasa's mantra is "follow the water".

A major task for the mission is to find out how many Earth-like planets there are beyond our own solar system, a question that has profound significance for the likelihood of life elsewhere.

"Finding that most stars have Earths implies that the conditions that support the development of life could be common throughout our galaxy," said William Borucki, Kepler's chief scientist at Nasa's Ames Research Center in California. "Finding few or no Earths indicates that we might be alone."

Mission scientists will use Kepler's 95 megapixel digital camera to survey the brightness of 100,000 stars in the constellations of Cygnus and Lyra every half an hour. Planets will reveal themselves as almost imperceptible reductions in brightness as they move across the faces of their stars.

"If Kepler were to look down at a small town on Earth at night from space, it would be able to detect the dimming of a porch light as somebody passed in front," said James Fanson, project manager at Nasa's Jet Propulsion Laboratory in California.

By watching for long enough, Kepler should be able to spot planets that take a year or longer to orbit their star. Most of the stars in the camera's field of view, which is as wide as two outstretched hands, are hundreds or thousands of light years from Earth.

Since before the days of Isaac Newton astronomers have speculated about planets in far-off solar systems, but the first confirmed sighting came only in the late 1980s. Since then more than 340 "exoplanets" have been discovered. The majority are what astronomers call "hot Jupiters": gas giants that orbit so close to their stars it only takes them a few days to complete an orbit.

Earlier this year, the French space agency's Corot spacecraft spotted the smallest exoplanet yet, in orbit around a star in the constellation Monoceros, 390 light years away. The planet, which is nearly twice the size of Earth, orbits so close to its parent star, temperatures on its molten surface are thought to approach 1500C.

"There's a very raw instinct to want to know if there are worlds like ours out there. It leads us to question are we alone, are there other forms of life out there, forms we might recognise as akin to our own civilised form of life? But it goes way beyond that. As scientists we're always trying to explore uncharted areas and we haven't been able to find planets like ours outside the Solar System, so this is a new frontier to explore," said Suzanne Aigrain, an astrophysicist at Exeter University in the UK who works on Corot.

Alan Boss, a member of the Kepler team and an astronomer at the Carnegie Institute in Washington DC, said last month there could be as many habitable, Earth-like planets in the Milky Way as there are Sun-like stars – around 100 billion. In the observable universe, there might be 10 billion trillion planets like ours. In other words, a one followed by 22 zeroes.

The Kepler scientists still have some formidable challenges ahead if they are to confirm there are Earth-like planets out there. For a claim to be taken seriously, they will have to work out the masses of any planets they find, and that cannot be done using the Kepler probe alone.

Traditionally, scientists work out a planet's mass using a ground-based telescope to measure how much its star wobbles as the planet goes around it. But Earth-sized planets exert such a tiny tug on their parent stars, the Kepler team will would need to spot shifts in a star's movement of less than one metre per second.

The most sensitive equipment for doing this is strapped to a telescope at the La Silla observatory in Chile, but this instrument cannot survey the starfield Kepler will be looking at. The Kepler team is now hoping to get a replica device built onto the William Herschel telescope in La Palma in the Canary Islands, which would help them to measure the mass of any planets that the spacecraft detects.

Though the Kepler mission should reveal Earth-sized planets in habitable orbits around stars, it will not be able to tell us if they are home to alien life. For that, we will have to wait for future missions that can analyse the atmospheres of the alien worlds Kepler finds.